Bugfix: Revealing a hidden update

[acdlite]

This fixes a bug I discovered related to revealing a hidden Offscreen tree. When this happens, we include in that render all the updates that had previously been deferred — that is, all the updates that would have already committed if the tree weren't hidden. This is necessary to avoid tearing with the surrounding contents. (This was the "flickering" Suspense bug we found a few years ago: #18411.)

The way we do this is by tracking the lanes of the updates that were deferred by a hidden tree. These are the "base" lanes. Then, in order to reveal the hidden tree, we process any update that matches one of those base lanes.

The bug I discovered is that some of these base lanes may include updates that were not present at the time the tree was hidden. We cannot flush those updates earlier that the surrounding contents — that, too, could cause tearing.

The crux of the problem is that we sometimes reuse the same lane for base updates and for non-base updates. So the lane alone isn't sufficient to distinguish between these cases. We must track this in some other way.

The solution I landed upon was to add an extra OffscreenLane bit to any update that is made to a hidden tree. Then later when we reveal the tree, we'll know not to treat them as base updates.

The extra OffscreenLane bit is removed as soon as that lane is committed by the root (markRootFinished) — at that point, it gets "upgraded" to a base update.

The trickiest part of this algorithm is reliably detecting when an update is made to a hidden tree. What makes this challenging is when the update is received during a concurrent event, while a render is already in progress — it's possible the work-in-progress render is about to flip the visibility of the tree that's being updated, leading to a race condition.

To avoid a race condition, we will wait to read the visibility of the tree until the current render has finished. In other words, this makes it an atomic operation. Most of this logic was already implemented in #24663.

Because this bugfix depends on a moderately risky refactor to the update queue (#24663), it only works in the "new" reconciler fork. We will roll it out gradually to www before landing in the main fork.

[sizebot]

Comparing: 7e8a020...5f66498

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	+0.05%	131.83 kB	131.89 kB	+0.06%	42.33 kB	42.36 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	+0.05%	137.10 kB	137.16 kB	+0.07%	43.92 kB	43.95 kB
facebook-www/ReactDOM-prod.classic.js	+0.04%	441.04 kB	441.21 kB	+0.05%	80.69 kB	80.73 kB
facebook-www/ReactDOM-prod.modern.js	+0.04%	426.35 kB	426.52 kB	+0.05%	78.49 kB	78.53 kB
facebook-www/ReactDOMForked-prod.classic.js	+0.31%	440.57 kB	441.92 kB	+0.42%	80.61 kB	80.95 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
facebook-www/ReactDOMForked-prod.modern.js	+0.32%	425.89 kB	427.24 kB	+0.43%	78.39 kB	78.73 kB
facebook-www/ReactDOMForked-dev.modern.js	+0.31%	1,131.90 kB	1,135.38 kB	+0.36%	250.34 kB	251.24 kB
facebook-www/ReactDOMForked-prod.classic.js	+0.31%	440.57 kB	441.92 kB	+0.42%	80.61 kB	80.95 kB
facebook-www/ReactDOMForked-dev.classic.js	+0.30%	1,155.41 kB	1,158.89 kB	+0.36%	254.76 kB	255.69 kB
facebook-www/ReactDOMForked-profiling.modern.js	+0.30%	456.65 kB	458.00 kB	+0.40%	82.90 kB	83.23 kB
facebook-www/ReactDOMForked-profiling.classic.js	+0.29%	471.39 kB	472.74 kB	+0.39%	85.19 kB	85.52 kB

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[rickhanlonii]

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