diff --git a/.changeset/resizable-detail-panels.md b/.changeset/resizable-detail-panels.md
new file mode 100644
index 0000000..a84b562
--- /dev/null
+++ b/.changeset/resizable-detail-panels.md
@@ -0,0 +1,5 @@
+---
+'@solidjs/start-devtools': minor
+---
+
+Make the detail panes in the Ownership and Reactivity panels resizable. Drag the left edge of a pane, or focus it and press the left and right arrow keys. Each pane keeps its own width across reloads, and a double-click on the edge resets it.
diff --git a/src/dev-toolbar/ownership/index.tsx b/src/dev-toolbar/ownership/index.tsx
index c19a430..b034575 100644
--- a/src/dev-toolbar/ownership/index.tsx
+++ b/src/dev-toolbar/ownership/index.tsx
@@ -3,7 +3,9 @@ import { createEffect, createMemo, createSignal, For, getOwner, Show } from 'sol
 import { Badge } from '../../ui/Badge.js';
 import IconButton from '../../ui/IconButton.js';
 import Placeholder from '../../ui/Placeholder.js';
+import { ResizeHandle } from '../../ui/ResizeHandle.js';
 import { Text } from '../../ui/Text.js';
+import { useResizableDetail } from '../../ui/use-resizable-detail.js';
 import { CollapseIcon, EditIcon, ExpandIcon, EyeIcon, PauseIcon, PlayIcon } from '../icons.js';
 import { openInEditor } from '../open-in-editor.js';
 import { CopyReportButton } from '../report/CopyReportButton.js';
@@ -194,6 +196,7 @@ export default function OwnershipViewer(props: OwnershipViewerProps): JSX.Elemen
   const [query, setQuery] = createSignal('');
   const [collapsed, setCollapsed] = createSignal<string[]>([]);
   const [selected, setSelected] = createSignal<string>();
+  const detail = useResizableDetail({ storageKey: 'solid-start-devtools:ownership-detail-width' });
 
   const firstSeen = new Map<string, number>();
   let appliedFocus: { owner: object } | undefined;
@@ -464,7 +467,8 @@ export default function OwnershipViewer(props: OwnershipViewerProps): JSX.Elemen
 
               <Show when={selectedNode()}>
                 {(node) => (
-                  <aside data-solid-ownership-detail>
+                  <aside data-solid-ownership-detail style={{ width: `${detail.width()}px` }}>
+                    <ResizeHandle detail={detail} />
                     <div data-solid-ownership-detail-content>
                       <div data-solid-ownership-detail-head>
                         <span data-solid-ownership-kind={node().kind} />
diff --git a/src/dev-toolbar/ownership/styles.css b/src/dev-toolbar/ownership/styles.css
index a881e6a..4f0ed28 100644
--- a/src/dev-toolbar/ownership/styles.css
+++ b/src/dev-toolbar/ownership/styles.css
@@ -251,10 +251,12 @@
 }
 
 [data-solid-ownership-detail] {
+  position: relative;
   display: flex;
   flex-direction: column;
 
-  width: 20rem;
+  /* Keeps the tree usable when the window shrinks below a saved width. */
+  max-width: calc(100% - 240px);
   flex-shrink: 0;
   min-height: 0;
 
