diff --git a/lua/codediff/ui/conflict/gutter/compute.lua b/lua/codediff/ui/conflict/gutter/compute.lua
new file mode 100644
index 00000000..d6093555
--- /dev/null
+++ b/lua/codediff/ui/conflict/gutter/compute.lua
@@ -0,0 +1,74 @@
+-- Compute conflict gutter markers from visual positions.
+local M = {}
+
+local function append_positions(positions, line, count)
+  for offset = 1, count do
+    positions[#positions + 1] = { line = line, offset = offset }
+  end
+end
+
+--- Return the visual positions covered by one buffer-side range.
+---
+--- Ranges are one-based and half-open. Filler counts are indexed by the
+--- one-based real-line anchor after which the filler is displayed. Anchor zero
+--- represents filler above the first buffer line.
+function M.visual_positions(range, filler_counts)
+  if not range or range.start_line > range.end_line then
+    return {}
+  end
+
+  filler_counts = filler_counts or {}
+  local positions = {}
+
+  if range.start_line == range.end_line then
+    local anchor = range.start_line - 1
+    local count = filler_counts[anchor] or 0
+    if count == 0 then
+      anchor = range.start_line
+      count = filler_counts[anchor] or 0
+    end
+    append_positions(positions, anchor, count)
+    return positions
+  end
+
+  append_positions(positions, range.start_line - 1, filler_counts[range.start_line - 1] or 0)
+  for line = range.start_line, range.end_line - 1 do
+    positions[#positions + 1] = { line = line, offset = 0 }
+    append_positions(positions, line, filler_counts[line] or 0)
+  end
+
+  return positions
+end
+
+local function set_marker(markers, position, text)
+  local line_markers = markers[position.line]
+  if not line_markers then
+    line_markers = {}
+    markers[position.line] = line_markers
+  end
+  line_markers[position.offset] = text
+end
+
+--- Compute one marker for every supplied visual position.
+function M.compute_markers(positions)
+  local markers = {}
+  local count = #positions
+
+  for index, position in ipairs(positions) do
+    local text
+    if count == 1 then
+      text = "["
+    elseif index == 1 then
+      text = "╭─"
+    elseif index == count then
+      text = "╰─"
+    else
+      text = "│ "
+    end
+    set_marker(markers, position, text)
+  end
+
+  return markers
+end
+
+return M
diff --git a/lua/codediff/ui/conflict/gutter/init.lua b/lua/codediff/ui/conflict/gutter/init.lua
new file mode 100644
index 00000000..8e30238f
--- /dev/null
+++ b/lua/codediff/ui/conflict/gutter/init.lua
@@ -0,0 +1,186 @@
+-- Coordinate conflict gutter computation and its Neovim renderers.
+local M = {}
+
+local lifecycle = require("codediff.ui.lifecycle")
+local tracking = require("codediff.ui.conflict.tracking")
+local filler = require("codediff.ui.filler")
+local compute = require("codediff.ui.conflict.gutter.compute")
+local signcolumn = require("codediff.ui.conflict.gutter.signcolumn")
+local statuscolumn = require("codediff.ui.conflict.gutter.statuscolumn")
+
+local function normalize_range(bufnr, range)
+  if not bufnr or not range or not vim.api.nvim_buf_is_valid(bufnr) then
+    return nil
+  end
+
+  local line_count = vim.api.nvim_buf_line_count(bufnr)
+  if line_count == 0 then
+    return nil
+  end
+
+  local start_line = math.max(1, math.min(range.start_line, line_count + 1))
+  local end_line = math.max(1, math.min(range.end_line, line_count + 1))
+  if end_line < start_line then
+    return nil
+  end
+
+  return { start_line = start_line, end_line = end_line }
+end
+
+local function compute_projection(bufnr, range, filler_counts, highlight)
+  local normalized = normalize_range(bufnr, range)
+  if not normalized then
+    return nil
+  end
+
+  local positions = compute.visual_positions(normalized, filler_counts)
+  if #positions == 0 then
+    return nil
+  end
+
+  return {
+    markers = compute.compute_markers(positions),
+    highlight = highlight,
+  }
+end
+
+local function accepted_highlights(session, block)
+  if tracking.is_block_active(session, block) then
+    return "CodeDiffConflictSign", "CodeDiffConflictSign", "CodeDiffConflictSign"
+  end
+
+  local accepted = tracking.get_accepted_side(session, block)
+  if accepted == "incoming" then
+    return "CodeDiffConflictSignAccepted", "CodeDiffConflictSignRejected", "CodeDiffConflictSignResolved"
+  elseif accepted == "current" then
+    return "CodeDiffConflictSignRejected", "CodeDiffConflictSignAccepted", "CodeDiffConflictSignResolved"
+  elseif accepted == "both" then
+    return "CodeDiffConflictSignAccepted", "CodeDiffConflictSignAccepted", "CodeDiffConflictSignResolved"
+  end
+  return "CodeDiffConflictSignResolved", "CodeDiffConflictSignResolved", "CodeDiffConflictSignResolved"
+end
+
+local function tracking_needs_reinit(session)
+  for _, block in ipairs(session.conflict_blocks) do
+    if not block.extmark_id then
+      return true
+    end
+    local mark = vim.api.nvim_buf_get_extmark_by_id(session.result_bufnr, tracking.tracking_ns, block.extmark_id, { details = true })
+    if not mark or #mark < 3 or not mark[3] then
+      return true
+    end
+    local expected_range = block.result_range or block.base_range
+    if mark[1] == 0 and expected_range.start_line - 1 > 0 then
+      return true
+    end
+  end
+  return false
+end
+
+local function clear_markers(session, namespace)
+  for _, bufnr in ipairs({ session.original_bufnr, session.modified_bufnr }) do
+    if bufnr and vim.api.nvim_buf_is_valid(bufnr) then
+      vim.api.nvim_buf_clear_namespace(bufnr, namespace, 0, -1)
+    end
+  end
+  if session.result_bufnr and vim.api.nvim_buf_is_valid(session.result_bufnr) then
+    vim.api.nvim_buf_clear_namespace(session.result_bufnr, tracking.result_signs_ns, 0, -1)
+  end
+end
+
+--- Recompute all conflict marker maps and render them in every buffer.
+function M.refresh(session)
+  if not session or not session.conflict_blocks then
+    return
+  end
+
+  local highlights = require("codediff.ui.highlights")
+  local ns_conflict = highlights.ns_conflict
+
+  if tracking_needs_reinit(session) and session.result_bufnr and vim.api.nvim_buf_is_valid(session.result_bufnr) then
+    tracking.initialize_tracking(session.result_bufnr, session.conflict_blocks)
+  end
+
+  clear_markers(session, ns_conflict)
+
+  local original_counts = filler.get_virt_line_counts(session.original_bufnr)
+  local modified_counts = filler.get_virt_line_counts(session.modified_bufnr)
+  local result_counts = filler.get_virt_line_counts(session.result_bufnr)
+  local original_projections = {}
+  local modified_projections = {}
+  local result_projections = {}
+
+  for _, block in ipairs(session.conflict_blocks) do
+    local left_highlight, right_highlight, result_highlight = accepted_highlights(session, block)
+
+    local original = compute_projection(session.original_bufnr, block.output1_range, original_counts, left_highlight)
+    if original then
+      original_projections[#original_projections + 1] = original
+    end
+
+    local modified = compute_projection(session.modified_bufnr, block.output2_range, modified_counts, right_highlight)
+    if modified then
+      modified_projections[#modified_projections + 1] = modified
+    end
+
+    if session.result_bufnr and vim.api.nvim_buf_is_valid(session.result_bufnr) and block.extmark_id then
+      local mark = vim.api.nvim_buf_get_extmark_by_id(session.result_bufnr, tracking.tracking_ns, block.extmark_id, { details = true })
+      if mark and #mark >= 3 and mark[3].end_row then
+        local result = compute_projection(session.result_bufnr, { start_line = mark[1] + 1, end_line = mark[3].end_row + 1 }, result_counts, result_highlight)
+        if result then
+          result_projections[#result_projections + 1] = result
+        end
+      end
+    end
+  end
+
+  signcolumn.render(session.original_bufnr, original_projections, ns_conflict)
+  signcolumn.render(session.modified_bufnr, modified_projections, ns_conflict)
+  signcolumn.render(session.result_bufnr, result_projections, tracking.result_signs_ns)
+  statuscolumn.update(session.original_win, original_projections, original_counts)
+  statuscolumn.update(session.modified_win, modified_projections, modified_counts)
+end
+
+--- Attach gutter rendering to the two conflict input windows.
+function M.attach(original_win, modified_win)
+  statuscolumn.attach(original_win)
+  statuscolumn.attach(modified_win)
+end
+
+--- Detach gutter rendering from one conflict input window.
+function M.detach(win)
+  statuscolumn.detach(win)
+end
+
+--- Release all gutter resources before a conflict view is retargeted or closed.
+function M.teardown(tabpage)
+  local session = lifecycle.get_session(tabpage)
+  if session then
+    clear_markers(session, require("codediff.ui.highlights").ns_conflict)
+    M.detach(session.original_win)
+    M.detach(session.modified_win)
+  end
+  pcall(vim.api.nvim_del_augroup_by_name, "CodeDiffConflictGutter_" .. tabpage)
+end
+
+--- Setup an autocmd to refresh gutter markers when the result changes.
+function M.setup_refresh_autocmd(tabpage, result_bufnr)
+  if not result_bufnr or not vim.api.nvim_buf_is_valid(result_bufnr) then
+    return
+  end
+
+  local group = vim.api.nvim_create_augroup("CodeDiffConflictGutter_" .. tabpage, { clear = true })
+
+  vim.api.nvim_create_autocmd({ "TextChanged", "TextChangedI" }, {
+    group = group,
+    buffer = result_bufnr,
+    callback = function()
+      local session = lifecycle.get_session(tabpage)
+      if session and not session.suspended and session.result_bufnr == result_bufnr then
+        M.refresh(session)
+      end
+    end,
+  })
+end
+
+return M
diff --git a/lua/codediff/ui/conflict/gutter/signcolumn.lua b/lua/codediff/ui/conflict/gutter/signcolumn.lua
new file mode 100644
index 00000000..c2b61d75
--- /dev/null
+++ b/lua/codediff/ui/conflict/gutter/signcolumn.lua
@@ -0,0 +1,24 @@
+-- Render precomputed conflict markers in the native signcolumn.
+local M = {}
+
+--- Render the real-line markers from one or more buffer projections.
+function M.render(bufnr, projections, namespace)
+  if not bufnr or not vim.api.nvim_buf_is_valid(bufnr) then
+    return
+  end
+
+  for _, projection in ipairs(projections or {}) do
+    for line, offsets in pairs(projection.markers or {}) do
+      local text = offsets[0]
+      if text and line >= 1 then
+        vim.api.nvim_buf_set_extmark(bufnr, namespace, line - 1, 0, {
+          sign_text = text,
+          sign_hl_group = projection.highlight,
+          priority = 50,
+        })
+      end
+    end
+  end
+end
+
+return M
diff --git a/lua/codediff/ui/conflict/gutter/statuscolumn.lua b/lua/codediff/ui/conflict/gutter/statuscolumn.lua
new file mode 100644
index 00000000..3b90b9d8
--- /dev/null
+++ b/lua/codediff/ui/conflict/gutter/statuscolumn.lua
@@ -0,0 +1,146 @@
+-- Render precomputed conflict markers for filler virtual lines.
+local M = {}
+
+local states = {}
+local expression = "%!v:lua.require'codediff.ui.conflict.gutter.statuscolumn'.render()"
+
+local function number_item(win)
+  local options = vim.wo[win]
+  if not options.number and not options.relativenumber then
+    return ""
+  end
+
+  -- %l preserves native relative numbers, hybrid alignment and wrapped rows.
+  -- With signcolumn=number, signs need right alignment even on the cursor row.
+  if options.signcolumn == "number" then
+    local cursor_number = options.number and options.relativenumber and vim.v.relnum == 0 and vim.v.virtnum == 0
+    if cursor_number then
+      local row = vim.v.lnum - 1
+      local signs = vim.api.nvim_buf_get_extmarks(vim.api.nvim_win_get_buf(win), -1, { row, 0 }, { row, -1 }, { type = "sign", limit = 1 })
+      cursor_number = #signs == 0
+    end
+    if not cursor_number then
+      return "%=%l "
+    end
+  end
+  return "%l "
+end
+
+local function default_statuscolumn(win)
+  return "%C%s" .. number_item(win)
+end
+
+local function render_marker(win, marker)
+  local marker_text = marker.text == "[" and "[ " or marker.text
+  local highlighted = "%#" .. marker.highlight .. "#" .. marker_text .. "%*"
+  local options = vim.wo[win]
+  if options.signcolumn == "number" and (options.number or options.relativenumber) then
+    return "%C%=" .. highlighted .. " "
+  end
+  return "%C" .. highlighted .. number_item(win)
+end
+
+--- Render in the drawing window's context, even if another pane has focus.
+function M.render()
+  local win = tonumber(vim.g.statusline_winid)
+  if not win or not vim.api.nvim_win_is_valid(win) then
+    return ""
+  end
+  local state = states[win]
+  if state and state.bufnr == vim.api.nvim_win_get_buf(win) and vim.v.virtnum < 0 and vim.wo[win].signcolumn ~= "no" then
+    local line_markers = state.markers[vim.v.lnum]
+    local marker = line_markers and line_markers[-vim.v.virtnum]
+    if marker then
+      return render_marker(win, marker)
+    end
+  end
+  return default_statuscolumn(win)
+end
+
+--- Attach only when the column is native or inherited from our renderer.
+function M.attach(win)
+  if vim.fn.exists("+statuscolumn") ~= 1 or not win or not vim.api.nvim_win_is_valid(win) then
+    return false
+  end
+
+  local current = vim.wo[win].statuscolumn
+  if current ~= "" and current ~= expression then
+    states[win] = nil
+    return false
+  end
+
+  local bufnr = vim.api.nvim_win_get_buf(win)
+  local state = states[win]
+  if not state or state.bufnr ~= bufnr then
+    states[win] = { bufnr = bufnr, markers = {} }
+  end
+  if current ~= expression then
+    vim.wo[win].statuscolumn = expression
+  end
+  return true
+end
+
+--- Index precomputed markers by Neovim's statuscolumn coordinates.
+function M.update(win, projections, filler_counts)
+  local state = states[win]
+  if not state or not vim.api.nvim_win_is_valid(win) or state.bufnr ~= vim.api.nvim_win_get_buf(win) or vim.wo[win].statuscolumn ~= expression then
+    M.detach(win)
+    return false
+  end
+
+  local markers = {}
+  local bof_count = filler_counts and filler_counts[0] or 0
+  for _, projection in ipairs(projections or {}) do
+    for line, offsets in pairs(projection.markers) do
+      for offset, text in pairs(offsets) do
+        if offset > 0 then
+          -- BOF fillers and fillers after line one share v:lnum=1; Neovim
+          -- numbers them consecutively, including off-screen BOF fillers.
+          local lnum = math.max(1, line)
+          local virtnum = offset + (line == 1 and bof_count or 0)
+          markers[lnum] = markers[lnum] or {}
+          markers[lnum][virtnum] = { text = text, highlight = projection.highlight }
+        end
+      end
+    end
+  end
+  state.markers = markers
+  return true
+end
+
+--- Restore the option only while we still own it.
+function M.detach(win)
+  local state = states[win]
+  if not state then
+    return
+  end
+  states[win] = nil
+  if vim.api.nvim_win_is_valid(win) and vim.wo[win].statuscolumn == expression then
+    vim.wo[win].statuscolumn = ""
+  end
+end
+
+if vim.fn.exists("+statuscolumn") == 1 then
+  local group = vim.api.nvim_create_augroup("CodeDiffConflictGutterWindows", { clear = true })
+  vim.api.nvim_create_autocmd("WinClosed", {
+    group = group,
+    callback = function(event)
+      states[tonumber(event.match)] = nil
+    end,
+  })
+  vim.api.nvim_create_autocmd("BufWinEnter", {
+    group = group,
+    callback = function()
+      local win = vim.api.nvim_get_current_win()
+      local state = states[win]
+      if state and state.bufnr ~= vim.api.nvim_win_get_buf(win) then
+        M.detach(win)
+      elseif not state and vim.wo[win].statuscolumn == expression then
+        -- A split can inherit the expression, but not ownership of its state.
+        vim.wo[win].statuscolumn = ""
+      end
+    end,
+  })
+end
+
+return M
diff --git a/lua/codediff/ui/conflict/init.lua b/lua/codediff/ui/conflict/init.lua
index 47e8e960..50a4773a 100644
--- a/lua/codediff/ui/conflict/init.lua
+++ b/lua/codediff/ui/conflict/init.lua
@@ -4,7 +4,7 @@ local M = {}
 
