diff --git a/AGENTS.md b/AGENTS.md
index 3f2a0fa850..60297f11da 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -36,7 +36,7 @@ website/              # demo site (Vite + TanStack Router)
 - **Dual class names** — components apply both a semantic class (`rdg-cell`) and a generated hash. Preserve both.
 - **Light/dark mode** — handled via CSS `light-dark()` + `color-scheme`, not JS.
 - **Accessibility first** — ARIA attributes (e.g., `aria-colindex`, `aria-rowindex`, `aria-selected`, roles) are required. Tests query by role.
-- **Build** — `vp pack` bundles the library to `lib/`, then lints the packed package with publint; `ecij` plugin prefixes classes with `rdg-{version}-` (dots→dashes) to avoid cross-version conflicts.
+- **Build** — `vp pack` bundles the library to `lib/`, then lints the packed package with publint; `ecij` plugin prefixes classes with `rdg-{version}-` (dots→dashes) to avoid cross-version conflicts. React Compiler compiles every component and hook, and `vp pack` fails in CI and agent sessions when it can't compile one.
 - **Workflows** — pin every action to a full commit SHA with its version in a comment (Dependabot updates both), pass expressions to `run` steps through `env` instead of inlining `${{ }}`, grant each job only the `permissions` it uses, and check out with `persist-credentials: false`. Jobs that install dependencies only get read permissions: publishing, pushing, and editing PRs happen in separate jobs that receive build output as artifacts and never install the project's dependencies.
 - **Releases** — the header of `.github/workflows/publish.yml` explains how to publish. Its read-only `build` job hands `lib/` to the `publish` job, so the project's dependencies never run with the `id-token: write` permission that npm's trusted publishing relies on, and a separate `release` job creates the GitHub release.
 
diff --git a/src/DataGrid.tsx b/src/DataGrid.tsx
index 76c1cf90f8..ca3f54e1c1 100644
--- a/src/DataGrid.tsx
+++ b/src/DataGrid.tsx
@@ -503,7 +503,6 @@ export function DataGrid<R, SR = unknown, K extends Key = Key>(props: DataGridPr
   const selectHeaderRowLatest = useLatestFunc(selectHeaderRow);
   const selectRowLatest = useLatestFunc(selectRow);
   const updateRowLatest = useLatestFunc(updateRow);
-  // oxlint-disable-next-line react/immutability
   const setPositionLatest = useLatestFunc(setPosition);
   const selectHeaderCellLatest = useLatestFunc(selectHeaderCell);
 
@@ -530,7 +529,7 @@ export function DataGrid<R, SR = unknown, K extends Key = Key>(props: DataGridPr
         setScrollToPosition({ idx: scrollToIdx, rowIdx: scrollToRowIdx });
       }
     },
-    setActivePosition: setPosition
+    setActivePosition: setPositionLatest
   }));
 
   /**
diff --git a/src/EditCell.tsx b/src/EditCell.tsx
index 548b74de56..1da46e4f63 100644
--- a/src/EditCell.tsx
+++ b/src/EditCell.tsx
@@ -50,6 +50,14 @@ export default function EditCell<R, SR>({
   const cancelScheduledTaskRef = useRef<() => void>(undefined);
   const commitOnOutsideClick = column.editorOptions?.commitOnOutsideClick ?? true;
 
+  function onClose(commitChanges = false, shouldFocus = true) {
+    if (commitChanges) {
+      onRowChange(row, true, shouldFocus);
+    } else {
+      closeEditor(shouldFocus);
+    }
+  }
+
   // We need to prevent the `useLayoutEffect` from cleaning up between re-renders,
   // as `onWindowCaptureMouseDown` might otherwise miss valid mousedown events.
   // To that end, we instead access the latest props via useEffectEvent.
@@ -57,6 +65,14 @@ export default function EditCell<R, SR>({
     onClose(true, false);
   });
 
+  function cancelTask() {
+    captureEventRef.current = undefined;
+    cancelScheduledTaskRef.current?.();
+    cancelScheduledTaskRef.current = undefined;
+  }
+
+  const cancelTaskEffect = useEffectEvent(cancelTask);
+
   useLayoutEffect(() => {
     if (!commitOnOutsideClick) return;
 
