diff --git a/src/lib/stores/focusTimer.test.ts b/src/lib/stores/focusTimer.test.ts
new file mode 100644
index 0000000..9a99356
--- /dev/null
+++ b/src/lib/stores/focusTimer.test.ts
@@ -0,0 +1,72 @@
+// Phase B.10 audit regression (2026-05-14). The Race-10 fix in commit
+// 5ba761a was documentation-only — it added a load-bearing comment
+// saying "startTick() is REQUIRED here" on the already-expired branch
+// of `rehydrate()`. Without that startTick call, the tick loop never
+// runs after rehydrating an expired timer, so `now >= completionFlashUntil`
+// never trips and the completion flash sticks until the user
+// interacts. The comment is the only thing standing between a future
+// edit and the regression — vitest wasn't wired in the workspace when
+// 5ba761a landed (vitest scaffold arrived in commit 206ac62 a day
+// later as Phase A.5). Now that vitest is wired, pin the invariant
+// with a test so a future drop of the startTick call surfaces as a
+// red gate rather than a stuck UI flash.
+
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+import { focusTimer } from "./focusTimer.svelte";
+
+const STORAGE_KEY = "lumotia.focusTimer.v1";
+
+beforeEach(() => {
+  // Drain the module-singleton's in-closure state from any prior test.
+  focusTimer.cancel();
+  localStorage.clear();
+  vi.useFakeTimers();
+});
+
+afterEach(() => {
+  vi.useRealTimers();
+  focusTimer.cancel();
+  localStorage.clear();
+});
+
+describe("focusTimer rehydrate of an already-expired timer", () => {
+  it("auto-clears the completion flash after the 3s window via the tick loop", () => {
+    // Seed localStorage with a timer that started a minute ago and was
+    // only 30 s long — it expired 30 s before "now" by wall-clock.
+    const now = Date.now();
+    const startedAt = now - 60_000;
+    const durationMs = 30_000;
+    localStorage.setItem(
+      STORAGE_KEY,
+      JSON.stringify({
+        startedAt,
+        durationMs,
+        taskId: null,
+        label: null,
+      }),
+    );
+
+    focusTimer.rehydrate();
+
+    // Immediately after rehydrate the completion flash must be visible
+    // (fireCompletion was called) and the timer must be reported as
+    // active (startedAt is still set until the flash clears).
+    expect(focusTimer.showingCompletionFlash).toBe(true);
+    expect(focusTimer.active).toBe(true);
+
+    // Advance past the 3-second flash window. If startTick() was dropped
+    // from the already-expired branch of rehydrate(), the tick loop
+    // would never run and the flash would stay visible forever —
+    // because tick() is the only path that observes
+    // `now >= completionFlashUntil` and calls clear().
+    vi.advanceTimersByTime(3_500);
+
+    expect(focusTimer.showingCompletionFlash).toBe(false);
+    expect(focusTimer.active).toBe(false);
+    expect(focusTimer.remainingMs).toBe(0);
+
+    // localStorage must also be wiped — clear() persists null.
+    expect(localStorage.getItem(STORAGE_KEY)).toBeNull();
+  });
+});