 -- Import submodules
 local tracking = require("codediff.ui.conflict.tracking")
-local signs = require("codediff.ui.conflict.signs")
+local gutter = require("codediff.ui.conflict.gutter")
 local resolution = require("codediff.ui.conflict.resolution")
 local navigation = require("codediff.ui.conflict.navigation")
 local keymaps = require("codediff.ui.conflict.keymaps")
@@ -13,9 +13,12 @@ local keymaps = require("codediff.ui.conflict.keymaps")
 M.run_repeatable_action = tracking.run_repeatable_action
 M.initialize_tracking = tracking.initialize_tracking
 
--- Delegate to signs module
-M.refresh_all_conflict_signs = signs.refresh_all_conflict_signs
-M.setup_sign_refresh_autocmd = signs.setup_sign_refresh_autocmd
+-- Delegate to the conflict renderer
+M.refresh = gutter.refresh
+M.setup_refresh_autocmd = gutter.setup_refresh_autocmd
+M.attach_gutter = gutter.attach
+M.detach_gutter = gutter.detach
+M.teardown_gutter = gutter.teardown
 
 -- Delegate to resolution module
 M.accept_incoming = resolution.accept_incoming
diff --git a/lua/codediff/ui/conflict/resolution/block.lua b/lua/codediff/ui/conflict/resolution/block.lua
index ae2e56a7..ad494020 100644
--- a/lua/codediff/ui/conflict/resolution/block.lua
+++ b/lua/codediff/ui/conflict/resolution/block.lua
@@ -4,7 +4,7 @@ local M = {}
 local lifecycle = require("codediff.ui.lifecycle")
 local auto_refresh = require("codediff.ui.auto_refresh")
 local tracking = require("codediff.ui.conflict.tracking")
-local signs = require("codediff.ui.conflict.signs")
+local gutter = require("codediff.ui.conflict.gutter")
 local apply_to_result = require("codediff.ui.conflict.resolution.replace").apply_to_result
 local combine = require("codediff.ui.conflict.resolution.combine")
 local smart_combine_inputs = combine.smart_combine_inputs
@@ -57,7 +57,7 @@ function M.accept_incoming(tabpage)
   end
 
   apply_to_result(result_bufnr, block, incoming_lines, base_lines)
-  signs.refresh_all_conflict_signs(session)
+  gutter.refresh(session)
   auto_refresh.refresh_result_now(result_bufnr)
   return true
 end
@@ -107,7 +107,7 @@ function M.accept_current(tabpage)
   end
 
   apply_to_result(result_bufnr, block, current_lines, base_lines)
-  signs.refresh_all_conflict_signs(session)
+  gutter.refresh(session)
   auto_refresh.refresh_result_now(result_bufnr)
   return true
 end
@@ -169,7 +169,7 @@ function M.accept_both(tabpage)
   end
 
   apply_to_result(result_bufnr, block, combined, base_lines)
-  signs.refresh_all_conflict_signs(session)
+  gutter.refresh(session)
   auto_refresh.refresh_result_now(result_bufnr)
   return true
 end
@@ -231,7 +231,7 @@ function M.discard(tabpage)
   -- apply_to_result indexes its base_lines parameter by result_range for the
   -- content-search fallback, so pass the Result seed (auto-merged content).
   apply_to_result(result_bufnr, block, base_content, session.result_base_lines or base_lines)
-  signs.refresh_all_conflict_signs(session)
+  gutter.refresh(session)
   auto_refresh.refresh_result_now(result_bufnr)
   return true
 end
diff --git a/lua/codediff/ui/conflict/resolution/diffget.lua b/lua/codediff/ui/conflict/resolution/diffget.lua
index 070e6407..8b92d413 100644
--- a/lua/codediff/ui/conflict/resolution/diffget.lua
+++ b/lua/codediff/ui/conflict/resolution/diffget.lua
@@ -4,7 +4,7 @@ local M = {}
 local lifecycle = require("codediff.ui.lifecycle")
 local auto_refresh = require("codediff.ui.auto_refresh")
 local tracking = require("codediff.ui.conflict.tracking")
-local signs = require("codediff.ui.conflict.signs")
+local gutter = require("codediff.ui.conflict.gutter")
 
 --- Apply text to result buffer at the conflict's range
 --- @param result_bufnr number Result buffer
@@ -105,7 +105,7 @@ function M.diffget_incoming(tabpage)
   end
 
   apply_to_result(result_bufnr, block, incoming_lines, base_lines)
-  signs.refresh_all_conflict_signs(session)
+  gutter.refresh(session)
   auto_refresh.refresh_result_now(result_bufnr)
   return true
 end
@@ -153,7 +153,7 @@ function M.diffget_current(tabpage)
   end
 
   apply_to_result(result_bufnr, block, current_lines, base_lines)
-  signs.refresh_all_conflict_signs(session)
+  gutter.refresh(session)
   auto_refresh.refresh_result_now(result_bufnr)
   return true
 end
diff --git a/lua/codediff/ui/conflict/resolution/file.lua b/lua/codediff/ui/conflict/resolution/file.lua
index 9608a071..84885faa 100644
--- a/lua/codediff/ui/conflict/resolution/file.lua
+++ b/lua/codediff/ui/conflict/resolution/file.lua
@@ -4,7 +4,7 @@ local M = {}
 local lifecycle = require("codediff.ui.lifecycle")
 local auto_refresh = require("codediff.ui.auto_refresh")
 local tracking = require("codediff.ui.conflict.tracking")
-local signs = require("codediff.ui.conflict.signs")
+local gutter = require("codediff.ui.conflict.gutter")
 local apply_to_result = require("codediff.ui.conflict.resolution.replace").apply_to_result
 
 --- Accept ALL incoming (left/input1) for all active conflicts
@@ -47,7 +47,7 @@ function M.accept_all_incoming(tabpage)
     end
   end)
 
-  signs.refresh_all_conflict_signs(session)
+  gutter.refresh(session)
   auto_refresh.refresh_result_now(result_bufnr)
   vim.notify(string.format("[codediff] Accepted %d incoming change(s)", count), vim.log.levels.INFO)
   return count > 0
@@ -91,7 +91,7 @@ function M.accept_all_current(tabpage)
     end
   end)
 
-  signs.refresh_all_conflict_signs(session)
+  gutter.refresh(session)
   auto_refresh.refresh_result_now(result_bufnr)
   vim.notify(string.format("[codediff] Accepted %d current change(s)", count), vim.log.levels.INFO)
   return count > 0
@@ -149,7 +149,7 @@ function M.accept_all_both(tabpage, first_input)
     end
   end)
 
-  signs.refresh_all_conflict_signs(session)
+  gutter.refresh(session)
   auto_refresh.refresh_result_now(result_bufnr)
   vim.notify(string.format("[codediff] Accepted %d combined change(s)", count), vim.log.levels.INFO)
   return count > 0
@@ -200,7 +200,7 @@ function M.discard_all(tabpage)
     end
   end)
 