diff --git a/src/dev-toolbar/reactivity/index.tsx b/src/dev-toolbar/reactivity/index.tsx
index 4adfb97..3040670 100644
--- a/src/dev-toolbar/reactivity/index.tsx
+++ b/src/dev-toolbar/reactivity/index.tsx
@@ -3,7 +3,9 @@ import { createEffect, createMemo, createSignal, For, getOwner, Show } from 'sol
 import { Badge } from '../../ui/Badge.js';
 import IconButton from '../../ui/IconButton.js';
 import Placeholder from '../../ui/Placeholder.js';
+import { ResizeHandle } from '../../ui/ResizeHandle.js';
 import { Text } from '../../ui/Text.js';
+import { useResizableDetail } from '../../ui/use-resizable-detail.js';
 import { FitIcon, PauseIcon, PlayIcon } from '../icons.js';
 import { CopyReportButton } from '../report/CopyReportButton.js';
 import { pageRoute } from '../report/markdown.js';
@@ -164,6 +166,10 @@ export default function ReactivityViewer(props: ReactivityViewerProps): JSX.Elem
   const [selected, setSelected] = createSignal<string>();
   const [hovered, setHovered] = createSignal<string>();
   const [view, setView] = createSignal<ViewTransform>({ x: 0, y: 0, k: 1 });
+  const detail = useResizableDetail({
+    storageKey: 'solid-start-devtools:reactivity-detail-width',
+    defaultWidth: 304,
+  });
 
   let viewport: HTMLDivElement | undefined;
   let fittedSize = '';
@@ -702,7 +708,8 @@ export default function ReactivityViewer(props: ReactivityViewerProps): JSX.Elem
 
               <Show when={selectedNode()}>
                 {(node) => (
-                  <aside data-solid-reactivity-detail>
+                  <aside data-solid-reactivity-detail style={{ width: `${detail.width()}px` }}>
+                    <ResizeHandle detail={detail} />
                     <div data-solid-reactivity-detail-content>
                       <NodeSummary
                         node={node()}
diff --git a/src/dev-toolbar/reactivity/styles.css b/src/dev-toolbar/reactivity/styles.css
index 1ec1f0d..82e9dd9 100644
--- a/src/dev-toolbar/reactivity/styles.css
+++ b/src/dev-toolbar/reactivity/styles.css
@@ -484,10 +484,12 @@
 }
 
 [data-solid-reactivity-detail] {
+  position: relative;
   display: flex;
   flex-direction: column;
 
-  width: 19rem;
+  /* Keeps the tree usable when the window shrinks below a saved width. */
+  max-width: calc(100% - 240px);
   flex-shrink: 0;
   min-height: 0;
 
