lastNativeEvent retains unmounted elements & fibers

[bgirard]

I'm seeing memory kept around for longer then it needs to by lastNativeEvent:

react/packages/react-dom/src/events/EnterLeaveEventPlugin.js

Line 49 in 26ba38a

let lastNativeEvent;

In my case it's holding on to detached DOM nodes/Fiber.

I think we should be able to use WeakSet since we're doing a === check. Otherwise we should consider timing it out in some way.

[bgirard]

Related #17006

[bgirard]

Fixed by #17585