-  signs.refresh_all_conflict_signs(session)
+  gutter.refresh(session)
   auto_refresh.refresh_result_now(result_bufnr)
   vim.notify(string.format("[codediff] Reset %d conflict(s) to base", count), vim.log.levels.INFO)
   return count > 0
diff --git a/lua/codediff/ui/conflict/signs.lua b/lua/codediff/ui/conflict/signs.lua
deleted file mode 100644
index cf1d7af3..00000000
--- a/lua/codediff/ui/conflict/signs.lua
+++ /dev/null
@@ -1,192 +0,0 @@
--- Conflict sign management for merge tool
--- Handles sign refresh and autocmd setup
-local M = {}
-
-local lifecycle = require("codediff.ui.lifecycle")
-local tracking = require("codediff.ui.conflict.tracking")
-
---- Refresh all conflict signs based on current state (event-driven approach)
---- Called on TextChanged to keep signs in sync with actual buffer content
---- Also used for initial sign setup after initialize_tracking
---- @param session table The diff session
-function M.refresh_all_conflict_signs(session)
-  if not session or not session.conflict_blocks then
-    return
-  end
-
-  local highlights = require("codediff.ui.highlights")
-  local ns_conflict = highlights.ns_conflict
-
-  -- Check if extmarks need re-initialization (e.g., after undo to original state)
-  -- If any extmark is missing or invalid, re-initialize all tracking
-  local needs_reinit = false
-  for _, block in ipairs(session.conflict_blocks) do
-    if not block.extmark_id then
-      needs_reinit = true
-      break
-    end
-    local mark = vim.api.nvim_buf_get_extmark_by_id(session.result_bufnr, tracking.tracking_ns, block.extmark_id, { details = true })
-    if not mark or #mark < 3 or not mark[3] then
-      needs_reinit = true
-      break
-    end
-    -- Check if extmark position is reasonable (not spanning entire buffer or starting at 0 when it shouldn't)
-    local mark_start = mark[1]
-    local mark_end = mark[3].end_row
-    -- The extmark is placed at the block's position in the auto-merged Result
-    -- buffer (result_range), so compare against that. Legacy blocks without
-    -- result_range fall back to base_range.
-    local expected_range = block.result_range or block.base_range
-    local expected_start = expected_range.start_line - 1
-    -- If extmark start moved to 0 but expected start is not 0, it's corrupted (common after undo/redo)
-    if mark_start == 0 and expected_start > 0 then
-      needs_reinit = true
-      break
-    end
-  end
-
-  if needs_reinit and session.result_bufnr and vim.api.nvim_buf_is_valid(session.result_bufnr) then
-    tracking.initialize_tracking(session.result_bufnr, session.conflict_blocks)
-  end
-
-  -- Helper to set signs for a buffer range (for non-empty ranges)
-  local function set_signs_for_range(bufnr, start_line, end_line, namespace, hl_group, is_active)
-    if not bufnr or not vim.api.nvim_buf_is_valid(bufnr) then
-      return
-    end
-
-    local line_count = vim.api.nvim_buf_line_count(bufnr)
-
-    if start_line == end_line then
-      -- Empty range: show a top-aligned horizontal bar sign to indicate "something goes here"
-      if start_line >= 0 and start_line < line_count then
-        local marks = vim.api.nvim_buf_get_extmarks(bufnr, namespace, { start_line, 0 }, { start_line, -1 }, {})
-        for _, mark in ipairs(marks) do
-          vim.api.nvim_buf_del_extmark(bufnr, namespace, mark[1])
-        end
-        vim.api.nvim_buf_set_extmark(bufnr, namespace, start_line, 0, {
-          sign_text = "▔▔", -- Upper block - appears at top of line, like between two lines
-          sign_hl_group = hl_group,
-          priority = 50,
-        })
-      end
-    else
-      for line = start_line, end_line - 1 do
-        if line >= 0 and line < line_count then
-          local marks = vim.api.nvim_buf_get_extmarks(bufnr, namespace, { line, 0 }, { line, -1 }, {})
-          for _, mark in ipairs(marks) do
-            vim.api.nvim_buf_del_extmark(bufnr, namespace, mark[1])
-          end
-          vim.api.nvim_buf_set_extmark(bufnr, namespace, line, 0, {
-            sign_text = "▌",
-            sign_hl_group = hl_group,
-            priority = 50,
-          })
-        end
-      end
-    end
-  end
-
-  -- Clear result buffer signs (they move with content, so clear and re-add)
-  if session.result_bufnr and vim.api.nvim_buf_is_valid(session.result_bufnr) then
-    vim.api.nvim_buf_clear_namespace(session.result_bufnr, tracking.result_signs_ns, 0, -1)
-  end
-
-  -- Update signs for each block based on is_block_active state
-  for _, block in ipairs(session.conflict_blocks) do
-    local is_active = tracking.is_block_active(session, block)
-
-    -- Determine highlight groups for left/right/result based on accepted side
-    local left_hl, right_hl, result_hl
-    if is_active then
-      -- Unresolved: all orange
-      left_hl = "CodeDiffConflictSign"
-      right_hl = "CodeDiffConflictSign"
-      result_hl = "CodeDiffConflictSign"
-    else
-      -- Resolved: check which side was accepted
-      local accepted = tracking.get_accepted_side(session, block)
-      if accepted == "incoming" then
-        left_hl = "CodeDiffConflictSignAccepted" -- Green (chosen)
-        right_hl = "CodeDiffConflictSignRejected" -- Red (not chosen)
-      elseif accepted == "current" then
-        left_hl = "CodeDiffConflictSignRejected" -- Red (not chosen)
-        right_hl = "CodeDiffConflictSignAccepted" -- Green (chosen)
-      elseif accepted == "both" then
-        left_hl = "CodeDiffConflictSignAccepted" -- Green (both chosen)
-        right_hl = "CodeDiffConflictSignAccepted" -- Green (both chosen)
-      else
-        -- Manual edit or unknown - use gray
-        left_hl = "CodeDiffConflictSignResolved"
-        right_hl = "CodeDiffConflictSignResolved"
-      end
-      -- Result buffer always uses gray for resolved
-      result_hl = "CodeDiffConflictSignResolved"
-    end
-
-    -- Update left buffer (incoming)
-    local left_start = block.output1_range.start_line - 1
-    local left_end = block.output1_range.end_line - 1
-    set_signs_for_range(session.original_bufnr, left_start, left_end, ns_conflict, left_hl, is_active)
-
-    -- Update right buffer (current)
-    local right_start = block.output2_range.start_line - 1
-    local right_end = block.output2_range.end_line - 1
-    set_signs_for_range(session.modified_bufnr, right_start, right_end, ns_conflict, right_hl, is_active)
-
-    -- Update result buffer (use tracked extmark position)
-    if session.result_bufnr and vim.api.nvim_buf_is_valid(session.result_bufnr) and block.extmark_id then
-      local mark = vim.api.nvim_buf_get_extmark_by_id(session.result_bufnr, tracking.tracking_ns, block.extmark_id, { details = true })
-      if mark and #mark >= 3 then
-        local result_start = mark[1]
-        local result_end = mark[3].end_row
-        local line_count = vim.api.nvim_buf_line_count(session.result_bufnr)
-
-        if result_start == result_end then
-          -- Empty conflict region: show a top-aligned horizontal bar sign
-          if result_start >= 0 and result_start < line_count then
-            vim.api.nvim_buf_set_extmark(session.result_bufnr, tracking.result_signs_ns, result_start, 0, {
-              sign_text = "▔▔", -- Upper block - appears at top of line, like between two lines
-              sign_hl_group = result_hl,
-              priority = 50,
-            })
-          end
-        else
-          for line = result_start, result_end - 1 do
-            if line >= 0 and line < line_count then
-              vim.api.nvim_buf_set_extmark(session.result_bufnr, tracking.result_signs_ns, line, 0, {
-                sign_text = "▌",
-                sign_hl_group = result_hl,
-                priority = 50,
-              })
-            end
-          end
-        end
-      end
-    end
-  end
-end
-
---- Setup autocmd to refresh signs when result buffer changes
---- @param tabpage number The tabpage ID
---- @param result_bufnr number The result buffer handle
-function M.setup_sign_refresh_autocmd(tabpage, result_bufnr)
-  if not result_bufnr or not vim.api.nvim_buf_is_valid(result_bufnr) then
-    return
-  end
-
-  local group = vim.api.nvim_create_augroup("CodeDiffConflictSigns_" .. tabpage, { clear = true })
-
-  vim.api.nvim_create_autocmd({ "TextChanged", "TextChangedI" }, {
-    group = group,
-    buffer = result_bufnr,
-    callback = function()
-      local session = lifecycle.get_session(tabpage)
-      if session then
-        M.refresh_all_conflict_signs(session)
-      end
-    end,
-  })
-end
-
-return M
diff --git a/lua/codediff/ui/conflict/view/result.lua b/lua/codediff/ui/conflict/view/result.lua
index 49c23905..b5e4839c 100644
--- a/lua/codediff/ui/conflict/view/result.lua
+++ b/lua/codediff/ui/conflict/view/result.lua
@@ -169,14 +169,15 @@ function M.setup_conflict_result_window(tabpage, session_config, original_win, m
   -- Initialize conflict tracking (keymaps setup separately after setup_all_keymaps)
   local conflict = require("codediff.ui.conflict")
   conflict.initialize_tracking(result_bufnr, conflict_diffs.conflict_blocks)
+  conflict.attach_gutter(original_win, modified_win)
 
-  -- Setup autocmd to refresh signs when result buffer changes (event-driven approach)
-  conflict.setup_sign_refresh_autocmd(tabpage, result_bufnr)
+  -- Setup autocmd to refresh markers when the result buffer changes.
+  conflict.setup_refresh_autocmd(tabpage, result_bufnr)
 
-  -- Initialize all conflict signs (uses refresh_all_conflict_signs for centralized logic)
+  -- Initialize all conflict markers through the centralized renderer.
   local session = lifecycle.get_session(tabpage)
   if session then
-    conflict.refresh_all_conflict_signs(session)
+    conflict.refresh(session)
   end
 
   -- Return focus to modified window
diff --git a/lua/codediff/ui/filler.lua b/lua/codediff/ui/filler.lua
index d1820893..2c5d2167 100644
--- a/lua/codediff/ui/filler.lua
+++ b/lua/codediff/ui/filler.lua
@@ -41,6 +41,25 @@ local function get_line()
   return cached_line
 end
 
+--- Return virtual-line counts indexed by the one-based line anchor.
+function M.get_virt_line_counts(bufnr)
+  local counts = {}
+  if not bufnr or not vim.api.nvim_buf_is_valid(bufnr) then
+    return counts
+  end
+
+  local marks = vim.api.nvim_buf_get_extmarks(bufnr, highlights.ns_filler, 0, -1, { details = true })
+  for _, mark in ipairs(marks) do
+    local details = mark[4]
+    local count = details and details.virt_lines and #details.virt_lines or 0
+    if count > 0 then
+      local anchor = details.virt_lines_above and mark[2] or mark[2] + 1
+      counts[anchor] = (counts[anchor] or 0) + count
+    end
+  end
+  return counts
+end
+
 -- Insert virtual filler lines using extmarks
 function M.place(bufnr, after_line_0idx, count, opts)
   if count <= 0 then
diff --git a/lua/codediff/ui/lifecycle/accessors.lua b/lua/codediff/ui/lifecycle/accessors.lua
index 0e86d2b2..aad75a6a 100644
--- a/lua/codediff/ui/lifecycle/accessors.lua
+++ b/lua/codediff/ui/lifecycle/accessors.lua
@@ -422,8 +422,15 @@ function M.set_result(tabpage, result_bufnr, result_win)
 
   -- Leaving conflict mode: retire the conflict mappings so do/dp and the
   -- ordinary view mappings can be claimed again on the next setup pass.
-  if result_bufnr == nil and sess.result_bufnr ~= nil and sess.keymaps then
-    sess.keymaps:release_scope("conflict")
+  if result_bufnr == nil and sess.result_bufnr ~= nil then
+    if sess.keymaps then
+      sess.keymaps:release_scope("conflict")
+    end
+    require("codediff.ui.conflict").teardown_gutter(tabpage)
+    require("codediff.ui.auto_refresh").disable_result(sess.result_bufnr)
+    sess.conflict_blocks = nil
+    sess.result_base_lines = nil
+    sess.merge_base_lines = nil
   end
 
   sess.result_bufnr = result_bufnr
diff --git a/lua/codediff/ui/lifecycle/cleanup.lua b/lua/codediff/ui/lifecycle/cleanup.lua
index c15940d0..c32f5736 100644
--- a/lua/codediff/ui/lifecycle/cleanup.lua
+++ b/lua/codediff/ui/lifecycle/cleanup.lua
@@ -38,6 +38,12 @@ local function cleanup_diff(tabpage)
   local auto_refresh = require("codediff.ui.auto_refresh")
   auto_refresh.disable(diff.original_bufnr)
   auto_refresh.disable(diff.modified_bufnr)
+  if diff.result_bufnr then
+    auto_refresh.disable_result(diff.result_bufnr)
+  end
+
+  -- Restore gutter options before deleting buffers can close or reuse windows.
+  require("codediff.ui.conflict").teardown_gutter(tabpage)
 
