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Bugfix: Don't rely on
finishedLanes
for passive effects (#21233)
I recently started using `pendingPassiveEffectsLanes` to check if there were any pending passive effects (530027a). `pendingPassiveEffectsLanes` is the value of `root.finishedLanes` at the beginning of the commit phase. When there are pending passive effects, it should always be a non-zero value, because it represents the lanes used to render the effects. But it turns out that `root.finishedLanes` isn't always correct. Sometimes it's `NoLanes` even when there's a new commit. I found this while investigating an internal bug report. The only repro I could get was via a headless e2e test runner; I couldn't get one in an actual browser, or other interactive environment. I used the e2e test to bisect and confirm the fix. But I don't know yet know how to write a regression test for the precise underlying scenario. I can probably reverse engineer one by studying the code; after a quick glance at `performConcurrentWorkOnRoot` and `performSyncWorkOnRoot`, it's not hard to see how this might happen. In the meantime, I'll revert the recent change that exposed the bug. I was surprised that this had never come up before, since the code that assigns `root.finishedLanes` is in an extremely hot path, and it hasn't changed in a while. The reason is that, before 530027a, `root.finishedLanes` was only used by the DevTools profiler, which is probably why we had never noticed any issues. In addition to fixing the inconsistency, we might also consider making `finishedLanes` a profiling-only field.
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