diff --git a/src/ui/ResizeHandle.css b/src/ui/ResizeHandle.css
new file mode 100644
index 0000000..97a996d
--- /dev/null
+++ b/src/ui/ResizeHandle.css
@@ -0,0 +1,62 @@
+[data-solid-resize-handle] {
+  position: absolute;
+  top: 0;
+  bottom: 0;
+  left: -4px;
+  width: 8px;
+  z-index: 10;
+
+  cursor: ew-resize;
+  touch-action: none;
+}
+
+/* Hitbox: wide invisible target area centered on the border */
+[data-solid-resize-handle]::before {
+  content: '';
+  position: absolute;
+  top: 0;
+  bottom: 0;
+  left: -2px;
+  width: 12px;
+}
+
+/* Visual line: centered on border, subtle hover, solid thicker line when dragging */
+[data-solid-resize-handle]::after {
+  content: '';
+  position: absolute;
+  top: 0;
+  bottom: 0;
+  left: 3px;
+  width: 2px;
+
+  background: transparent;
+  transition:
+    background-color 150ms ease-out,
+    width 150ms ease-out,
+    left 150ms ease-out;
+  pointer-events: none;
+}
+
+/* Hover state: visible, semi-transparent accent line */
+[data-solid-resize-handle]:hover::after {
+  background: var(--start-dt-accent);
+  opacity: 0.6;
+}
+
+/* Active / Dragging state: thicker (3px), full solid opacity without glow */
+[data-solid-resize-handle]:active::after,
+[data-solid-resize-handle][data-dragging]::after {
+  left: 2.5px;
+  width: 3px;
+  background: var(--start-dt-accent);
+  opacity: 1;
+}
+
+[data-solid-resize-handle]:focus-visible {
+  outline: none;
+}
+
+[data-solid-resize-handle]:focus-visible::after {
+  background: var(--start-dt-accent);
+  opacity: 1;
+}
diff --git a/src/ui/ResizeHandle.tsx b/src/ui/ResizeHandle.tsx
new file mode 100644
index 0000000..30adb83
--- /dev/null
+++ b/src/ui/ResizeHandle.tsx
@@ -0,0 +1,74 @@
+import type { JSX } from '@solidjs/web';
+import { createSignal } from 'solid-js';
+import { KEYBOARD_STEP, maxDetailWidth, type ResizableDetail } from './use-resizable-detail.js';
+import './ResizeHandle.css';
+
+export interface ResizeHandleProps {
+  detail: ResizableDetail;
+  title?: string;
+}
+
+/** The left edge of a detail pane. The pane is the handle's parent, inside the panel. */
+export function ResizeHandle(props: ResizeHandleProps): JSX.Element {
+  const [dragging, setDragging] = createSignal(false);
+  const [panelWidth, setPanelWidth] = createSignal(0);
+  let handle: HTMLDivElement | undefined;
+
+  function measurePanel(): number {
+    const width = handle?.parentElement?.parentElement?.clientWidth ?? 0;
+    setPanelWidth(width);
+    return width;
+  }
+
+  function onPointerDown(event: PointerEvent): void {
+    if (event.button !== 0) return;
+    event.preventDefault();
+    setDragging(true);
+    props.detail.start(measurePanel());
+    const startX = event.clientX;
+
+    function onPointerMove(next: PointerEvent): void {
+      props.detail.drag(startX - next.clientX);
+    }
+
+    function onPointerUp(): void {
+      setDragging(false);
+      window.removeEventListener('pointermove', onPointerMove);
+      window.removeEventListener('pointerup', onPointerUp);
+      window.removeEventListener('pointercancel', onPointerUp);
+      props.detail.end();
+    }
+
+    window.addEventListener('pointermove', onPointerMove);
+    window.addEventListener('pointerup', onPointerUp);
+    window.addEventListener('pointercancel', onPointerUp);
+  }
+
+  // The handle is the pane's left edge, so the left arrow widens the pane.
+  function onKeyDown(event: KeyboardEvent): void {
+    if (event.key === 'ArrowLeft') props.detail.step(KEYBOARD_STEP, measurePanel());
+    else if (event.key === 'ArrowRight') props.detail.step(-KEYBOARD_STEP, measurePanel());
+    else return;
+    event.preventDefault();
+  }
+
+  return (
+    <div
+      ref={handle}
+      role="separator"
+      tabindex="0"
+      aria-orientation="vertical"
+      aria-label="Resize the detail pane"
