e2e on a canvas

apps/game/e2e/playwright.config.ts: Chromium, webServer = vp preview (port 8081) on the build. So: vp -C apps/game build && vp -C apps/game run e2e (the ready script does both). Results and traces: .local/test-results/ (gitignored). Chromium once per machine: vp -C apps/game exec playwright install chromium (CI: --with-deps).

No false green

A canvas has no DOM to query: "the canvas is visible" passes on a black screen. Assert on pixels, and prove the test can fail:

• Compare page.screenshot({ clip }) buffers. game.e2e.ts: the center square differs from a background-only corner (the hero is drawn), then after holding ArrowRight it equals it (the hero left). Wait with expect.poll, never a fixed delay before the first assertion.
• Collect pageerror and console errors for the whole test; assert [] at the end.
• After writing a test, break the game on purpose (e.g. speed 0), rebuild, see it fail, revert.

Traps

Symptom	Cause	Fix
Lint no-conditional-in-test on an if in a page.on handler	any condition inside test()	a module-level helper (collectErrors(page))
require-await on expect.poll(async () => helper())	the arrow only returns a promise	expect.poll(() => helper())
The test sees the old game	e2e runs on dist/, not the dev server	vp -C apps/game build first
Port 8081 is already in use	a vp preview left running (locally reuseExistingServer)	stop it by PID, skill vp ("Dev server under WSL")
"BEWARE: your OS is not officially supported by Playwright"	WSL distro newer than Playwright's list	harmless: the ubuntu24.04 build runs

Just looking at the game

Take a screenshot to see a visual change: a one-off test in e2e/ that calls page.screenshot({ path: "../../.local/screenshots/x.png" }), run it, read the PNG, delete the test.