[DevTools] Stop mounting empty roots #34467
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| if (prevFiber === null) { | ||
| throw new Error( | ||
| 'Expected previous fiber when updating an existing root', | ||
| ); | ||
| } |
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This never runs, since wasMounted implies that prevFiber !== null.
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Yeah it's for Flow.js. We have this in a lot of places in the backend. Especially to guard against moving this code further apart in the future.
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Stacked on #34464
DevTools previously always mounted the Root Fiber when it first encountered it. However, it also considered it newly mounted when we went from no children to some children. Duplicate mounts are not supported by the Store.
This was especially noticeable if you error in the shell e.g.
render(<BrokenRender>)and then recover later (or just go fromrender(null)torender(<Something />)). This case is already covered by our runtime tests:react/packages/react-dom/src/__tests__/ReactErrorBoundaries-test.internal.js
Lines 698 to 700 in 436bee1
The alternate to this would be to always consider commits as mounts or updates and bring back
handleCommitFiberUnmount. Since we never fully removedhandleCommitFiberUnmount, that may be save? Though I wouldn't know if that's compatible with all renderers. So not mounting new, empty roots seems simpler considering we already treated updates to empty roots as unmounts.