[Bug]: clock.fastForward() and runFor() never return while the page has a zero-delay setInterval() #43947

Version

1.65.0

Steps to reproduce

1. In an empty folder, run npm i -D @playwright/test@1.65.0 and npx playwright install chromium.
2. Save this test as tests/clock-interval.spec.ts:

import { test } from '@playwright/test';

test('fastForward with an interval that has no delay', async ({ page }) => {
  test.setTimeout(10_000);
  await page.clock.install({ time: 0 });
  await page.clock.pauseAt(1000);
  await page.evaluate(() => {
    setInterval(() => {});
  });
  await page.clock.fastForward(1000);
});

3. Run npx playwright test.

Expected behavior

page.clock.fastForward(1000) calls the interval callback and returns, as it does with setInterval(() => {}, 1).

Actual behavior

page.clock.fastForward(1000) does not return. The test fails with Test timeout of 10000ms exceeded.

The result is the same:

• with setInterval(fn, 0),
• with page.clock.runFor(10) instead of fastForward(1000),
• with a running clock (page.clock.install() without pauseAt()).

With a running clock, other timers stop too: a setTimeout(fn, 10) that the page sets together with the interval has not fired after page.waitForTimeout(1000).

With setInterval(fn, 1), all of these work.

I see this on 1.65.0 with Chromium, and on main (58d69dc) with Chromium, Firefox and WebKit.

Additional context

In packages/injected/src/clock.ts, _takeFirstTimer() moves an interval forward by its delay (lines 355-356):

if (timer.type === TimerType.Interval)
  timer.callAt = shiftTicks(timer.callAt, timer.delay);

With a delay of 0, the interval is due again at the same tick, so the loop in _runTo() always finds a due timer and does not end. With a running clock, the real-time timer runs this loop, so later timers do not fire, and fastForward() and runFor() wait for the loop to end when they pause the clock.

Browsers and Node.js give such an interval a minimum delay: in the HTML timer initialization steps, a timeout below 4 becomes 4 when the nesting level is greater than 5, and Node.js sets a delay below 1 to 1.

I found this with Toast.ProgressFill of @kobalte/core, which calls setInterval() without a delay while a toast is shown.

I would like to work on this. I have a one-line fix and a regression test. The fix makes _takeFirstTimer() move an interval forward by at least 1 ms (Math.max(timer.delay, 1)). This is the same 1 ms step that addTimer() already uses for a zero-delay timer that is set during a timer callback. The first call is not delayed, as with setTimeout(fn). The regression test times out without the fix and passes with it. With the fix, tests/library/unit/clock.spec.ts and tests/library/page-clock.spec.ts pass on Chromium, Firefox and WebKit. I will wait for the issue to be assigned before I open a pull request.

Environment

System:
  OS: Windows 11 10.0.26300
  CPU: (12) x64 12th Gen Intel(R) Core(TM) i5-12400F
Binaries:
  Node: 26.12.1
  npm: 12.0.2
npmPackages:
  @playwright/test: 1.65.0 => 1.65.0