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Implement Partial Hydration for Activity (facebook#32863)
Stacked on facebook#32862 and facebook#32842. This means that Activity boundaries now act as boundaries which can have their effects mounted independently. Just like Suspense boundaries, we hydrate the outer content first and then start hydrating the content in an Offscreen lane. Flowing props or interacting with the content increases the priority just like Suspense boundaries. This skips emitting even the comments for `<Activity mode="hidden">` so we don't hydrate those. Instead those are deferred to a later client render. The implementation are just forked copies of the SuspenseComponent branches and then carefully going through each line and tweaking it. The main interesting bit is that, unlike Suspense, Activity boundaries don't have fallbacks so all those branches where you might commit a suspended tree disappears. Instead, if something suspends while hydration, we can just leave the dehydrated content in place. However, if something does suspend during client rendering then it should bubble up to the parent. Therefore, we have to be careful to only pushSuspenseHandler when hydrating. That's really the main difference. This just uses the existing basic Activity tests but I've started work on port all of the applicable Suspense tests in SelectiveHydration-test and PartialHydration-test to Activity versions.
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‎packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js‎

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@@ -3746,11 +3746,7 @@ describe('ReactDOMServerPartialHydration', () => {
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`);
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// Passive effects.
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await waitForPaint([]);
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}
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// Subsequently, the hidden child is prerendered on the client
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// along with hydrating the Suspense boundary outside the Activity.
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await waitForPaint(['HiddenChild']);
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