[DevTools] Fix handling of host roots on mount #34400
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eps1lon
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Sep 5, 2025
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| throw new Error( | ||
| 'There should always be an Offscreen Fiber child in a hydrated Suspense boundary.', | ||
| ); |
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We were in the host root branch here. We're forking earlier now and only enter this branch for <Suspense>
eps1lon
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Sep 5, 2025
| // This Suspense Fiber is still dehydrated. It won't have any children | ||
| // until hydration. | ||
| } | ||
| const isTimedOut = fiber.memoizedState !== null; |
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This was always true for host roots since their state is non-nullable. We now unconditionally measure host roots.
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rickhanlonii
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Fixes a bug that was encountered at Meta when empty roots were mounted. We accidentally attributed empty roots to hydrated Suspense Components and expected an Offscreen Fiber.
While fixing this I noticed that we're also accidentally considering host roots as timed out (their state is always non-nullable) and therefore didn't measure them on mount. Not measuring the root means their children may not be aligned properly depending on the implementation for painting the rects.
I included rects of the root in the snapshot serializer to make the issue more obvious. The PR is split into previous and fixed behavior to make the change more obvious.