dillon1000/react

Regression test: SuspenseList causes lost unmount (#20433)

@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 #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 #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.