   -- Clear highlights from both buffers
   state.clear_buffer_highlights(diff.original_bufnr)
@@ -84,19 +90,12 @@ local function cleanup_diff(tabpage)
     vim.w[diff.result_win].codediff_restore = nil
   end
 
-  -- Clear result buffer signs (conflict mode)
-  if diff.result_bufnr and vim.api.nvim_buf_is_valid(diff.result_bufnr) then
-    local result_signs_ns = vim.api.nvim_create_namespace("codediff-result-signs")
-    vim.api.nvim_buf_clear_namespace(diff.result_bufnr, result_signs_ns, 0, -1)
-  end
-
   -- Clear conflict file tracking (buffers remain, just not tracked)
   diff.conflict_files = {}
 
   -- Clear tab-specific autocmd groups
   pcall(vim.api.nvim_del_augroup_by_name, "codediff_lifecycle_tab_" .. tabpage)
   pcall(vim.api.nvim_del_augroup_by_name, "codediff_working_sync_" .. tabpage)
-  pcall(vim.api.nvim_del_augroup_by_name, "CodeDiffConflictSigns_" .. tabpage)
 
   -- Tear down the scroll-sync group for this tab
   pcall(function()
diff --git a/lua/codediff/ui/lifecycle/state.lua b/lua/codediff/ui/lifecycle/state.lua
index 908e0dfb..9c938325 100644
--- a/lua/codediff/ui/lifecycle/state.lua
+++ b/lua/codediff/ui/lifecycle/state.lua
@@ -124,6 +124,7 @@ local function resume_diff(tabpage)
   -- orphan its registry, leaving codediff's mappings installed on whichever
   -- pane survived and its saved user mappings unreachable.
   if not vim.api.nvim_buf_is_valid(diff.original_bufnr) or not vim.api.nvim_buf_is_valid(diff.modified_bufnr) then
+    require("codediff.ui.conflict").teardown_gutter(tabpage)
     if diff.keymaps then
       diff.keymaps:dispose()
       diff.keymaps = nil
@@ -194,7 +195,27 @@ local function resume_diff(tabpage)
 
   -- Render with fresh content and (possibly reused) diff result
   if lines_diff then
-    if diff.layout == "inline" then
+    if diff.result_bufnr and diff.merge_base_lines and diff.conflict_blocks then
+      -- Conflict inputs are two diffs against BASE, not a diff against each
+      -- other. Restore that same geometry without resetting Result or its
+      -- tracking extmarks, so accepted edits and gutter colors survive tabs.
+      local diff_module = require("codediff.core.diff")
+      local options = require("codediff.config").options.diff
+      local diff_options = {
+        max_computation_time_ms = options.max_computation_time_ms,
+        ignore_trim_whitespace = options.ignore_trim_whitespace,
+        compute_moves = options.compute_moves,
+      }
+      local original_diff = diff_module.compute_diff(diff.merge_base_lines, original_lines, diff_options)
+      local modified_diff = diff_module.compute_diff(diff.merge_base_lines, modified_lines, diff_options)
+      if original_diff and modified_diff then
+        require("codediff.ui.core").render_merge_view(diff.original_bufnr, diff.modified_bufnr, original_diff, modified_diff, diff.merge_base_lines, original_lines, modified_lines)
+        diff.stored_diff_result = modified_diff
+        local conflict = require("codediff.ui.conflict")
+        conflict.attach_gutter(diff.original_win, diff.modified_win)
+        conflict.refresh(diff)
+      end
+    elseif diff.layout == "inline" then
       local inline_mod = require("codediff.ui.inline")
       inline_mod.render_inline_diff(diff.modified_bufnr, lines_diff, original_lines, modified_lines)
     else
diff --git a/lua/codediff/ui/view/side_by_side/update.lua b/lua/codediff/ui/view/side_by_side/update.lua
index cc8edf26..2ba676ad 100644
--- a/lua/codediff/ui/view/side_by_side/update.lua
+++ b/lua/codediff/ui/view/side_by_side/update.lua
@@ -133,6 +133,12 @@ function M.update(tabpage, session_config, auto_scroll_to_first_hunk)
     return false
   end
 
+  -- Retire the old conflict renderer before asynchronous buffer loading starts.
+  if session.result_bufnr then
+    require("codediff.ui.conflict").teardown_gutter(tabpage)
+    auto_refresh.disable_result(session.result_bufnr)
+  end
+
   -- Disable auto-refresh temporarily
   auto_refresh.disable(old_original_buf)
   auto_refresh.disable(old_modified_buf)
diff --git a/tests/README.md b/tests/README.md
index cbe9ceb2..74fa66bd 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -96,6 +96,39 @@ they opened files from, and a watcher left pointing at a deleted path prints
 | `CODEDIFF_TEST_TIMEOUT` | `300000` | Per-spec timeout in ms; guards against a hung spec stalling CI. |
 | `NO_COLOR` / `CODEDIFF_TEST_NO_COLOR` | unset | Disable ANSI colors. |
 
+## Screen-grid regressions
+
+`tests/framework/screen.lua` starts a separate `nvim --embed` and attaches an
+RPC UI. Assertions read actual `grid_line` cells and highlight colors, not
+extmark metadata or screen functions in the headless test process. It uses
+Neovim's bundled MessagePack and libuv, with no external dependencies.
+
+The conflict gutter has three automatically discovered specs:
+
+- `ui/conflict/gutter_grid_spec.lua`: hand-authored block shapes from
+  `fixtures/conflict_gutter.lua`, including empty/filler-only blocks, BOF/EOF,
+  partial scrolling, unrelated blank rows, multiple blocks and Result
+  projections. Checks both glyph cells and colors in all three panes under
+  each focus state. Fixtures go through the production conflict renderer;
+  expected rows do not call or duplicate the gutter calculator.
+- `ui/conflict/gutter_options_spec.lua`: compares ordinary columns against
+  Neovim's native rendering, including relative/hybrid numbers, folds, wrapped
+  rows and other signs; checks custom-option ownership and restoration.
+- `ui/conflict/gutter_lifecycle_spec.lua`: real Git merges, file/window/tab
+  transitions, resizing, accept/undo/redo/discard, manual Result edits and
+  teardown of stale callbacks and invalid sessions.
+
+Run these like any other spec, for example:
+
+```bash
+nvim --headless --noplugin -u tests/init.lua \
+  -c "lua require('tests.framework').run_and_exit('tests/ui/conflict/gutter_grid_spec.lua')"
+```
+
+Always close the embedded UI in `after_each`, including when an assertion
+fails. A grid failure reports the pane/focus context, display row, expected
+text and actual text. These are cell-level checks, not font or pixel snapshots.
+
 ## Test Philosophy
 
 Focus on **integration points** that C tests cannot validate:
@@ -107,4 +140,4 @@ Focus on **integration points** that C tests cannot validate:
 ## What's NOT Covered
 