@@ -104,18 +120,10 @@ export default function EditCell<R, SR>({
         win.removeEventListener('mousedown', onWindowCaptureMouseDown, { capture: true });
         win.removeEventListener('mousedown', onWindowMouseDown);
       }
-      cancelTask();
+      cancelTaskEffect();
     };
   }, [commitOnOutsideClick]);
 
-  // TODO: report React Compiler bug
-  // oxlint-disable-next-line react/invariant
-  function cancelTask() {
-    captureEventRef.current = undefined;
-    cancelScheduledTaskRef.current?.();
-    cancelScheduledTaskRef.current = undefined;
-  }
-
   function handleKeyDown(event: React.KeyboardEvent<HTMLDivElement>) {
     if (onKeyDown) {
       const cellEvent = createCellEvent(event);
@@ -145,14 +153,6 @@ export default function EditCell<R, SR>({
     }
   }
 
-  function onClose(commitChanges = false, shouldFocus = true) {
-    if (commitChanges) {
-      onRowChange(row, true, shouldFocus);
-    } else {
-      closeEditor(shouldFocus);
-    }
-  }
-
   function onEditorRowChange(row: R, commitChangesAndFocus = false) {
     onRowChange(row, commitChangesAndFocus, commitChangesAndFocus);
   }
diff --git a/src/GroupCell.tsx b/src/GroupCell.tsx
index 8354f167c0..206f05377d 100644
--- a/src/GroupCell.tsx
+++ b/src/GroupCell.tsx
@@ -13,8 +13,9 @@ interface GroupCellProps<R, SR> {
   column: CalculatedColumn<R, SR>;
   row: GroupRow<R>;
   isCellActive: boolean;
-  groupColumnIndex: number;
   isGroupByColumn: boolean;
+  // Only the group-by column of the row's level renders the group cell and is clickable
+  isLevelMatching: boolean;
 }
 
 function GroupCell<R, SR>({
@@ -25,8 +26,8 @@ function GroupCell<R, SR>({
   isCellActive,
   column,
   row,
-  groupColumnIndex,
   isGroupByColumn,
+  isLevelMatching,
   toggleGroup: toggleGroupWrapper
 }: GroupCellProps<R, SR>) {
   const { tabIndex, childTabIndex, onFocus } = useRovingTabIndex(isCellActive);
@@ -35,9 +36,6 @@ function GroupCell<R, SR>({
     toggleGroupWrapper(id);
   }
 
-  // Only make the cell clickable if the group level matches
-  const isLevelMatching = isGroupByColumn && groupColumnIndex === column.idx;
-
   return (
     <div
       key={column.key}
diff --git a/src/GroupedRow.tsx b/src/GroupedRow.tsx
index feb8dde296..8f48ec6c1b 100644
--- a/src/GroupedRow.tsx
+++ b/src/GroupedRow.tsx
@@ -4,7 +4,6 @@ import { css } from 'ecij';
 import { RowSelectionContext, type RowSelectionContextValue } from './hooks';
 import { classnames } from './utils';
 import type { BaseRenderRowProps, GroupRow, Omit } from './types';
-import { SELECT_COLUMN_KEY } from './Columns';
 import GroupCell from './GroupCell';
 import { cell, cellFrozenBase } from './style/cell';
 import { rowClassname, rowActiveClassname } from './style/row';
@@ -43,8 +42,8 @@ function GroupedRow<R, SR>({
   ...props
 }: GroupedRowProps<R, SR>) {
   const isPositionOnRow = activeCellIdx === -1;
-
-  let idx = row.level;
+  // the group-by columns follow the `groupBy` order, so each level has its own column
+  const levelColumnKey = groupBy[row.level];
 
