Commits on Dec 11, 2020

1. Regression test: SuspenseList causes lost unmount …

   @sebmarkbage reminded me that the complete phase of SuspenseList
   will sometimes enter the begin phase of the children without calling
   `createWorkInProgress` again, instead calling `resetWorkInProgress`.
   
   This was raised in the context of considering whether facebook#20398 might
   have accidentally caused a SuspenseList bug. (I did look at this code
   at the time, but considering how complicated SuspenseList is it's not
   hard to imagine that I made a mistake.)
   
   Anyway, I think that PR is fine; however, reviewing it again did lead me
   to find a different bug. This new bug is actually a variant of the bug
   fixed by facebook#20398.
   
   `resetWorkInProgress` clears a fiber's static flags. That's wrong, since
   static flags -- like PassiveStatic -- are meant to last the lifetime of
   the component.
   
   In more practical terms, what happens is that if a direct child of
   SuspenseList contains a `useEffect`, then SuspenseList will cause the
   child to "forget" that it contains passive effects. When the child
   is deleted, its unmount effects are not fired :O
   
   This is the second of this type of bug I've found, which indicates to me
   that it's too easy to accidentally clear static flags.
   
   Maybe we should only update the `flags` field using helper functions,
   like we do with `lanes`.
   
   Or perhaps we add an internal warning somewhere that detects when a
   fiber has different static flags than its alternate.

   

   acdlite committed Dec 11, 2020
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