fix(web): PR Code tab no longer requests the diff of a commit the PR dropped

apps/web/src/components/pullRequest/PullRequestCodeTab.browser.test.tsx

@@ -50,7 +50,7 @@ const { diffAtom, pagedDiffAtom } = await vi.hoisted(async () => {
   };
 });
 
-const mocks = vi.hoisted(() => ({ paged: false }));
+const mocks = vi.hoisted(() => ({ paged: false, diffCommits: [] as Array<string | undefined> }));
 
 vi.mock(import("~/state/use-atom-command"), async (original) => ({
   ...(await original()),
@@ -62,8 +62,10 @@ vi.mock(import("~/state/pullRequests"), async (original) => {
     ...actual,
     pullRequestEnvironment: {
       ...actual.pullRequestEnvironment,
-      diff: (({ input }: { input: { cursor?: string } }) =>
-        mocks.paged ? pagedDiffAtom(input.cursor ?? "first") : diffAtom) as never,
+      diff: (({ input }: { input: { cursor?: string; commit?: string } }) => {
+        mocks.diffCommits.push(input.commit);
+        return mocks.paged ? pagedDiffAtom(input.cursor ?? "first") : diffAtom;
+      }) as never,
     },
   };
 });
@@ -125,6 +127,7 @@ const detail: PullRequestDetailView = {
 
 function setup({ paged }: { paged: boolean }) {
   mocks.paged = paged;
+  mocks.diffCommits = [];
 }
 
 const tab = (
@@ -199,3 +202,37 @@ it("pages forward to a file asked for that is not loaded yet, then scrolls to it
 
   await expectFileHeaderShown(screen, container, "late.ts");
 });
+
+it("never requests the diff of a selected commit the pull request no longer has", async () => {
+  setup({ paged: false });
+  const container = document.createElement("div");
+  container.className = "flex h-[700px] w-[900px]";
+  // The timeline can open a commit that a force-push removed from the pull request.
+  await using screen = await render(
+    <PullRequestCodeTab
+      scrollerRef={() => {}}
+      environmentId={EnvironmentId.make("environment")}
+      reference={detail}
+      detail={{
+        ...detail,
+        commits: [
+          {
+            oid: "kept",
+            messageHeadline: "Kept commit",
+            committedDate: "2026-09-01T00:00:00Z",
+          },
+        ],
+      }}
+      selectedCommitOid="gone"
+      onSelectedCommitChange={() => {}}
+      onRefresh={() => {}}
+    />,
+    { container },
+  );
+
+  await expect.element(screen.getByText("1 file", { exact: true })).toBeVisible();
+  await expect
+    .element(screen.getByRole("button", { name: "Diff scope: All commits" }))
+    .toBeVisible();
+  expect(mocks.diffCommits).not.toContain("gone");
+});

apps/web/src/components/pullRequest/PullRequestCodeTab.tsx

@@ -314,7 +314,12 @@ function PullRequestCodeTab({
   }>({ key: "", cursor: null, slices: NO_SLICES });
 
   const referenceKey = pullRequestReviewKey(reference);
-  const commit = selectedCommitOid;
+  // A commit opened from the timeline or left selected across a force-push may no longer be
+  // part of the pull request; read it as "All commits" instead of requesting its diff.
+  const commit =
+    selectedCommitOid !== null && detail.commits.some((entry) => entry.oid === selectedCommitOid)
+      ? selectedCommitOid
+      : null;
   const scopeKey = JSON.stringify([environmentId, referenceKey, commit]);
   // The panel keeps this mounted across pull requests, so an open composer would otherwise
   // survive the switch and attach its comment to whichever one is on screen when it is sent.
@@ -983,7 +988,6 @@ function PullRequestCodeTab({
     </div>
   );
 
-  // Clear obsolete commit scopes even when an empty list hides the dropdown.
   const selectedCommit = orderedCommits.find((entry) => entry.oid === commit);
   const scopeLabel = selectedCommit ? selectedCommit.messageHeadline : "All commits";
   const changeIgnoreWhitespace = (next: boolean) => {
@@ -1297,11 +1301,6 @@ function PullRequestCodeTab({
     answerReveal();
     // oxlint-disable-next-line react/exhaustive-effect-dependencies -- Re-checked as pages load until the requested file arrives.
   }, [canLoadNextSlice, items, revealRequest]);
-  useEffect(() => {
-    if (commit !== null && selectedCommit === undefined) {
-      onSelectedCommitChange(null);
-    }
-  }, [commit, onSelectedCommitChange, selectedCommit]);
   const withToolbar = (body: ReactNode) => (
     <div className="flex h-full min-h-0 flex-col">
       {toolbar}