   function handleSelectGroup() {
     setActivePosition({ idx: -1, rowIdx }, { shouldFocus: true });
@@ -75,29 +74,21 @@ function GroupedRow<R, SR>({
         style={{ gridRowStart }}
         {...props}
       >
-        {mapViewportColumns(activeCellIdx, undefined, (column, isCellActive, colSpan, index) => {
-          // Select is always the first column
-          if (index === 0 && column.key === SELECT_COLUMN_KEY) {
-            // oxlint-disable-next-line react/immutability
-            idx += 1;
-          }
-
-          return (
-            <GroupCell
-              key={column.key}
-              id={row.id}
-              groupKey={row.groupKey}
-              childRows={row.childRows}
-              isExpanded={row.isExpanded}
-              isCellActive={isCellActive}
-              column={column}
-              row={row}
-              groupColumnIndex={idx}
-              toggleGroup={toggleGroup}
-              isGroupByColumn={groupBy.includes(column.key)}
-            />
-          );
-        })}
+        {mapViewportColumns(activeCellIdx, undefined, (column, isCellActive) => (
+          <GroupCell
+            key={column.key}
+            id={row.id}
+            groupKey={row.groupKey}
+            childRows={row.childRows}
+            isExpanded={row.isExpanded}
+            isCellActive={isCellActive}
+            column={column}
+            row={row}
+            toggleGroup={toggleGroup}
+            isGroupByColumn={groupBy.includes(column.key)}
+            isLevelMatching={column.key === levelColumnKey}
+          />
+        ))}
       </div>
     </RowSelectionContext>
   );
diff --git a/src/hooks/useCalculatedColumns.ts b/src/hooks/useCalculatedColumns.ts
index 433371c36e..fbb7f2808b 100644
--- a/src/hooks/useCalculatedColumns.ts
+++ b/src/hooks/useCalculatedColumns.ts
@@ -136,7 +136,7 @@ export function useCalculatedColumns<R, SR>({
         columns.push(column);
 
         if (isStartFrozen(frozen)) {
-          lastStartFrozenColumnIndex++;
+          lastStartFrozenColumnIndex += 1;
         }
 
         if (level > headerRowsCount) {
diff --git a/src/hooks/useColumnWidths.ts b/src/hooks/useColumnWidths.ts
index 5a2388504d..fdc8560300 100644
--- a/src/hooks/useColumnWidths.ts
+++ b/src/hooks/useColumnWidths.ts
@@ -65,7 +65,9 @@ export function useColumnWidths<R, SR>(
 
     for (const key of columnsToMeasure) {
       const measuredWidth = measureColumnWidth(gridRef, key);
-      hasChanges ||= measuredWidth !== columnWidths.get(key)?.width;
+      if (measuredWidth !== columnWidths.get(key)?.width) {
+        hasChanges = true;
+      }
       if (measuredWidth === undefined) {
         newColumnWidths.delete(key);
       } else {
diff --git a/src/hooks/useViewportRows.ts b/src/hooks/useViewportRows.ts
index ca149b4425..c277571998 100644
--- a/src/hooks/useViewportRows.ts
+++ b/src/hooks/useViewportRows.ts
@@ -50,7 +50,7 @@ export function useViewportRows<R>({
         repeatCount = 1;
       } else if (currentHeight === currentRowHeight) {
         // If the current row height is the same as the previous one, increment the repeat count
-        repeatCount++;
+        repeatCount += 1;
       } else {
         if (repeatCount > 1) {
           gridTemplateRows += `repeat(${repeatCount}, ${currentHeight}px) `;
diff --git a/vite.config.ts b/vite.config.ts
index 415f693cc5..97757b6b39 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -53,6 +53,15 @@ async function getTscPath(): Promise<string> {
   return getExePath();
 }
 
+// shared by Vite and `vp pack`
+const reactPlugin = react({
+  // warn about the code React Compiler can't compile, which fails `vp pack` with `failOnWarn`
+  compiler: { reportDiagnostics: true },
+  // `vp pack` passes the declaration files it generates through the plugin,
+  // and the compiler's TypeScript transform would strip all of their declarations
+  exclude: [/\/node_modules\//, /\.d\.ts$/]
+});
+
 const actionTimeout = 2000;
 const viewport = { width: 1920, height: 1080 } as const;
 const playwrightOptions: PlaywrightProviderOptions = {
@@ -190,7 +199,7 @@ export default defineConfig({
           autoCodeSplitting: true
         })
       ),
-    react({ compiler: true })
+    reactPlugin
   ]),
 
   server: {
@@ -239,10 +248,7 @@ export default defineConfig({
         // between multiple versions of RDG on the same page
         classPrefix: `rdg-${pkg.version.replaceAll('.', '-')}-`
       }),
-      react({
-        compiler: true,
-        exclude: [/node_modules/, /\.d\.ts$/]
-      })
+      reactPlugin
     ]
   },
 
@@ -677,7 +683,7 @@ copy(
       'react/static-components': 'warn',
       'react/style-prop-object': 'warn',
       'react/syntax': 'warn',
-      'react/todo': 'off',
+      'react/todo': 'warn',
       'react/unsupported-syntax': 'warn',
       'react/use-memo': 'warn',
       'react/void-dom-elements-no-children': 'warn',