 ❌ **Diff algorithm** - Validated by C tests in `c-diff-core/tests/` (3,490 lines)
-❌ **Visual correctness** - Manual testing required
+❌ **Other visual features and third-party UI integrations** - Manual testing unless covered by a dedicated grid spec
diff --git a/tests/fixtures/conflict_gutter.lua b/tests/fixtures/conflict_gutter.lua
new file mode 100644
index 00000000..3dcfc771
--- /dev/null
+++ b/tests/fixtures/conflict_gutter.lua
@@ -0,0 +1,118 @@
+-- Hand-authored screen rows for a six-line buffer: one, two, three, four, five, six.
+-- These are visual expectations, not output from the gutter calculator.
+return {
+  {
+    name = "single real position",
+    range = { start_line = 2, end_line = 3 },
+    fillers = {},
+    rows = { "  one", "[ two", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "one real line and one trailing filler",
+    range = { start_line = 2, end_line = 3 },
+    fillers = { [2] = 1 },
+    rows = { "  one", "╭─two", "╰─╱╱╱╱", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "one real line and multiple trailing fillers",
+    range = { start_line = 2, end_line = 3 },
+    fillers = { [2] = 2 },
+    rows = { "  one", "╭─two", "│ ╱╱╱╱", "╰─╱╱╱╱", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "ordinary multiline range",
+    range = { start_line = 2, end_line = 5 },
+    fillers = {},
+    rows = { "  one", "╭─two", "│ three", "╰─four", "  five", "  six" },
+  },
+  {
+    name = "leading filler",
+    range = { start_line = 2, end_line = 5 },
+    fillers = { [1] = 1 },
+    rows = { "  one", "╭─╱╱╱╱", "│ two", "│ three", "╰─four", "  five", "  six" },
+  },
+  {
+    name = "interior filler",
+    range = { start_line = 2, end_line = 5 },
+    fillers = { [3] = 2 },
+    rows = { "  one", "╭─two", "│ three", "│ ╱╱╱╱", "│ ╱╱╱╱", "╰─four", "  five", "  six" },
+  },
+  {
+    name = "trailing filler",
+    range = { start_line = 2, end_line = 5 },
+    fillers = { [4] = 2 },
+    rows = { "  one", "╭─two", "│ three", "│ four", "│ ╱╱╱╱", "╰─╱╱╱╱", "  five", "  six" },
+  },
+  {
+    name = "empty without filler",
+    range = { start_line = 3, end_line = 3 },
+    fillers = {},
+    rows = { "  one", "  two", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "empty with one filler",
+    range = { start_line = 3, end_line = 3 },
+    fillers = { [2] = 1 },
+    rows = { "  one", "  two", "[ ╱╱╱╱", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "empty with multiple fillers",
+    range = { start_line = 3, end_line = 3 },
+    fillers = { [2] = 2 },
+    rows = { "  one", "  two", "╭─╱╱╱╱", "╰─╱╱╱╱", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "empty with filler on the fallback anchor",
+    range = { start_line = 3, end_line = 3 },
+    fillers = { [3] = 1 },
+    rows = { "  one", "  two", "  three", "[ ╱╱╱╱", "  four", "  five", "  six" },
+  },
+  {
+    name = "BOF leading filler",
+    range = { start_line = 1, end_line = 3 },
+    fillers = { [0] = 2 },
+    rows = { "╭─╱╱╱╱", "│ ╱╱╱╱", "│ one", "╰─two", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "fillers both before and after line one",
+    range = { start_line = 1, end_line = 3 },
+    fillers = { [0] = 2, [1] = 1 },
+    rows = { "╭─╱╱╱╱", "│ ╱╱╱╱", "│ one", "│ ╱╱╱╱", "╰─two", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "one filler-only position at BOF",
+    range = { start_line = 1, end_line = 1 },
+    fillers = { [0] = 1 },
+    rows = { "[ ╱╱╱╱", "  one", "  two", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "multiple filler-only positions at BOF",
+    range = { start_line = 1, end_line = 1 },
+    fillers = { [0] = 2 },
+    rows = { "╭─╱╱╱╱", "╰─╱╱╱╱", "  one", "  two", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "unmarked BOF fillers must not shift marked fillers after line one",
+    range = { start_line = 2, end_line = 3 },
+    fillers = { [0] = 2, [1] = 1 },
+    rows = { "  ╱╱╱╱", "  ╱╱╱╱", "  one", "╭─╱╱╱╱", "╰─two", "  three", "  four", "  five", "  six" },
+  },
+  {
+    name = "trailing filler at EOF",
+    range = { start_line = 6, end_line = 7 },
+    fillers = { [6] = 2 },
+    rows = { "  one", "  two", "  three", "  four", "  five", "╭─six", "│ ╱╱╱╱", "╰─╱╱╱╱" },
+  },
+  {
+    name = "empty projection at EOF",
+    range = { start_line = 7, end_line = 7 },
+    fillers = { [6] = 2 },
+    rows = { "  one", "  two", "  three", "  four", "  five", "  six", "╭─╱╱╱╱", "╰─╱╱╱╱" },
+  },
+  {
+    name = "unrelated fillers outside the block stay blank",
+    range = { start_line = 3, end_line = 5 },
+    fillers = { [1] = 1, [5] = 1 },
+    rows = { "  one", "  ╱╱╱╱", "  two", "╭─three", "╰─four", "  five", "  ╱╱╱╱", "  six" },
+  },
+}
diff --git a/tests/framework/screen.lua b/tests/framework/screen.lua
new file mode 100644
index 00000000..ca69ca2c
--- /dev/null
+++ b/tests/framework/screen.lua
@@ -0,0 +1,220 @@
+-- An embedded Neovim UI for assertions on rendered cells, not extmark metadata.
+-- Uses Neovim's bundled MessagePack and libuv; no Python or external UI dependency.
+local Screen = {}
+Screen.__index = Screen
+
+local uv = vim.uv or vim.loop
+
+local function blank_row(width)
+  local row = {}
+  for col = 1, width do
+    row[col] = { " ", 0 }
+  end
+  return row
+end
+
+function Screen:_redraw(events)
+  for _, event in ipairs(events) do
+    local name = event[1]
+    for i = 2, #event do
+      local args = event[i]
+      if name == "grid_resize" and args[1] == 1 then
+        self.width, self.height = args[2], args[3]
+        self.grid = {}
+        for row = 1, self.height do
+          self.grid[row] = blank_row(self.width)
+        end
+      elseif name == "grid_clear" and args[1] == 1 then
+        for row = 1, self.height do
+          self.grid[row] = blank_row(self.width)
+        end
+      elseif name == "grid_line" and args[1] == 1 then
+        local row, col, highlight = self.grid[args[2] + 1], args[3] + 1, 0
+        for _, cell in ipairs(args[4]) do
+          highlight = cell[2] or highlight
+          for _ = 1, cell[3] or 1 do
+            row[col] = { cell[1], highlight }
+            col = col + 1
+          end
+        end
+      elseif name == "grid_scroll" and args[1] == 1 then
+        local top, bottom, left, right, rows, cols = unpack(args, 2)
+        local old = {}
+        for row = top + 1, bottom do
+          old[row] = {}
+          for col = left + 1, right do
+            old[row][col] = self.grid[row][col]
+          end
+        end
+        for row = top + 1, bottom do
+          for col = left + 1, right do
+            local source = old[row + rows]
+            self.grid[row][col] = source and source[col + cols] or { " ", 0 }
+          end
+        end
+      elseif name == "hl_attr_define" then
+        self.highlights[args[1]] = args[2]
+      elseif name == "flush" then
+        self.flushes = self.flushes + 1
+      end
+    end
+  end
+end
+
+function Screen.new(width, height)
+  local self = setmetatable({ next_id = 0, responses = {}, grid = {}, highlights = {}, flushes = 0, stderr = "" }, Screen)
+  self.stdin, self.stdout, self.errpipe = uv.new_pipe(false), uv.new_pipe(false), uv.new_pipe(false)
+  local err
+  self.process, err = uv.spawn(vim.v.progpath, {
+    args = { "--embed", "--noplugin", "-u", "tests/init.lua", "-i", "NONE" },
+    cwd = vim.fn.getcwd(),
+    stdio = { self.stdin, self.stdout, self.errpipe },
+  }, function(code)
+    self.exit_code = code
+  end)
+  if not self.process then
+    self:close()
+    error("Cannot start embedded Neovim: " .. tostring(err))
+  end
+
+  local unpacker = vim.mpack.Unpacker()
+  self.stdout:read_start(function(read_error, data)
+    if read_error then
+      self.error = read_error
+    end
+    if not data then
+      return
+    end
+    local ok, decode_error = pcall(function()
+      local pos = 1
+      while pos <= #data do
+        local message
+        message, pos = unpacker(data, pos)
+        if message then
+          if message[1] == 1 then
+            self.responses[message[2]] = message
+          elseif message[1] == 2 and message[2] == "redraw" then
+            self:_redraw(message[3])
+          end
+        end
+      end
+    end)
+    if not ok then
+      self.error = decode_error
+    end
+  end)
+  self.errpipe:read_start(function(_, data)
+    self.stderr = self.stderr .. (data or "")
+  end)
+
+  local ok, start_error = pcall(function()
+    self:request("nvim_ui_attach", { width or 100, height or 30, { rgb = true, ext_linegrid = true } })
+    self:exec([[
+      vim.o.showtabline = 0
+      vim.o.laststatus = 0
+      vim.o.showmode = false
+      vim.o.ruler = false
+      vim.o.shortmess = 'atIF'
+      vim.o.scrolloff = 0
+      vim.o.sidescrolloff = 0
+    ]])
+    self:flush()
+  end)
+  if not ok then
+    self:close()
+    error(start_error)
+  end
+  return self
+end
+
+function Screen:request(method, args)
+  self.next_id = self.next_id + 1
+  local id = self.next_id
+  self.stdin:write(vim.mpack.encode({ 0, id, method, args or {} }))
+  local ready = vim.wait(10000, function()
+    return self.responses[id] ~= nil or self.error ~= nil or self.exit_code ~= nil
+  end, 1)
+  local response = self.responses[id]
+  self.responses[id] = nil
+  assert(ready and response and not self.error, string.format("RPC %s failed: %s\n%s", method, tostring(self.error or self.exit_code or "timeout"), self.stderr))
+  assert(response[3] == vim.NIL, string.format("RPC %s: %s", method, vim.inspect(response[3])))
+  if response[4] ~= vim.NIL then
+    return response[4]
+  end
+end
+
+function Screen:exec(code, args)
+  return self:request("nvim_exec_lua", { code, args or {} })
+end
+
+function Screen:command(command)
+  return self:request("nvim_command", { command })
+end
+
+function Screen:input(keys)
+  return self:request("nvim_input", { keys })
+end
+
+function Screen:flush()
+  local before = self.flushes
+  self:command("redraw!")
+  self:request("nvim_eval", { "0" })
+  assert(
+    vim.wait(5000, function()
+      return self.flushes > before
+    end, 1),
+    "No UI flush after redraw"
+  )
+end
+
+function Screen:wait_for(predicate, message)
+  assert(vim.wait(10000, predicate, 10), (message or "Embedded Neovim did not become ready") .. "\n" .. self.stderr)
+  self:flush()
+end
+
+-- Screen coordinates here are one-based, including UI columns (not UTF-8 bytes).
+function Screen:text(row, col, width)
+  local text = {}
+  for index = col, col + width - 1 do
+    text[#text + 1] = self.grid[row][index][1]
+  end
+  return table.concat(text)
+end
+
+function Screen:highlight(row, col)
+  return self.highlights[self.grid[row][col][2]] or {}
+end
+
+function Screen:foreground(row, col)
+  return self:highlight(row, col).foreground
+end
+
+function Screen:expect_rows(row, col, expected, label)
+  for index, text in ipairs(expected) do
+    assert.equals(text, self:text(row + index - 1, col, vim.fn.strdisplaywidth(text)), string.format("%s, display row %d", label or "screen", index))
+  end
+end
+
+function Screen:close()
+  if self.process and not self.process:is_closing() then
+    if self.exit_code == nil then
+      self.stdin:write(vim.mpack.encode({ 0, self.next_id + 1, "nvim_command", { "qa!" } }))
+      if not vim.wait(2000, function()
+        return self.exit_code ~= nil
+      end, 10) then
+        self.process:kill("sigterm")
+        vim.wait(2000, function()
+          return self.exit_code ~= nil
+        end, 10)
+      end
+    end
+    self.process:close()
+  end
+  for _, pipe in ipairs({ self.stdin, self.stdout, self.errpipe }) do
+    if not pipe:is_closing() then
+      pipe:close()
+    end
+  end
+end
+
+return Screen
diff --git a/tests/module_loading_spec.lua b/tests/module_loading_spec.lua
index dc81d410..84ce167f 100644
--- a/tests/module_loading_spec.lua
+++ b/tests/module_loading_spec.lua
@@ -349,9 +349,9 @@ describe("Conflict submodules", function()
       assert.is_not_nil(mod)
     end)
 
-    it("loads signs module", function()
-      local ok, mod = pcall(require, "codediff.ui.conflict.signs")
-      assert.is_true(ok, "Failed to require codediff.ui.conflict.signs")
+    it("loads gutter module", function()
+      local ok, mod = pcall(require, "codediff.ui.conflict.gutter")
+      assert.is_true(ok, "Failed to require codediff.ui.conflict.gutter")
       assert.is_not_nil(mod)
     end)
 
@@ -423,16 +423,23 @@ describe("Conflict submodules", function()
       assert.is_function(mod.navigate_prev_conflict)
     end)
 
-    it("signs exports expected functions", function()
-      local mod = require("codediff.ui.conflict.signs")
-      assert.is_function(mod.refresh_all_conflict_signs)
-      assert.is_function(mod.setup_sign_refresh_autocmd)
+    it("gutter modules export expected functions", function()
+      local gutter = require("codediff.ui.conflict.gutter")
+      local compute = require("codediff.ui.conflict.gutter.compute")
+      local signcolumn = require("codediff.ui.conflict.gutter.signcolumn")
+      local statuscolumn = require("codediff.ui.conflict.gutter.statuscolumn")
+      assert.is_function(gutter.refresh)
+      assert.is_function(gutter.setup_refresh_autocmd)
+      assert.is_function(compute.visual_positions)
+      assert.is_function(compute.compute_markers)
+      assert.is_function(signcolumn.render)
+      assert.is_function(statuscolumn.render)
     end)
 
     it("init facade delegates expected functions", function()
       local mod = require("codediff.ui.conflict")
       local tracking = require("codediff.ui.conflict.tracking")
-      local signs = require("codediff.ui.conflict.signs")
+      local gutter = require("codediff.ui.conflict.gutter")
       local resolution = require("codediff.ui.conflict.resolution")
       local navigation = require("codediff.ui.conflict.navigation")
       local keymaps = require("codediff.ui.conflict.keymaps")
@@ -443,8 +450,8 @@ describe("Conflict submodules", function()
       assert.is_function(mod.navigate_next_conflict)
       assert.equal(tracking.run_repeatable_action, mod.run_repeatable_action)
       assert.equal(tracking.initialize_tracking, mod.initialize_tracking)
-      assert.equal(signs.refresh_all_conflict_signs, mod.refresh_all_conflict_signs)
-      assert.equal(signs.setup_sign_refresh_autocmd, mod.setup_sign_refresh_autocmd)
+      assert.equal(gutter.refresh, mod.refresh)
+      assert.equal(gutter.setup_refresh_autocmd, mod.setup_refresh_autocmd)
       assert.equal(resolution.accept_incoming, mod.accept_incoming)
       assert.equal(resolution.accept_current, mod.accept_current)
       assert.equal(resolution.accept_both, mod.accept_both)
diff --git a/tests/ui/conflict/conflict_signs_spec.lua b/tests/ui/conflict/conflict_signs_spec.lua
index fec1f09b..ee26223a 100644
--- a/tests/ui/conflict/conflict_signs_spec.lua
+++ b/tests/ui/conflict/conflict_signs_spec.lua
@@ -58,7 +58,7 @@ describe("conflict signs", function()
   end)
 
