I'm seeing memory kept around for longer then it needs to by lastNativeEvent:
https://github.com/facebook/react/blob/26ba38ae4b65375b97b561ec2d6a58ef597647ed/packages/react-dom/src/events/EnterLeaveEventPlugin.js#L49

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.