fix(web): Diff pane expands unmodified lines again

apps/web/src/components/DiffPanel.browser.test.tsx

@@ -32,6 +32,7 @@ const state = vi.hoisted(() => ({
   localRefs: null as unknown as ResultAtom<VcsListRefsResult>,
   remoteRefs: null as unknown as ResultAtom<VcsListRefsResult>,
   commits: null as unknown as ResultAtom<ReviewBranchCommitsResult>,
+  viewerFiles: [] as Array<ReadonlyArray<{ fileDiff: object; collapsed: boolean }>>,
 }));
 
 vi.mock("~/state/server", () => ({
@@ -76,8 +77,14 @@ vi.mock("~/state/use-atom-command", () => ({ useAtomCommand: () => async () => {
 vi.mock("~/editorPreferences", () => ({ useOpenInPreferredEditor: () => async () => {} }));
 vi.mock("~/fileContextMenu", () => ({ useFileContextMenuHandler: () => () => {} }));
 vi.mock("~/browser/useOpenLink", () => ({ useOpenLink: () => async () => {} }));
-// The test drives comparison controls, without mounting virtualized diff viewers or their workers.
-vi.mock("./diffs/AnnotatableCodeView", () => ({ AnnotatableCodeView: () => null }));
+// The tests drive comparison controls and record the viewer's files, without mounting virtualized
+// diff viewers or their workers.
+vi.mock("./diffs/AnnotatableCodeView", () => ({
+  AnnotatableCodeView: ({ files }: { files: (typeof state.viewerFiles)[number] }) => {
+    state.viewerFiles.push(files);
+    return null;
+  },
+}));
 vi.mock("./diffs/DiffFileTree", () => ({ DiffFileTree: () => null }));
 vi.mock("./DiffFilePathCopyButton", () => ({ DiffFilePathCopyButton: () => null }));
 
@@ -87,7 +94,20 @@ import { useDiffPanelSessionStore, useDiffPanelStore } from "~/diffPanelStore";
 const cwd = "/project-worktree";
 const errorMessage = "Could not find a common commit between 'origin/orphan' and HEAD.";
 
-function preview(): ReviewDiffPreviewResult {
+const valuesPatch = [
+  "diff --git a/src/values.ts b/src/values.ts",
+  "index 1111111..2222222 100644",
+  "--- a/src/values.ts",
+  "+++ b/src/values.ts",
+  "@@ -40,3 +40,3 @@",
+  " export const line40 = 40;",
+  "-export const line41 = 41;",
+  "+export const line41 = 410;",
+  " export const line42 = 42;",
+  "",
+].join("\n");
+
+function preview(diff: string): ReviewDiffPreviewResult {
   return {
     cwd,
     generatedAt: DateTime.makeUnsafe("2026-10-30T00:00:00Z"),
@@ -98,15 +118,15 @@ function preview(): ReviewDiffPreviewResult {
         title: "Changes",
         baseRef: "main",
         headRef: "dev",
-        diff: "",
-        diffHash: "empty",
+        diff,
+        diffHash: diff === "" ? "empty" : "values",
         truncated: false,
       },
     ],
   };
 }
 