   it("changes input and Result signs after accepting one side", function()
-    conflict.refresh_all_conflict_signs(session)
+    conflict.refresh(session)
     assert.equals("CodeDiffConflictSign", sign_highlight(session.original_bufnr, highlights.ns_conflict))
     assert.equals("CodeDiffConflictSign", sign_highlight(session.modified_bufnr, highlights.ns_conflict))
     assert.equals("CodeDiffConflictSign", sign_highlight(session.result_bufnr, tracking.result_signs_ns))
diff --git a/tests/ui/conflict/gutter_grid_spec.lua b/tests/ui/conflict/gutter_grid_spec.lua
new file mode 100644
index 00000000..bb493f82
--- /dev/null
+++ b/tests/ui/conflict/gutter_grid_spec.lua
@@ -0,0 +1,241 @@
+local Screen = require("tests.framework.screen")
+local gutter = require("tests.ui.conflict.gutter_helpers")
+local cases = require("tests.fixtures.conflict_gutter")
+
+describe("conflict block gutter on the screen grid", function()
+  local screen
+
+  after_each(function()
+    if screen then
+      screen:close()
+      screen = nil
+    end
+  end)
+
+  for index, case in ipairs(cases) do
+    it(case.name .. " in all panes under each focus state", function()
+      screen = Screen.new(120, 24)
+      local panes = gutter.open_block(screen, index)
+      local expected = gutter.block_rows(case)
+
+      for _, focus in ipairs({ "original", "modified", "result" }) do
+        screen:exec("vim.api.nvim_set_current_win(...)", { panes[focus] })
+        screen:flush()
+        for _, side in ipairs({ "original", "modified", "result" }) do
+          gutter.expect_pane(screen, panes[side], expected[side], case.name .. ": " .. side .. " while focused on " .. focus, panes.foreground)
+        end
+      end
+    end)
+  end
+
+  it("keeps BOF offsets correct when only part of the leading filler is visible", function()
+    screen = Screen.new(120, 24)
+    local panes = gutter.open_block(screen, 13)
+    screen:exec(
+      [[
+      local win = ...
+      vim.api.nvim_set_current_win(win)
+      vim.fn.winrestview({ topline = 1, topfill = 1 })
+    ]],
+      { panes.original }
+    )
+    screen:flush()
+    gutter.expect_pane(screen, panes.original, {
+      "│ ╱╱╱╱",
+      "│ one",
+      "│ ╱╱╱╱",
+      "╰─two",
+      "  three",
+      "  four",
+      "  five",
+      "  six",
+    }, "partially scrolled BOF", panes.foreground)
+  end)
+
+  it("keeps after-line-one markers correct when all BOF filler is scrolled out", function()
+    screen = Screen.new(120, 24)
+    local panes = gutter.open_block(screen, 13)
+    screen:exec(
+      [[
+      vim.api.nvim_set_current_win(...)
+      vim.fn.winrestview({ topline = 1, topfill = 0 })
+    ]],
+      { panes.original }
+    )
+    screen:flush()
+    gutter.expect_pane(screen, panes.original, {
+      "│ one",
+      "│ ╱╱╱╱",
+      "╰─two",
+      "  three",
+      "  four",
+      "  five",
+      "  six",
+    }, "hidden BOF", panes.foreground)
+  end)
+
+  for _, options in ipairs({
+    { name = "number and fold columns", number = true, relativenumber = true, numberwidth = 6, foldcolumn = "2", signcolumn = "yes" },
+    { name = "multiple sign slots", number = true, relativenumber = false, numberwidth = 4, foldcolumn = "0", signcolumn = "yes:2" },
+    { name = "signs in the number column", number = true, relativenumber = true, numberwidth = 4, foldcolumn = "0", signcolumn = "number" },
+    { name = "hidden signs", number = true, relativenumber = true, numberwidth = 4, foldcolumn = "0", signcolumn = "no" },
+  }) do
+    it("changes only filler sign cells with " .. options.name, function()
+      screen = Screen.new(120, 24)
+      local panes = gutter.open_block(screen, 3)
+      local rect = screen:exec(
+        [[
+        local win, options = ...
+        vim.api.nvim_set_current_win(win)
+        require('codediff.ui.conflict').detach_gutter(win)
+        for name, value in pairs(options) do
+          if name ~= 'name' then vim.wo[win][name] = value end
+        end
+        local p = vim.api.nvim_win_get_position(win)
+        return { p[1] + 1, p[2] + 1, vim.api.nvim_win_get_width(win), vim.fn.getwininfo(win)[1].textoff }
+      ]],
+        { panes.original, options }
+      )
+      screen:flush()
+      local expected = {}
+      for row = 1, 8 do
+        expected[row] = screen:text(rect[1] + row - 1, rect[2], rect[3])
+      end
+      if options.signcolumn ~= "no" then
+        local sign_col = options.signcolumn == "number" and rect[4] - 3 or tonumber(options.foldcolumn)
+        for row, glyph in pairs({ [3] = "│ ", [4] = "╰─" }) do
+          expected[row] = vim.fn.strcharpart(expected[row], 0, sign_col) .. glyph .. vim.fn.strcharpart(expected[row], sign_col + 2)
+        end
+      end
+      screen:exec(
+        [[
+        local win, tab = ...
+        local conflict = require('codediff.ui.conflict')
+        conflict.attach_gutter(win)
+        conflict.refresh(require('codediff.ui.lifecycle').get_session(tab))
+      ]],
+        { panes.original, panes.tab }
+      )
+      screen:flush()
+      screen:expect_rows(rect[1], rect[2], expected, options.name)
+    end)
+  end
+
+  it("composes separate blocks and removes one without damaging the other", function()
+    screen = Screen.new(120, 24)
+    local panes = gutter.open_block(screen, 3)
+    screen:exec(
+      [[
+      local s = require('codediff.ui.lifecycle').get_session(...)
+      s.conflict_blocks[2] = {
+        base_range = { start_line = 7, end_line = 8 }, result_range = { start_line = 7, end_line = 8 },
+        output1_range = { start_line = 5, end_line = 6 }, output2_range = { start_line = 7, end_line = 8 },
+      }
+      local conflict = require('codediff.ui.conflict')
+      conflict.initialize_tracking(s.result_bufnr, s.conflict_blocks)
+      conflict.refresh(s)
+    ]],
+      { panes.tab }
+    )
+    screen:flush()
+    local expected = gutter.block_rows(cases[3])
+    expected.original = { "  one", "╭─two", "│ ╱╱╱╱", "╰─╱╱╱╱", "  three", "  four", "[ five", "  six" }
+    expected.modified[9] = "[ five"
+    expected.result[7] = "[ base seven"
+    for _, side in ipairs({ "original", "modified", "result" }) do
+      gutter.expect_pane(screen, panes[side], expected[side], "two blocks: " .. side, panes.foreground)
+    end
+    screen:exec(
+      [[
+      local tab = ...
+      local lifecycle = require('codediff.ui.lifecycle')
+      local s = lifecycle.get_session(tab)
+      lifecycle.set_conflict_blocks(tab, { s.conflict_blocks[2] })
+      require('codediff.ui.conflict').refresh(s)
+    ]],
+      { panes.tab }
+    )
+    screen:flush()
+    gutter.expect_pane(screen, panes.original, {
+      "  one",
+      "  two",
+      "  ╱╱╱╱",
+      "  ╱╱╱╱",
+      "  three",
+      "  four",
+      "[ five",
+      "  six",
+    }, "only the second block remains", panes.foreground)
+  end)
+
+  it("renders no Result marker for an empty Result projection", function()
+    screen = Screen.new(120, 24)
+    local panes = gutter.open_block(screen, 3)
+    screen:exec(
+      [[
+      local s = require('codediff.ui.lifecycle').get_session(...)
+      s.conflict_blocks[1].result_range = { start_line = 4, end_line = 4 }
+      local conflict = require('codediff.ui.conflict')
+      conflict.initialize_tracking(s.result_bufnr, s.conflict_blocks)
+      conflict.refresh(s)
+    ]],
+      { panes.tab }
+    )
+    screen:flush()
+    gutter.expect_pane(screen, panes.result, {
+      "  base one",
+      "  base two",
+      "  base three",
+      "  base four",
+      "  base five",
+      "  base six",
+      "  base seven",
+    }, "empty Result projection")
+    gutter.expect_pane(screen, panes.original, cases[3].rows, "nonempty input projection", panes.foreground)
+  end)
+
+  it("uses updated highlight colors on real and filler markers", function()
+    screen = Screen.new(120, 24)
+    local panes = gutter.open_block(screen, 3)
+    screen:exec("vim.api.nvim_set_hl(0, 'CodeDiffConflictSign', { fg = 0x12ab34, bg = 0x345678, bold = true })")
+    screen:flush()
+    local expected = gutter.block_rows(cases[3])
+    for _, side in ipairs({ "original", "modified", "result" }) do
+      gutter.expect_pane(screen, panes[side], expected[side], "updated highlight: " .. side, { foreground = 0x12ab34, background = 0x345678, bold = true })
+    end
+  end)
+
+  it("clears a removed block on both real and filler rows", function()
+    screen = Screen.new(120, 24)
+    local panes = gutter.open_block(screen, 3)
+    screen:exec(
+      [[
+      local tab = ...
+      local lifecycle = require('codediff.ui.lifecycle')
+      lifecycle.set_conflict_blocks(tab, {})
+      require('codediff.ui.conflict').refresh(lifecycle.get_session(tab))
+    ]],
+      { panes.tab }
+    )
+    screen:flush()
+    gutter.expect_pane(screen, panes.original, {
+      "  one",
+      "  two",
+      "  ╱╱╱╱",
+      "  ╱╱╱╱",
+      "  three",
+      "  four",
+      "  five",
+      "  six",
+    }, "removed block")
+    gutter.expect_pane(screen, panes.result, {
+      "  base one",
+      "  base two",
+      "  base three",
+      "  base four",
+      "  base five",
+      "  base six",
+      "  base seven",
+    }, "removed Result block")
+  end)
+end)
diff --git a/tests/ui/conflict/gutter_helpers.lua b/tests/ui/conflict/gutter_helpers.lua
new file mode 100644
index 00000000..d77d9049
--- /dev/null
+++ b/tests/ui/conflict/gutter_helpers.lua
@@ -0,0 +1,254 @@
+local M = {}
+
+-- Build a block with deliberately different buffer coordinates in each pane.
+-- Only the public conflict entry points render it; no calculator/backend calls.
+function M.open_block(screen, case_index)
+  return screen:exec(
+    [[
+    local case_index = ...
+    local case = require('tests.fixtures.conflict_gutter')[case_index]
+    local conflict = require('codediff.ui.conflict')
+    local lifecycle = require('codediff.ui.lifecycle')
+    local path = require('codediff.core.path')
+    local filler = require('codediff.ui.filler')
+    require('codediff').setup({ diff = { filler_text = '╱' } })
+    local words = { 'one', 'two', 'three', 'four', 'five', 'six' }
+    local function buffer(lines)
+      local buf = vim.api.nvim_create_buf(false, true)
+      vim.api.nvim_buf_set_lines(buf, 0, -1, false, lines)
+      return buf
+    end
+    local function window(buf, split)
+      if split then vim.cmd('rightbelow vsplit') end
+      local win = vim.api.nvim_get_current_win()
+      vim.api.nvim_win_set_buf(win, buf)
+      for name, value in pairs({
+        number = false, relativenumber = false, foldcolumn = '0',
+        signcolumn = 'yes', statuscolumn = '', wrap = false,
+        cursorline = false, cursorcolumn = false, winbar = '',
+      }) do
+        vim.wo[win][name] = value
+      end
+      return win
+    end
+    local original_buf = buffer(words)
+    local original_win = window(original_buf)
+    local modified_lines = { 'prefix', 'prefix' }
+    vim.list_extend(modified_lines, words)
+    local modified_buf = buffer(modified_lines)
+    local modified_win = window(modified_buf, true)
+    local base_lines = { 'base one', 'base two', 'base three', 'base four', 'base five', 'base six', 'base seven' }
+    local result_buf = buffer(base_lines)
+    local result_win = window(result_buf, true)
+    local tab = vim.api.nvim_get_current_tabpage()
+    lifecycle.create_session(tab, {
+      conflict = true, original = path.make_ref('', nil), modified = path.make_ref('', nil),
+    }, {
+      original_bufnr = original_buf, modified_bufnr = modified_buf,
+      original_win = original_win, modified_win = modified_win,
+    })
+    lifecycle.set_result(tab, result_buf, result_win)
+    lifecycle.set_result_base_lines(tab, base_lines)
