fix(server): Changes totals no longer diff against a guessed base after a branch rename

apps/server/src/vcs/GitVcsDriverCore.test.ts

@@ -2879,6 +2879,70 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
       }),
     );
 
+    it.effect("does not guess a Changes base for a branch renamed during the status read", () =>
+      Effect.gen(function* () {
+        const cwd = yield* makeTmpDir();
+        const fork = yield* makeTmpDir();
+        const upstream = yield* makeTmpDir();
+        yield* initRepoWithCommit(cwd);
+        yield* git(cwd, ["branch", "-M", "main"]);
+        yield* git(fork, ["init", "--bare"]);
+        yield* git(upstream, ["init", "--bare"]);
+        yield* git(cwd, ["remote", "add", "origin", fork]);
+        yield* git(cwd, ["remote", "add", "upstream", upstream]);
+        yield* git(cwd, ["push", "origin", "main"]);
+        yield* writeTextFile(cwd, "upstream.txt", "work missing from the fork\n");
+        yield* git(cwd, ["add", "upstream.txt"]);
+        yield* git(cwd, ["commit", "-m", "upstream work"]);
+        yield* git(cwd, ["push", "upstream", "main"]);
+        yield* git(cwd, ["fetch", "origin"]);
+        yield* git(cwd, ["fetch", "upstream"]);
+        yield* git(cwd, ["checkout", "-b", "t3/temporary"]);
+        yield* git(cwd, ["config", "branch.t3/temporary.t3-merge-base", "main"]);
+        yield* git(cwd, [
+          "config",
+          "branch.t3/temporary.t3-merge-base-ref",
+          "refs/remotes/upstream/main",
+        ]);
+        yield* writeTextFile(cwd, "feature.txt", "thread work\n");
+
+        const outer = yield* GitVcsDriver.GitVcsDriver;
+        const delegate = yield* ChildProcessSpawner.ChildProcessSpawner;
+        const renameClaimed = yield* Ref.make(false);
+        const renamed = yield* Deferred.make<void>();
+        // Rename after `git status` has read HEAD, as the thread launcher does after the first turn.
+        // Base config reads start only after status completes; hold every one until the rename lands.
+        const spawner = ChildProcessSpawner.make((command) =>
+          Effect.gen(function* () {
+            if (ChildProcess.isStandardCommand(command) && command.args[0] === "config") {
+              if (!(yield* Ref.getAndSet(renameClaimed, true))) {
+                yield* git(cwd, ["branch", "-m", "t3/temporary", "feature/named"]).pipe(
+                  Effect.provideService(GitVcsDriver.GitVcsDriver, outer),
+                  Effect.orDie,
+                );
+                yield* Deferred.succeed(renamed, undefined);
+              }
+              yield* Deferred.await(renamed);
+            }
+            return yield* delegate.spawn(command);
+          }),
+        );
+        const driver = yield* makeGitVcsDriverCore().pipe(
+          Effect.provideService(ChildProcessSpawner.ChildProcessSpawner, spawner),
+          Effect.provide(layerServerConfig),
+        );
+
+        const status = yield* driver.statusDetailsLocal(cwd, { includeBranchChanges: true });
+
+        assert.strictEqual(status.branch, "t3/temporary");
+        assert.deepStrictEqual(status.branchChanges, {
+          baseRef: null,
+          insertions: 1,
+          deletions: 0,
+        });
+      }),
+    );
+
     it.effect("keeps local branch bases consistent when tags share their names", () =>
       Effect.gen(function* () {
         const cwd = yield* makeTmpDir();

apps/server/src/vcs/GitVcsDriverCore.ts

@@ -3144,12 +3144,17 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
       readonly untrackedRoots: ReadonlyArray<string>;
     },
   ) {
-    const { baseRef, mergeBase } = yield* resolveReviewMergeBase(
+    const resolved = yield* resolveReviewMergeBase(
       cwd,
       branch,
       undefined,
       options?.knownOriginDefaultBranch ?? null,
     );
+    // A rename after `git status` read HEAD moves the base config away, so the base was guessed.
+    const { baseRef, mergeBase } =
+      branch === null || (yield* branchExists(cwd, branch))
+        ? resolved
+        : { baseRef: null, mergeBase: "HEAD" };
     const roots = options?.untrackedRoots ?? [];
     // Bound pathspec matching work and stay below Windows' command-line limit.
     const pathspecs =