dillon1000/react

Never attach ping listeners in legacy Suspense (#22407)

I noticed a weird branch where we attach a ping listener even in legacy
mode. It's weird because this shouldn't be necessary. Fallbacks always
synchronously commit in legacy mode, so pings never happen. (A "ping" is
when a suspended promise resolves before the fallback has committed.)

It took me a moment to remember why this case exists, but it's related
to React.lazy.

There's a special case where we suspend while reconciling the children
of a Suspense boundary's inner Offscreen wrapper fiber. This happens
when a React.lazy component is a direct child of a Suspense boundary.

Suspense boundaries are implemented as multiple fibers, but they are a
single conceptual unit. The legacy mode behavior where we pretend the
suspended fiber committed as `null` won't work, because in this case the
"suspended" fiber is the inner Offscreen wrapper.

Because the contents of the boundary haven't started rendering yet (i.e.
nothing in the tree has partially rendered) we can switch to the
regular, concurrent mode behavior: mark the boundary with ShouldCapture
and enter the unwind phase.

However, even though we're switching to the concurrent mode behavior, we
don't need to attach a ping listener. So I refactored the logic so that
it doesn't escape back into the regular path.

It's not really a big deal that we attach an unncessary ping listener,
since this case is so unusual. The motivation is not performance related
— it's to make the logic clearer, because I'm about to add another case
where we trigger a Suspense boundary without attaching a ping listener.