fix(web): namespace the diff file tree by repo in multi-repo turn diffs

apps/web/src/components/DiffPanel.tsx

@@ -63,7 +63,11 @@ import { DiffPanelLoadingState, DiffPanelShell, type DiffPanelMode } from "./Dif
 import { DiffStatLabel } from "./chat/DiffStatLabel";
 import { AnnotatableCodeView, type AnnotatableCodeViewHandle } from "./diffs/AnnotatableCodeView";
 import { DiffFileTree } from "./diffs/DiffFileTree";
-import { diffFileTreeEntries } from "./diffs/diffFileTree.logic";
+import {
+  diffFileTreeEntries,
+  groupedDiffFileTreeEntries,
+  groupedDiffFileTreePath,
+} from "./diffs/diffFileTree.logic";
 import { Button } from "./ui/button";
 import { ToggleGroup, Toggle } from "./ui/toggle-group";
 import { Switch } from "./ui/switch";
@@ -643,7 +647,6 @@ export default function DiffPanel({
   const diffFileKeys = useMemo(() => codeViewFiles.map((file) => file.fileKey), [codeViewFiles]);
   const allDiffFilesCollapsed = areAllDiffFilesCollapsed(diffFileKeys, collapsedDiffFileKeys);
   const diffLineStat = useMemo(() => getDiffLineStat(renderableFiles), [renderableFiles]);
-  const fileTreeEntries = useMemo(() => diffFileTreeEntries(renderableFiles), [renderableFiles]);
   const selectedDiffFileKey = selectedFilePath
     ? (codeViewFiles.find((candidate) => candidate.filePath === selectedFilePath)?.fileKey ?? null)
     : null;
@@ -703,9 +706,34 @@ export default function DiffPanel({
   const visibleDiffTargets = effectiveRepoFilter
     ? diffRepoTargets.filter((entry) => repoRootBaseName(entry.repoRoot) === effectiveRepoFilter)
     : diffRepoTargets;
-  const visibleGroups = effectiveRepoFilter
-    ? renderableGroups.filter((group) => repoRootBaseName(group.repoRoot) === effectiveRepoFilter)
-    : renderableGroups;
+  const visibleGroups = useMemo(
+    () =>
+      effectiveRepoFilter
+        ? renderableGroups.filter(
+            (group) => repoRootBaseName(group.repoRoot) === effectiveRepoFilter,
+          )
+        : renderableGroups,
+    [effectiveRepoFilter, renderableGroups],
+  );
+
+  // The tree mirrors what the diff draws. A grouped view gets one folder per
+  // repo section (named like the section header), so two roots that changed the
+  // same relative path are two rows instead of a duplicate-path crash.
+  const fileTreeGroups = useMemo(
+    () => visibleGroups.map((group) => ({ label: group.displayName, files: group.files })),
+    [visibleGroups],
+  );
+  const fileTreeEntries = useMemo(
+    () =>
+      isGroupedDiffView
+        ? groupedDiffFileTreeEntries(fileTreeGroups)
+        : diffFileTreeEntries(renderableFiles),
+    [fileTreeGroups, isGroupedDiffView, renderableFiles],
+  );
+  const selectedFileTreePath =
+    selectedFilePath && isGroupedDiffView
+      ? groupedDiffFileTreePath(fileTreeGroups, selectedFilePath)
+      : selectedFilePath;
 
   useEffect(() => {
     if (!selectedDiffFileKey || !codeView?.getInstance()) return;
@@ -1463,7 +1491,7 @@ export default function DiffPanel({
                     <DiffFileTree
                       ariaLabel={`${reviewSectionTitle} files`}
                       entries={fileTreeEntries}
-                      selectedPath={selectedFilePath}
+                      selectedPath={selectedFileTreePath}
                       revealRequestId={selectedFileRevealRequestId}
                       onSelectFile={revealDiffFile}
                     />

apps/web/src/components/diffs/diffFileTree.logic.test.ts

@@ -5,6 +5,8 @@ import {
   buildDiffFileTreeUpdates,
   collectDirectoryPaths,
   diffFileTreeEntries,
+  groupedDiffFileTreeEntries,
+  groupedDiffFileTreePath,
 } from "./diffFileTree.logic";
 
