Bug: Adding StrictMode component leads to cases that can't happen in concurrent mode. about react

[rickhanlonii]

Is it really possible that useEffect with empty array can be called twice with reserving the component state in concurrent mode? (Without the offscreen API)?

Yes, this happens today with Fast Refresh. And it also happens today with layout effects when you re-suspend a tree on update. In that case we move the existing content "offscreen" (unmounting the layout effects), and when the content is ready to commit again re-mount it using the same state and re-creating the layout effects. This is important for performance of re-committing suspended trees (so we don't need to destroy and re-create all of the state like DOM elements). Since the content may have changed, it means we need to re-run layout side-effects, like setting the hight of a tooltip, so it's not out of date.

Even outside of these cases, if you don't clean up the resources like websockets or subscriptions during unmount, then you'll leak resources or connections, with Strict Mode helps you find (as described in the docs for this behavior here).

[Szymon20000]

@rickhanlonii Thanks for taking the time and responding!

[zhangshichun]

LGTM