+      aria-valuenow={props.detail.width()}
+      aria-valuemin={props.detail.minWidth}
+      aria-valuemax={panelWidth() ? maxDetailWidth(panelWidth(), props.detail.minWidth) : undefined}
+      data-solid-resize-handle
+      data-dragging={dragging() ? '' : undefined}
+      title={props.title ?? 'Drag to resize, double-click to reset'}
+      onPointerDown={onPointerDown}
+      onKeyDown={onKeyDown}
+      onFocus={measurePanel}
+      onDblClick={() => props.detail.reset()}
+    />
+  );
+}
diff --git a/src/ui/use-resizable-detail.test.ts b/src/ui/use-resizable-detail.test.ts
new file mode 100644
index 0000000..b6b0c21
--- /dev/null
+++ b/src/ui/use-resizable-detail.test.ts
@@ -0,0 +1,66 @@
+import { createRoot } from 'solid-js';
+import { describe, expect, it } from 'vitest';
+import {
+  clampDetailWidth,
+  maxDetailWidth,
+  MIN_DETAIL_WIDTH,
+  MIN_MAIN_WIDTH,
+  useResizableDetail,
+} from './use-resizable-detail.js';
+
+describe('clampDetailWidth', () => {
+  it('keeps width within bounds', () => {
+    expect(clampDetailWidth(300, 200, 600)).toBe(300);
+    expect(clampDetailWidth(150, 200, 600)).toBe(200);
+    expect(clampDetailWidth(750, 200, 600)).toBe(600);
+  });
+
+  it('rounds non-integer widths', () => {
+    expect(clampDetailWidth(300.4, 200, 600)).toBe(300);
+    expect(clampDetailWidth(300.6, 200, 600)).toBe(301);
+  });
+});
+
+describe('maxDetailWidth', () => {
+  it('leaves room for the tree beside the pane', () => {
+    expect(maxDetailWidth(1000)).toBe(1000 - MIN_MAIN_WIDTH);
+  });
+
+  it('never goes below the minimum width', () => {
+    expect(maxDetailWidth(300)).toBe(MIN_DETAIL_WIDTH);
+  });
+});
+
+describe('useResizableDetail', () => {
+  function detail() {
+    return createRoot(() => useResizableDetail({ storageKey: 'test', defaultWidth: 320 }));
+  }
+
+  it('keeps the edge under the pointer after passing a limit', () => {
+    const pane = detail();
+    pane.start(1000);
+    // 100px past the minimum, then 50px back: still past it.
+    pane.drag(-(320 - MIN_DETAIL_WIDTH) - 100);
+    expect(pane.width()).toBe(MIN_DETAIL_WIDTH);
+    pane.drag(-(320 - MIN_DETAIL_WIDTH) - 50);
+    expect(pane.width()).toBe(MIN_DETAIL_WIDTH);
+    pane.drag(40);
+    expect(pane.width()).toBe(360);
+  });
+
+  it('stops at the width the panel allows', () => {
+    const pane = detail();
+    pane.start(800);
+    pane.drag(1000);
+    expect(pane.width()).toBe(800 - MIN_MAIN_WIDTH);
+  });
+
+  it('steps from the current width', () => {
+    const pane = detail();
+    pane.step(16, 1000);
+    pane.step(16, 1000);
+    expect(pane.width()).toBe(352);
+    pane.reset();
+    expect(pane.width()).toBe(320);
+  });
+});
diff --git a/src/ui/use-resizable-detail.ts b/src/ui/use-resizable-detail.ts
new file mode 100644
index 0000000..e16aa03
--- /dev/null
+++ b/src/ui/use-resizable-detail.ts
@@ -0,0 +1,100 @@
+import { createSignal, onSettled } from 'solid-js';
+
+export const DEFAULT_DETAIL_WIDTH = 320;
+export const MIN_DETAIL_WIDTH = 240;
+/** The narrowest the tree or graph beside a detail pane may get. */
+export const MIN_MAIN_WIDTH = 240;
+/** How far one arrow key press moves the edge. */
+export const KEYBOARD_STEP = 16;
+
+function readSavedWidth(key: string): number | undefined {
+  try {
+    const raw = localStorage.getItem(key);
+    if (!raw) return undefined;
+    const parsed = Number(raw);
+    return Number.isFinite(parsed) ? parsed : undefined;
+  } catch {
+    return undefined;
+  }
+}
+
+function writeSavedWidth(key: string, width: number): void {
+  try {
+    localStorage.setItem(key, String(width));
+  } catch {
+    // Storage can be blocked.
+  }
+}
+
+/** The widest a detail pane may get inside a panel `panelWidth` wide. */
+export function maxDetailWidth(panelWidth: number, min = MIN_DETAIL_WIDTH): number {
