Remove recoverable error when a sync update flows into a dehydrated boundary

[sebmarkbage]

This just removes the error but the underlying issue is still there, and it's likely that the best course of action is to not update in effects and to wrap most updates in startTransition. However, that's more of a performance concern which is not something we generally do even in recoverable errors since they're less actionable and likely belong in another channel. It is also likely that in many cases this happens so rarely because you have to interact quickly enough that it can often be ignored.

After changes to other parts of the model, this only happens for sync/discrete updates. There are three scenarios that can happen:

• We replace a server rendered fallback with a client rendered fallback. Other than this potentially causing some flickering in the loading state, it's not a big deal.
• We replace the server rendered content with a client side fallback if this suspends on the client. This is in line with what would happen anyway. We will loose state of forms which is not intended semantics. State and animations etc would've been lost anyway if it was client-side so that's not a concern.
• We replace the server rendered content with a client side rendered tree and lose selection/state and form state. While the content looks the same, which is unfortunate.

In most scenarios it's a bad loading state but it's the same scenario as flushing sync client-side. So it's not so bad.

The big change here is that we consider this a bug of React that we should fix. Therefore it's not actionable to users today because it should just get fixed. So we're removing the error early. Although anyone that has fixed these issues already are probably better off for it.

To fix this while still hydrating we need to be able to rewind a sync tree and then replay it.

@tyao1 is going to add a Sync hydration lane. This is will allow us to rewind the tree when we hit this state, and replay it given the previous Context, hydrate and then reapply the update. The reason we didn't do this originally is because it causes sync mode to unwind where as for backwards compatibility we didn't want to cause that breaking semantic - outside Suspense boundaries - and we don't want that semantic longer term. We're only do this as a short term fix.

We should also have a way to leave a partial tree in place. If the sync hydration lane suspends, we should be able to switch to a client side fallback without throwing away the state of the DOM and then hydrate later.

We now know how we want to fix this longer term. We're going to move all Contexts into resumable trees like what Fizz/Flight does. That way we can leave the original Context at the hydration boundaries and then resume from there. That way the rewinding would never happen even in the existence of a sync hydration lane which would only apply locally to the dehydrated tree.

So the steps are 1) remove the error 2) add the sync hydration lane with rewinding 3) Allow hiding server-rendered content while still not hydrated 4) add resumable contexts at these boundaries.

Fixes #25625 and #24959.

[sizebot]

Comparing: d1e35c7...2157c76

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	=	153.65 kB	153.64 kB	=	48.90 kB	48.90 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	155.57 kB	155.56 kB	=	49.51 kB	49.51 kB
facebook-www/ReactDOM-prod.classic.js	=	530.48 kB	530.41 kB	=	94.68 kB	94.67 kB
facebook-www/ReactDOM-prod.modern.js	=	515.74 kB	515.66 kB	=	92.50 kB	92.49 kB
facebook-www/ReactDOMForked-prod.classic.js	=	530.48 kB	530.41 kB	=	94.68 kB	94.67 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-experimental/react-test-renderer/cjs/react-test-renderer.production.min.js	=	99.62 kB	99.39 kB	=	30.67 kB	30.58 kB
oss-stable/react-test-renderer/cjs/react-test-renderer.production.min.js	=	99.56 kB	99.33 kB	=	30.63 kB	30.54 kB
oss-stable-semver/react-test-renderer/cjs/react-test-renderer.production.min.js	=	99.54 kB	99.30 kB	=	30.63 kB	30.54 kB
oss-experimental/react-test-renderer/umd/react-test-renderer.production.min.js	=	99.86 kB	99.62 kB	=	31.10 kB	30.98 kB
oss-stable/react-test-renderer/umd/react-test-renderer.production.min.js	=	99.80 kB	99.56 kB	=	31.07 kB	30.97 kB
oss-stable-semver/react-test-renderer/umd/react-test-renderer.production.min.js	=	99.77 kB	99.54 kB	=	31.07 kB	30.97 kB

Generated by 🚫 dangerJS against 2157c76

[OliverJAsh]

Has this change been released yet? Or is it part of the forthcoming v19?