-function setup(failedBaseRef: string | null) {
+function setup({ failedBaseRef, diff = "" }: { failedBaseRef?: string | null; diff?: string }) {
   const reset = () => {
     useDiffPanelStore.setState({
       byThreadKey: {},
@@ -128,9 +148,18 @@ function setup(failedBaseRef: string | null) {
     worktreePath: cwd,
   });
   state.previews.clear();
-  state.previews.set(null, Atom.make(AsyncResult.success(preview())));
-  state.previews.set("main", Atom.make(AsyncResult.success(preview())));
-  state.previews.set(failedBaseRef, Atom.make(AsyncResult.fail(new Error(errorMessage))));
+  state.previews.set(
+    null,
+    Atom.make(() => AsyncResult.success(preview(diff))),
+  );
+  state.previews.set(
+    "main",
+    Atom.make(() => AsyncResult.success(preview(diff))),
+  );
+  if (failedBaseRef !== undefined) {
+    state.previews.set(failedBaseRef, Atom.make(AsyncResult.fail(new Error(errorMessage))));
+  }
+  state.viewerFiles = [];
   state.config = Atom.make(null);
   state.status = Atom.make(
     AsyncResult.success({
@@ -163,7 +192,7 @@ function setup(failedBaseRef: string | null) {
   state.localRefs = Atom.make(AsyncResult.success(refs(["dev", "main"], false)));
   state.remoteRefs = Atom.make(AsyncResult.success(refs(["origin/orphan"], true)));
   state.commits = Atom.make(AsyncResult.success({ commits: [], truncated: false }));
-  useDiffPanelStore.getState().selectBranchBaseRef(state.threadRef, failedBaseRef);
+  useDiffPanelStore.getState().selectBranchBaseRef(state.threadRef, failedBaseRef ?? null);
   const registry = AtomRegistry.make();
   return {
     registry,
@@ -191,7 +220,7 @@ it.each([
 ])(
   "recovers from $name without leaving the diff panel",
   async ({ failedBaseRef, recovery, baseRef }) => {
-    using context = setup(failedBaseRef);
+    using context = setup({ failedBaseRef });
     await using screen = await render(
       <RegistryContext.Provider value={context.registry}>
         <DiffPanel composerDraftTarget={context.threadRef} workspaceMutationId={null} visible />
@@ -212,3 +241,20 @@ it.each([
     });
   },
 );
+
+it("hands the viewer the same parsed file when it expands after a refetch", async () => {
+  using context = setup({ diff: valuesPatch });
+  await using screen = await render(
+    <RegistryContext.Provider value={context.registry}>
+      <DiffPanel composerDraftTarget={context.threadRef} workspaceMutationId={null} visible />
+    </RegistryContext.Provider>,
+  );
+  await vi.waitFor(() => expect(state.viewerFiles.at(-1)?.[0]?.collapsed).toBe(true));
+  const collapsedFile = state.viewerFiles.at(-1)![0]!.fileDiff;
+  // A refetch returns the same patch in a new result.
+  context.registry.refresh(state.previews.get(null)!);
+  await userEvent.click(screen.getByRole("button", { name: "Expand all files" }));
+  await vi.waitFor(() => expect(state.viewerFiles.at(-1)?.[0]?.collapsed).toBe(false));
+  // Pierre loads unmodified lines into the parsed file it was given; a re-parse would drop them.
+  expect(state.viewerFiles.at(-1)![0]!.fileDiff).toBe(collapsedFile);
+});

apps/web/src/components/DiffPanel.tsx

@@ -50,6 +50,7 @@ import {
   getRenderablePatch,
   resolveDiffThemeName,
   resolveFileDiffPath,
+  type RenderablePatch,
 } from "../lib/diffRendering";
 import { PREFERRED_HIGHLIGHTER } from "../lib/syntaxHighlighting";
 import { areAllDiffFilesCollapsed, toggleAllDiffFiles } from "../lib/diffCollapse";
@@ -129,6 +130,27 @@ function getCachedFileEntry(fileDiff: FileDiffMetadata) {
   return entry;
 }
 
+// Pierre loads expanded lines into the parsed file it was given, so a patch has to keep its parse
+// across re-renders and refetches that return the same text. The four most recent are kept.
+const recentPatchParses: Array<{ key: string; patch: string; parse: RenderablePatch | null }> = [];
+
+function getCachedRenderablePatch(
+  patch: string | undefined,
+  cacheScope: string,
+  options: { compactPartialHunkOffsets: boolean; ignoreWhitespace?: boolean },
+) {
+  if (patch === undefined) return null;
+  const key = `${cacheScope}:${options.compactPartialHunkOffsets}:${options.ignoreWhitespace}`;
+  const index = recentPatchParses.findIndex((entry) => entry.key === key && entry.patch === patch);
+  const entry =
+    index === -1
+      ? { key, patch, parse: getRenderablePatch(patch, cacheScope, options) }
+      : recentPatchParses.splice(index, 1)[0]!;
+  recentPatchParses.unshift(entry);
+  recentPatchParses.splice(4);
+  return entry.parse;
+}
+
 const EMPTY_COLLAPSED_DIFF_FILE_KEYS: ReadonlySet<string> = new Set();
 
 function DiffFileCollapseToggle({
@@ -477,7 +499,7 @@ export default function DiffPanel({
     isGitSelection && selectedGitSource?.truncated && selectedGitSource.files
       ? selectedGitSource
       : null;
-  const renderablePatch = getRenderablePatch(
+  const renderablePatch = getCachedRenderablePatch(
     lazySource ? undefined : selectedPatch,
     `diff-panel:${resolvedTheme}`,
     {