+  return Math.max(min, panelWidth - MIN_MAIN_WIDTH);
+}
+
+export function clampDetailWidth(width: number, min: number, max: number): number {
+  return Math.round(Math.min(Math.max(width, min), max));
+}
+
+export interface UseResizableDetailOptions {
+  /** Each pane keeps its own width. */
+  storageKey: string;
+  defaultWidth?: number;
+  minWidth?: number;
+}
+
+export interface ResizableDetail {
+  width: () => number;
+  minWidth: number;
+  /** Starts a drag. `panelWidth` is the width of the panel the pane sits in. */
+  start: (panelWidth: number) => void;
+  /** Moves the edge `offsetX` pixels to the left of where the drag started. */
+  drag: (offsetX: number) => void;
+  /** Moves the edge `deltaX` pixels to the left now, and saves the width. */
+  step: (deltaX: number, panelWidth: number) => void;
+  end: () => void;
+  reset: () => void;
+}
+
+export function useResizableDetail(options: UseResizableDetailOptions): ResizableDetail {
+  const key = options.storageKey;
+  const initial = options.defaultWidth ?? DEFAULT_DETAIL_WIDTH;
+  const min = options.minWidth ?? MIN_DETAIL_WIDTH;
+
+  const [width, setWidth] = createSignal(initial);
+  let startWidth = initial;
+  let max = Number.POSITIVE_INFINITY;
+
+  onSettled(() => {
+    const saved = readSavedWidth(key);
+    if (typeof saved === 'number') setWidth(Math.max(Math.round(saved), min));
+  });
+
+  function start(panelWidth: number): void {
+    max = maxDetailWidth(panelWidth, min);
+    // A saved width can be wider than the panel allows now; CSS shows it clamped.
+    startWidth = clampDetailWidth(width(), min, max);
+  }
+
+  // Measured from the start of the drag, so the edge stays under the pointer
+  // after it passes a limit and comes back.
+  function drag(offsetX: number): void {
+    setWidth(clampDetailWidth(startWidth + offsetX, min, max));
+  }
+
+  function step(deltaX: number, panelWidth: number): void {
+    start(panelWidth);
+    drag(deltaX);
+    end();
+  }
+
+  function end(): void {
+    writeSavedWidth(key, width());
+  }
+
+  function reset(): void {
+    setWidth(initial);
+    writeSavedWidth(key, initial);
+  }
+
+  return { width, minWidth: min, start, drag, step, end, reset };
+}
diff --git a/tests/e2e/devtools.spec.ts b/tests/e2e/devtools.spec.ts
index 718b2ba..bd87aac 100644
--- a/tests/e2e/devtools.spec.ts
+++ b/tests/e2e/devtools.spec.ts
@@ -456,6 +456,32 @@ test('maps the ownership tree', async ({ page }) => {
     .locator('[data-solid-ownership-label]')
     .filter({ hasText: /^doubled$/ })
     .click();
+
+  // The ownership detail pane can be resized horizontally by dragging its left edge handle.
+  const handle = detail.locator('[data-solid-resize-handle]');
+  await expect(handle).toBeVisible();
+  const initialWidth = (await detail.boundingBox())!.width;
+
+  const handleBox = (await handle.boundingBox())!;
+  await page.mouse.move(handleBox.x + handleBox.width / 2, handleBox.y + 100);
+  await page.mouse.down();
+  // Drag left by 100px to widen the detail pane
+  await page.mouse.move(handleBox.x + handleBox.width / 2 - 100, handleBox.y + 100);
+  await page.mouse.up();
+
+  const expandedWidth = (await detail.boundingBox())!.width;
+  expect(expandedWidth).toBeGreaterThan(initialWidth + 80);
+
+  // Double click resets the width
+  await handle.dblclick();
+  const resetWidth = (await detail.boundingBox())!.width;
+  expect(Math.abs(resetWidth - 320)).toBeLessThanOrEqual(2);
+
+  // The left arrow key widens the pane from the keyboard.
+  await handle.focus();
+  await page.keyboard.press('ArrowLeft');
+  await expect(handle).toHaveAttribute('aria-valuenow', '336');
+
   await detail
     .locator('[data-solid-ownership-detail-head]')
     .getByRole('button', { name: 'View in graph' })