+    local blocks = {{
+      base_range = { start_line = 4, end_line = 6 },
+      result_range = { start_line = 4, end_line = 6 },
+      output1_range = vim.deepcopy(case.range),
+      output2_range = { start_line = case.range.start_line + 2, end_line = case.range.end_line + 2 },
+    }}
+    lifecycle.set_conflict_blocks(tab, blocks)
+    for anchor, count in pairs(case.fillers) do
+      filler.place(original_buf, anchor - 1, count)
+      filler.place(modified_buf, anchor + 1, count)
+    end
+    conflict.initialize_tracking(result_buf, blocks)
+    conflict.attach_gutter(original_win, modified_win)
+    conflict.setup_refresh_autocmd(tab, result_buf)
+    conflict.refresh(lifecycle.get_session(tab))
+    vim.api.nvim_win_call(original_win, function()
+      vim.fn.winrestview({ topline = 1, topfill = case.fillers[0] or 0 })
+    end)
+    return {
+      tab = tab, original = original_win, modified = modified_win, result = result_win,
+      foreground = vim.api.nvim_get_hl(0, { name = 'CodeDiffConflictSign', link = false }).fg,
+    }
+  ]],
+    { case_index }
+  )
+end
+
+function M.expect_pane(screen, win, rows, label, highlight)
+  local position = screen:exec(
+    [[
+    local win = ...
+    local position = vim.api.nvim_win_get_position(win)
+    if vim.wo[win].winbar ~= '' then position[1] = position[1] + 1 end
+    return position
+  ]],
+    { win }
+  )
+  local row, col = position[1] + 1, position[2] + 1
+  screen:expect_rows(row, col, rows, label)
+  if highlight then
+    local expected = type(highlight) == "table" and highlight or { foreground = highlight }
+    for index, text in ipairs(rows) do
+      local first = vim.fn.strcharpart(text, 0, 1)
+      if first == "[" or first == "╭" or first == "│" or first == "╰" then
+        for offset = 0, 1 do
+          local actual = screen:highlight(row + index - 1, col + offset)
+          for attr, value in pairs(expected) do
+            assert.equals(value, actual[attr], label .. ": marker " .. attr .. " on display row " .. index)
+          end
+        end
+      end
+    end
+  end
+end
+
+function M.block_rows(case)
+  local modified = { "  prefix", "  prefix" }
+  vim.list_extend(modified, case.rows)
+  return {
+    original = case.rows,
+    modified = modified,
+    result = { "  base one", "  base two", "  base three", "╭─base four", "╰─base five", "  base six", "  base seven" },
+  }
+end
+
+-- A small real merge with an interior filler in incoming, plus a second file
+-- whose filler is on the opposite side. Expectations are authored below.
+function M.create_repo()
+  local repo = require("tests.helpers").create_temp_git_repo()
+  repo.write_file("conf.txt", { "before", "base1", "base2", "base3", "after", "tail" })
+  repo.write_file("other.txt", { "beforeB", "baseB", "afterB" })
+  repo.write_file("deleted.txt", { "deleted one", "deleted two" })
+  repo.git("add -A")
+  repo.git("commit -m base")
+  repo.git("checkout -b incoming")
+  repo.write_file("conf.txt", { "before", "base1", "THEIRS2", "THEIRS3", "after", "tail" })
+  repo.write_file("other.txt", { "beforeB", "THEIRS_B", "extraB", "afterB" })
+  repo.git("commit -am incoming")
+  repo.git("checkout main")
+  repo.write_file("conf.txt", { "before", "OURS1", "inserted", "OURS2", "base3", "after", "tail" })
+  repo.write_file("other.txt", { "beforeB", "OURS_B", "afterB" })
+  repo.git("commit -am current")
+  local _, code = repo.git("merge incoming --no-edit")
+  assert.equals(1, code, "fixture must produce a merge conflict")
+  repo.write_file("fresh.txt", { "fresh one", "fresh two" })
+  repo.write_file("plain-left.txt", { "before", "same", "keep", "after" })
+  repo.write_file("plain-right.txt", { "before", "same", "keep", "inserted", "after" })
+  return repo
+end
+
+M.merge_rows = {
+  original = { "  before", "╭─base1", "│ THEIRS2", "│ ╱╱╱╱", "╰─THEIRS3", "  after", "  tail" },
+  modified = { "  before", "╭─OURS1", "│ inserted", "│ OURS2", "╰─base3", "  after", "  tail" },
+  result = { "  before", "╭─base1", "│ base2", "╰─base3", "  after", "  tail" },
+}
+
+M.other_merge_rows = {
+  original = { "  beforeB", "╭─THEIRS_B", "╰─extraB", "  afterB" },
+  modified = { "  beforeB", "╭─╱╱╱╱", "╰─OURS_B", "  afterB" },
+  result = { "  beforeB", "[ baseB", "  afterB" },
+}
+
+function M.open_merge(screen, repo)
+  screen:exec(
+    [[
+    local root = ...
+    require('codediff').setup({ diff = { filler_text = '╱', jump_to_first_change = false } })
+    vim.o.number = false
+    vim.o.relativenumber = false
+    vim.o.foldcolumn = '0'
+    local path = require('codediff.core.path')
+    _G.gutter_test_config = function(file)
+      return {
+        git_root = root, conflict = true,
+        original = path.make_ref(file or 'conf.txt', root),
+        modified = path.make_ref(file or 'conf.txt', root),
+        original_revision = ':3', modified_revision = ':2',
+      }
+    end
+    require('codediff.ui.view').create(gutter_test_config(), '', function()
+      _G.gutter_test_ready = true
+      _G.gutter_test_tab = vim.api.nvim_get_current_tabpage()
+    end)
+  ]],
+    { repo.dir }
+  )
+  screen:wait_for(function()
+    return screen:exec("return _G.gutter_test_ready == true")
+  end, "merge view did not open")
+  return M.panes(screen)
+end
+
+function M.panes(screen)
+  return screen:exec([[
+    local session = require('codediff.ui.lifecycle').get_session(gutter_test_tab)
+    return {
+      tab = gutter_test_tab, original = session.original_win, modified = session.modified_win,
+      result = session.result_win, result_buf = session.result_bufnr,
+    }
+  ]])
+end
+
+function M.return_to_merge(screen, file)
+  screen:exec(
+    [[
+    local file = ...
+    require('codediff.ui.view').update(gutter_test_tab, gutter_test_config(file), false)
+  ]],
+    { file or "conf.txt" }
+  )
+  screen:wait_for(function()
+    return screen:exec(
+      [[
+      local file = ...
+      local s = require('codediff.ui.lifecycle').get_session(gutter_test_tab)
+      return s and s.stored_diff_result ~= nil and s.result_win ~= nil
+        and vim.api.nvim_buf_get_name(s.result_bufnr):sub(-#file) == file
+    ]],
+      { file or "conf.txt" }
+    )
+  end, "merge view did not return")
+  return M.panes(screen)
+end
+
+function M.open_plain(screen, repo)
+  screen:exec(
+    [[
+    local root = ...
+    local path = require('codediff.core.path')
+    require('codediff.ui.view').update(gutter_test_tab, {
+      original = path.make_ref(root .. '/plain-left.txt', nil),
+      modified = path.make_ref(root .. '/plain-right.txt', nil),
+    }, false)
+  ]],
+    { repo.dir }
+  )
+  screen:wait_for(function()
+    return screen:exec([[
+      local s = require('codediff.ui.lifecycle').get_session(gutter_test_tab)
+      return s and s.stored_diff_result ~= nil and s.result_win == nil
+    ]])
+  end, "plain diff did not open")
+  return M.panes(screen)
+end
+
+function M.expect_no_conflict_markers(screen, win)
+  local rect = screen:exec(
+    [[
+    local win = ...
+    local p = vim.api.nvim_win_get_position(win)
+    return { p[1] + 1, p[2] + 1, vim.api.nvim_win_get_height(win), vim.api.nvim_win_get_width(win) }
+  ]],
+    { win }
+  )
+  for row = rect[1], rect[1] + rect[3] - 1 do
+    local text = screen:text(row, rect[2], rect[4])
+    for _, glyph in ipairs({ "╭", "│", "╰", "[" }) do
+      assert.is_nil(text:find(glyph, 1, true), "unexpected conflict marker in window " .. win .. ": " .. text)
+    end
+  end
+end
+
+return M
diff --git a/tests/ui/conflict/gutter_lifecycle_spec.lua b/tests/ui/conflict/gutter_lifecycle_spec.lua
new file mode 100644
index 00000000..4603dbc3
--- /dev/null
+++ b/tests/ui/conflict/gutter_lifecycle_spec.lua
@@ -0,0 +1,243 @@
+local Screen = require("tests.framework.screen")
+local gutter = require("tests.ui.conflict.gutter_helpers")
+
+describe("conflict gutter lifecycle on the screen grid", function()
+  local screen, repo, panes
+
+  before_each(function()
+    repo = gutter.create_repo()
+    screen = Screen.new(120, 40)
+    panes = gutter.open_merge(screen, repo)
+  end)
+
+  after_each(function()
+    if screen then
+      screen:close()
+      screen = nil
+    end
+    if repo then
+      repo.cleanup()
+      repo = nil
+    end
+  end)
+
+  local function expect_merge(rows, colors)
+    rows = rows or gutter.merge_rows
+    for _, focus in ipairs({ "original", "modified", "result" }) do
+      screen:exec("vim.api.nvim_set_current_win(...)", { panes[focus] })
+      screen:flush()
+      for _, side in ipairs({ "original", "modified", "result" }) do
+        gutter.expect_pane(screen, panes[side], rows[side], side .. " while focused on " .. focus, colors and colors[side])
+      end
+    end
+  end
+
+  local function colors(original, modified, result)
+    return screen:exec(
+      [[
+      local names = ...
+      local values = {}
+      for side, name in pairs(names) do
+        values[side] = vim.api.nvim_get_hl(0, { name = 'CodeDiffConflictSign' .. name, link = false }).fg
+      end
+      return values
+    ]],
+      { { original = original, modified = modified, result = result } }
+    )
+  end
+
+  it("renders a real merge with the full block shape in all three panes", function()
+    expect_merge(nil, colors("", "", ""))
+  end)
+
+  it("removes conflict markers when the same windows show an ordinary diff", function()
+    panes = gutter.open_plain(screen, repo)
+    gutter.expect_no_conflict_markers(screen, panes.original)
+    gutter.expect_no_conflict_markers(screen, panes.modified)
+  end)
+
+  it("survives repeated ordinary-diff round trips", function()
+    for _ = 1, 3 do
+      panes = gutter.open_plain(screen, repo)
+      gutter.expect_no_conflict_markers(screen, panes.original)
+      gutter.expect_no_conflict_markers(screen, panes.modified)
+      panes = gutter.return_to_merge(screen)
+      expect_merge()
+    end
+  end)
+
+  it("restores the surviving window's option in single-file mode", function()
+    screen:exec(
+      [[
+      require('codediff.ui.view.side_by_side').show_untracked_file(gutter_test_tab, ...)
+    ]],
+      { repo.path("fresh.txt") }
+    )
+    assert.equals("", screen:exec("return vim.wo[...].statuscolumn", { panes.modified }))
+  end)
+
+  it("rebinds a recreated incoming pane after an untracked file", function()
+    for _ = 1, 2 do
+      screen:exec(
+        [[
+        require('codediff.ui.view.side_by_side').show_untracked_file(gutter_test_tab, ...)
+      ]],
+        { repo.path("fresh.txt") }
+      )
+      panes = gutter.return_to_merge(screen)
+      expect_merge()
+    end
+  end)
+
+  it("rebinds a recreated current pane after a deleted file", function()
+    panes = gutter.return_to_merge(screen, "other.txt")
+    expect_merge(gutter.other_merge_rows)
+    screen:exec(
+      [[
+      local root = ...
+      require('codediff.ui.view.side_by_side').show_deleted_file(gutter_test_tab, root, 'deleted.txt', root .. '/deleted.txt', 'unstaged')
+    ]],
+      { repo.dir }
+    )
+    panes = gutter.return_to_merge(screen, "other.txt")
+    expect_merge(gutter.other_merge_rows)
+  end)
+
+  it("replaces marker maps when switching between two conflict files", function()
+    panes = gutter.return_to_merge(screen, "other.txt")
+    expect_merge(gutter.other_merge_rows)
+    panes = gutter.return_to_merge(screen)
+    expect_merge()
+  end)
+
+  it("restores conflict geometry and markers after leaving and returning to the tab", function()
+    screen:command("tabnew")
+    screen:exec("vim.api.nvim_set_current_tabpage(gutter_test_tab)")
+    screen:wait_for(function()
+      return screen:exec("return not require('codediff.ui.lifecycle').get_session(gutter_test_tab).suspended")
