dillon1000/react

[Offscreen] Mount/unmount layout effects (#21386)

* [Offscreen] Mount/unmount layout effects

Exposes the Offscreen component type and implements basic support for
mount/unmounting layout effects when the visibility is toggled.

Mostly it works the same way as hidden Suspense trees, which use the
same internal fiber type. I had to add an extra bailout, though, that
doesn't apply to the Suspense case but does apply to Offscreen
components: a hidden Offscreen tree will eventually render at low
priority, and when we it does, its `subtreeTag` will have effects
scheduled on it. So I added a check to the layout phase where, if the
subtree is hidden, we skip over the subtree entirely. An alternate
design would be to clear the subtree flags in the render phase, but I
prefer doing it this way since it's harder to mess up.

We also need an API to enable the same thing for passive effects. This
is not yet implemented.

* Add test starting from hidden

Co-authored-by: Rick Hanlon <[email protected]>