Bugfix: Offscreen instance is null during setState #24734

During a setState, we traverse up the return path and check if any parent Offscreen components are currently hidden. To do that, we must access the Offscreen fiber's `stateNode` field. On a mounted Offscreen fiber, the `stateNode` is never null, so usually we don't need to refine the type. When a fiber is unmounted, though, we null out its `stateNode` field to prevent memory cycles. However, we also null out its `return` field, so I had assumed that an unmounted Offscreen fiber would never be reachable. What I didn't consider is that it's possible to call `setState` on a fiber that never finished mounting. Because it never mounted, it was never deleted. Because it was never deleted, its `return` field was never disconnected. This pattern is always accompanied by a warning but we still need to account for it. There may also be other patterns where an unmounted Offscreen instance is reachable, too. The discovery also suggests it may be better for memory usage if we don't attach the `return` pointer until the commit phase, though in order to do that we'd need some other way to track the return pointer during initial render, like on the stack. The fix is to add a null check before reading the instance during setState.

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Bugfix: Offscreen instance is null during setState #24734

Bugfix: Offscreen instance is null during setState #24734

Commits on Jun 16, 2022