Killed a 15% flaky test by swapping real sleeps for fake timers and clearing shared state

This test is flaky because it asserts on wall-clock timing and shares state through a module-level singleton. Failure rate ~15%, worse on a loaded CI runner. **Flakiness category:** async/timing + shared mutable state. **Root cause:** the test does `await sleep(100)` then expects the debounced callback to have fired — on a slow runner the timer hasn't elapsed yet, so the assertion races the event loop. Separately, a previous test leaves a stale entry in the shared `eventBus`, so order matters. ```typescript // BEFORE — racy: real timers + sleep guesswork it("debounces the search", async () => { input.type("hello"); await sleep(100); // flaky: may be too short under load expect(onSearch).toHaveBeenCalledOnce(); }); ``` **Fixed — fake timers make time deterministic; reset state per test:** ```typescript import { vi, beforeEach, it, expect } from "vitest"; beforeEach(() => { vi.useFakeTimers(); eventBus.clear(); // kill cross-test pollution }); it("debounces the search", () => { input.type("hello"); vi.advanceTimersByTime(300); // advance virtual clock past the debounce expect(onSearch).toHaveBeenCalledOnce(); vi.useRealTimers(); }); ``` **Why it's stable now:** `advanceTimersByTime` moves the clock synchronously, so there is no race with the runner's speed; the `eventBus.clear()` removes the ordering dependency. **Detection:** run `vitest --run --repeat 100 search.test.ts` — pre-fix it fails intermittently, post-fix it is green 100/100. **Policy:** ban `sleep()` in tests; require fake timers for anything time-dependent, and isolate shared singletons in `beforeEach`.