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Summary: `advanceTimersByTime` fired every timer due in the window first and ran their callbacks afterwards, in one work loop at the end. That is not what an idle event loop does over that much time: per the [HTML timer initialization steps](https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#timer-initialisation-steps), each timer's task runs when it becomes due, a repeating timer is re-armed only after its callback has run, and timers scheduled by a callback can come due before later ones. Tests could not observe a callback running at its due time, and a timer implementation that re-arms after the callback (as the spec says) could not be tested at all, because its next fire would only be scheduled after the whole advance. Here, the mock fires one timer at a time and runs the work loop after each one, for both `advanceTimersByTime` and `runAllTimers`. `NativeFantom.advanceTimers` / `runAllTimers` become `advanceTimersToNextDue` (fires the earliest timer due within the window and returns the time left) and `runNextTimer`; the loop and its 100,000-fire safety bound move to `TimerMock.js`. Changelog: [Internal] Differential Revision: D124119880
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Summary:
advanceTimersByTimefired every timer due in the window first and ran their callbacks afterwards, in one work loop at the end. That is not what an idle event loop does over that much time: per the HTML timer initialization steps, each timer's task runs when it becomes due, a repeating timer is re-armed only after its callback has run, and timers scheduled by a callback can come due before later ones. Tests could not observe a callback running at its due time, and a timer implementation that re-arms after the callback (as the spec says) could not be tested at all, because its next fire would only be scheduled after the whole advance.Here, the mock fires one timer at a time and runs the work loop after each one, for both
advanceTimersByTimeandrunAllTimers.NativeFantom.advanceTimers/runAllTimersbecomeadvanceTimersToNextDue(fires the earliest timer due within the window and returns the time left) andrunNextTimer; the loop and its 100,000-fire safety bound move toTimerMock.js.Changelog: [Internal]
Differential Revision: D124119880