+    end)
+    expect_merge()
+  end)
+
+  it("keeps both panes correct after resizing the attached UI", function()
+    screen:request("nvim_ui_try_resize", { 100, 36 })
+    screen:flush()
+    expect_merge()
+  end)
+
+  it("updates shapes and colors on accept, undo, redo and discard", function()
+    screen:exec([[
+      local s = require('codediff.ui.lifecycle').get_session(gutter_test_tab)
+      vim.api.nvim_set_current_win(s.modified_win)
+      vim.api.nvim_win_set_cursor(s.modified_win, { 2, 0 })
+      assert(require('codediff.ui.conflict').accept_current(gutter_test_tab))
+    ]])
+    local resolved = vim.deepcopy(gutter.merge_rows)
+    resolved.result = { "  before", "╭─OURS1", "│ inserted", "│ OURS2", "╰─base3", "  after", "  tail" }
+    expect_merge(resolved, colors("Rejected", "Accepted", "Resolved"))
+
+    screen:exec("vim.api.nvim_set_current_win(...)", { panes.result })
+    screen:input("u")
+    screen:wait_for(function()
+      return screen:exec("return vim.api.nvim_buf_line_count(...) == 6", { panes.result_buf })
+    end, "undo did not restore the seed")
+    expect_merge(nil, colors("", "", ""))
+
+    screen:exec("vim.api.nvim_set_current_win(...)", { panes.result })
+    screen:input("<C-r>")
+    screen:wait_for(function()
+      return screen:exec("return vim.api.nvim_buf_line_count(...) == 7", { panes.result_buf })
+    end, "redo did not restore the resolution")
+    expect_merge(resolved, colors("Rejected", "Accepted", "Resolved"))
+
+    screen:exec([[
+      local s = require('codediff.ui.lifecycle').get_session(gutter_test_tab)
+      vim.api.nvim_set_current_win(s.modified_win)
+      vim.api.nvim_win_set_cursor(s.modified_win, { 2, 0 })
+      assert(require('codediff.ui.conflict').discard(gutter_test_tab))
+    ]])
+    expect_merge(nil, colors("", "", ""))
+  end)
+
+  it("preserves resolved Result content and colors across a tab round trip", function()
+    screen:exec([[
+      local s = require('codediff.ui.lifecycle').get_session(gutter_test_tab)
+      vim.api.nvim_set_current_win(s.modified_win)
+      vim.api.nvim_win_set_cursor(s.modified_win, { 2, 0 })
+      assert(require('codediff.ui.conflict').accept_current(gutter_test_tab))
+      vim.cmd('tabnew')
+      vim.api.nvim_set_current_tabpage(gutter_test_tab)
+    ]])
+    screen:wait_for(function()
+      return screen:exec("return not require('codediff.ui.lifecycle').get_session(gutter_test_tab).suspended")
+    end)
+    local rows = vim.deepcopy(gutter.merge_rows)
+    rows.result = { "  before", "╭─OURS1", "│ inserted", "│ OURS2", "╰─base3", "  after", "  tail" }
+    expect_merge(rows, colors("Rejected", "Accepted", "Resolved"))
+  end)
+
+  it("removes Result markers when manual editing empties its tracked range", function()
+    screen:exec("vim.api.nvim_set_current_win(...)", { panes.result })
+    screen:input("2G3dd")
+    screen:wait_for(function()
+      return screen:exec("return vim.api.nvim_buf_line_count(...) == 3", { panes.result_buf })
+    end)
+    local rows = vim.deepcopy(gutter.merge_rows)
+    rows.result = { "before", "after", "tail" }
+    expect_merge(rows, colors("Resolved", "Resolved", "Resolved"))
+  end)
+
+  it("refreshes marker colors after a manual Result edit", function()
+    screen:exec("vim.api.nvim_set_current_win(...); vim.api.nvim_win_set_cursor(0, { 2, 0 })", { panes.result })
+    screen:input("ccMANUAL<Esc>")
+    screen:wait_for(function()
+      return screen:exec("return vim.api.nvim_buf_get_lines(..., 1, 2, false)[1] == 'MANUAL'", { panes.result_buf })
+    end)
+    local rows = vim.deepcopy(gutter.merge_rows)
+    rows.result[2] = "╭─MANUAL"
+    expect_merge(rows, colors("Resolved", "Resolved", "Resolved"))
+  end)
+
+  it("does not run the old Result refresh against a retargeted session", function()
+    local old_result = panes.result_buf
+    panes = gutter.open_plain(screen, repo)
+    screen:exec(
+      [[
+      local buf = ...
+      vim.api.nvim_buf_set_lines(buf, 0, 1, false, { 'changed after leaving conflict' })
+      local ok, err = pcall(vim.api.nvim_exec_autocmds, 'TextChanged', { buffer = buf })
+      assert(ok, tostring(err))
+    ]],
+      { old_result }
+    )
+    screen:flush()
+    gutter.expect_no_conflict_markers(screen, panes.original)
+    gutter.expect_no_conflict_markers(screen, panes.modified)
+  end)
+
+  it("releases the surviving pane when a suspended session loses an input buffer", function()
+    local value = screen:exec([[
+      local s = require('codediff.ui.lifecycle').get_session(gutter_test_tab)
+      local state = require('codediff.ui.lifecycle.state')
+      state.suspend_diff(gutter_test_tab)
+      vim.api.nvim_buf_delete(s.original_bufnr, { force = true })
+      state.resume_diff(gutter_test_tab)
+      return vim.wo[s.modified_win].statuscolumn
+    ]])
+    assert.equals("", value)
+  end)
+
+  it("restores statuscolumns when the session is explicitly cleaned up", function()
+    screen:exec("require('codediff.ui.lifecycle').cleanup(gutter_test_tab)")
+    assert.is_nil(screen:exec("return require('codediff.ui.lifecycle').get_session(gutter_test_tab)"))
+    local columns = screen:exec([[
+      local values = {}
+      for _, win in ipairs(vim.api.nvim_list_wins()) do values[#values + 1] = vim.wo[win].statuscolumn end
+      return values
+    ]])
+    assert.is_true(#columns > 0)
+    for _, value in ipairs(columns) do
+      assert.equals("", value)
+    end
+  end)
+end)
diff --git a/tests/ui/conflict/gutter_options_spec.lua b/tests/ui/conflict/gutter_options_spec.lua
new file mode 100644
index 00000000..91c35f0b
--- /dev/null
+++ b/tests/ui/conflict/gutter_options_spec.lua
@@ -0,0 +1,185 @@
+local Screen = require("tests.framework.screen")
+
+describe("conflict gutter preserves window options", function()
+  local screen, win
+
+  before_each(function()
+    screen = Screen.new(80, 24)
+    win = screen:exec([[
+      local lines = {}
+      for i = 1, 20 do lines[i] = 'content_' .. i end
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, lines)
+      vim.wo.signcolumn = 'yes'
+      vim.wo.foldcolumn = '0'
+      vim.wo.wrap = false
+      vim.wo.cursorline = false
+      vim.api.nvim_win_set_cursor(0, { 6, 0 })
+      return vim.api.nvim_get_current_win()
+    ]])
+  end)
+
+  after_each(function()
+    if screen then
+      screen:close()
+      screen = nil
+    end
+  end)
+
+  local function attach()
+    screen:exec("require('codediff.ui.conflict').attach_gutter(...)", { win })
+  end
+
+  local function detach()
+    screen:exec("require('codediff.ui.conflict').detach_gutter(...)", { win })
+  end
+
+  local function set_column(value)
+    screen:exec("vim.wo.statuscolumn = ...", { value })
+  end
+
+  local function column()
+    return screen:exec("return vim.wo.statuscolumn")
+  end
+
+  local function snapshot()
+    screen:flush()
+    local rows = {}
+    for row = 1, 12 do
+      rows[row] = screen:text(row, 1, 35)
+    end
+    return rows
+  end
+
+  for _, options in ipairs({
+    { name = "no numbers", number = false, relativenumber = false },
+    { name = "absolute numbers", number = true, relativenumber = false },
+    { name = "relative numbers", number = false, relativenumber = true },
+    { name = "hybrid numbers", number = true, relativenumber = true },
+    { name = "wide hybrid numbers", number = true, relativenumber = true, numberwidth = 7 },
+    { name = "fold column", number = true, relativenumber = true, foldcolumn = "2" },
+    { name = "multiple sign slots", number = true, relativenumber = true, signcolumn = "yes:2" },
+    { name = "signs in the number column", number = true, relativenumber = true, signcolumn = "number" },
+    { name = "a sign on the hybrid cursor row", number = true, relativenumber = true, signcolumn = "number", cursor = 3 },
+    { name = "hidden signs", number = true, relativenumber = true, signcolumn = "no" },
+  }) do
+    it("matches native rendering with " .. options.name, function()
+      screen:exec(
+        [[
+        local options = ...
+        for name, value in pairs(options) do
+          if name ~= 'name' and name ~= 'cursor' then vim.wo[name] = value end
+        end
+        if options.cursor then vim.api.nvim_win_set_cursor(0, { options.cursor, 0 }) end
+        local ns = vim.api.nvim_create_namespace('test-other-signs')
+        vim.api.nvim_buf_set_extmark(0, ns, 2, 0, { sign_text = '!!', sign_hl_group = 'DiagnosticWarn' })
+      ]],
+        { options }
+      )
+      local native = snapshot()
+      attach()
+      assert.same(native, snapshot(), "attaching must not change native columns")
+      detach()
+      assert.same(native, snapshot(), "detaching must restore native columns")
+    end)
+  end
+
+  it("preserves folded and wrapped rows", function()
+    screen:exec([[
+      vim.wo.number = true
+      vim.wo.relativenumber = true
+      vim.wo.foldcolumn = '1'
+      vim.wo.foldmethod = 'manual'
+      vim.wo.wrap = true
+      vim.api.nvim_buf_set_lines(0, 0, 1, false, { string.rep('wrapped ', 15) })
+      vim.cmd('3,5fold')
+      vim.api.nvim_win_set_cursor(0, { 6, 0 })
+    ]])
+    local native = snapshot()
+    attach()
+    assert.same(native, snapshot())
+  end)
+
+  it("follows number option changes after attaching", function()
+    attach()
+    screen:exec("vim.wo.number = true; vim.wo.relativenumber = true")
+    local attached = snapshot()
+    detach()
+    assert.same(snapshot(), attached)
+  end)
+
+  it("uses each drawing window's number options and cursor", function()
+    local other = screen:exec([[
+      vim.wo.number = true
+      vim.wo.relativenumber = false
+      vim.cmd('rightbelow vsplit')
+      vim.wo.number = false
+      vim.wo.relativenumber = true
+      vim.api.nvim_win_set_cursor(0, { 10, 0 })
+      return vim.api.nvim_get_current_win()
+    ]])
+    for _, focus in ipairs({ win, other }) do
+      screen:exec(
+        [[
+        local first, second, focus = ...
+        local conflict = require('codediff.ui.conflict')
+        conflict.detach_gutter(first)
+        conflict.detach_gutter(second)
+        vim.api.nvim_set_current_win(focus)
+      ]],
+        { win, other, focus }
+      )
+      screen:flush()
+      local native = {}
+      for row = 1, 12 do
+        native[row] = screen:text(row, 1, 80)
+      end
+      screen:exec("require('codediff.ui.conflict').attach_gutter(...)", { win, other })
+      screen:flush()
+      screen:expect_rows(1, 1, native, "different per-window number options")
+    end
+  end)
+
+  it("leaves an existing custom statuscolumn untouched", function()
+    set_column("USER %C%s%l")
+    local native = snapshot()
+    attach()
+    assert.equals("USER %C%s%l", column())
+    assert.same(native, snapshot())
+    detach()
+    assert.equals("USER %C%s%l", column())
+  end)
+
+  it("does not restore a configuration it declined to own", function()
+    set_column("USER_A %l")
+    attach()
+    set_column("USER_B %l")
+    detach()
+    assert.equals("USER_B %l", column())
+  end)
+
+  it("does not overwrite a replacement installed while attached", function()
+    attach()
+    set_column("NEW_OWNER %l")
+    detach()
+    assert.equals("NEW_OWNER %l", column())
+  end)
+
+  it("does not reclaim a user's replacement on a repeated attach", function()
+    attach()
+    set_column("NEW_OWNER %l")
+    attach()
+    assert.equals("NEW_OWNER %l", column())
+    detach()
+    assert.equals("NEW_OWNER %l", column())
+  end)
+
+  it("attaches and detaches repeatedly without losing the original option", function()
+    for _ = 1, 3 do
+      attach()
+      attach()
+      detach()
+      detach()
+      assert.equals("", column())
+    end
+  end)
+end)