 function file(type: FileDiffMetadata["type"], name: string, prevName = name): FileDiffMetadata {
@@ -29,6 +31,45 @@ describe("diffFileTreeEntries", () => {
       { path: "README.md", status: "modified" },
     ]);
   });
+
+  it("keeps the first of two files at the same path so the tree never throws on a duplicate", () => {
+    expect(diffFileTreeEntries([file("change", "README.md"), file("new", "README.md")])).toEqual([
+      { path: "README.md", status: "modified" },
+    ]);
+  });
+});
+
+describe("groupedDiffFileTreeEntries", () => {
+  const groups = [
+    { label: "api", files: [file("change", "README.md"), file("new", "src/a.ts")] },
+    { label: "web", files: [file("change", "README.md")] },
+  ];
+
+  it("files each repo's changes under a folder named for the repo", () => {
+    expect(groupedDiffFileTreeEntries(groups)).toEqual([
+      { path: "api/README.md", status: "modified" },
+      { path: "api/src/a.ts", status: "added" },
+      { path: "web/README.md", status: "modified" },
+    ]);
+  });
+
+  it("drops a repeat when two roots share a folder name", () => {
+    expect(
+      groupedDiffFileTreeEntries([
+        { label: "app", files: [file("change", "README.md")] },
+        { label: "app", files: [file("change", "README.md"), file("new", "b.ts")] },
+      ]),
+    ).toEqual([
+      { path: "app/README.md", status: "modified" },
+      { path: "app/b.ts", status: "added" },
+    ]);
+  });
+
+  it("resolves a repo-relative path to the first group that changed it", () => {
+    expect(groupedDiffFileTreePath(groups, "README.md")).toBe("api/README.md");
+    expect(groupedDiffFileTreePath(groups, "src/a.ts")).toBe("api/src/a.ts");
+    expect(groupedDiffFileTreePath(groups, "missing.ts")).toBeNull();
+  });
 });
 
 describe("collectDirectoryPaths", () => {

apps/web/src/components/diffs/diffFileTree.logic.ts

@@ -23,11 +23,65 @@ function toGitStatus(file: FileDiffMetadata): GitStatus {
   }
 }
 
-/** Maps parsed diff files to tree entries, keeping the diff's own order. */
+/**
+ * Maps parsed diff files to tree entries, keeping the diff's own order. A path that repeats
+ * keeps its first entry: Pierre's path store throws on a duplicate, and a tree missing a row
+ * beats a diff panel that cannot render at all.
+ */
 export function diffFileTreeEntries(
   files: ReadonlyArray<FileDiffMetadata>,
 ): ReadonlyArray<DiffFileTreeEntry> {
-  return files.map((file) => ({ path: resolveFileDiffPath(file), status: toGitStatus(file) }));
+  const entries: DiffFileTreeEntry[] = [];
+  appendDiffFileTreeEntries(entries, new Set(), files, "");
+  return entries;
+}
+
+function appendDiffFileTreeEntries(
+  entries: DiffFileTreeEntry[],
+  seen: Set<string>,
+  files: ReadonlyArray<FileDiffMetadata>,
+  pathPrefix: string,
+): void {
+  for (const file of files) {
+    const path = `${pathPrefix}${resolveFileDiffPath(file)}`;
+    if (seen.has(path)) continue;
+    seen.add(path);
+    entries.push({ path, status: toGitStatus(file) });
+  }
+}
+
+/** A group of changed files under one repo root of a multi-repo diff. */
+export interface DiffFileTreeGroup {
+  /** Folder name the group's files sit under in the tree, matching the diff's section header. */
+  readonly label: string;
+  readonly files: ReadonlyArray<FileDiffMetadata>;
+}
+
+/**
+ * Tree entries for a diff that spans several repo roots. Each root's files sit under a folder
+ * named for that root, so two roots that both changed `README.md` stay two rows, the same way
+ * the diff draws one section per root.
+ */
+export function groupedDiffFileTreeEntries(
+  groups: ReadonlyArray<DiffFileTreeGroup>,
+): ReadonlyArray<DiffFileTreeEntry> {
+  const entries: DiffFileTreeEntry[] = [];
+  const seen = new Set<string>();
+  for (const group of groups) {
+    appendDiffFileTreeEntries(entries, seen, group.files, `${group.label}/`);
+  }
+  return entries;
+}
+
+/** The tree path a repo-relative file takes inside a grouped tree, or null when no group has it. */
+export function groupedDiffFileTreePath(
+  groups: ReadonlyArray<DiffFileTreeGroup>,
+  filePath: string,
+): string | null {
+  const group = groups.find((candidate) =>
+    candidate.files.some((file) => resolveFileDiffPath(file) === filePath),
+  );
+  return group ? `${group.label}/${filePath}` : null;
 }
 
 /**