dillon1000/react

Bugfix: Missing mode check in resetChildLanes (#18961)

Deferring a hidden tree is only supported in Concurrent Mode.

The missing check leads to an infinite loop when an update is scheduled
inside a hidden tree, because the pending work never gets reset.

This "accidentally" worked in the old reconciler because the heurstic
we used to detect offscreen trees was if `childExpirationTime`
was `Never`.

In the new reconciler, we check the tag instead. Which means we also
need to check the mode, like we do in the begin phase.

We should move this check out of the hot path. It shouldn't have been
in the hot path of the old reconciler, either.

Probably by moving `resetChildLanes` into the switch statement
in ReactFiberCompleteWork.