Consider removing mouseenter/mouseleave polyfill

[gaearon]

As suggested in #10247.
Not sure we want to do it, but I decided to create an issue to track future attempts (the PR is stale).

[gaearon]

Also related: #10269

[jquense]

The main difficulty with removing the polyfill is the portal support. Unlike all other events, it doesn't fall out naturally from the synthetic event system. #10269 has (i think) the minimum basic code to add portal support back in. The question is whether it's better than the current polyfill /shrug

[aldendaniels]

I was pleasantly surprised that React correctly avoids firing onMouseEnter / onMouseLeave events on ancestors when moving the mouse between a portal host and its content - simulating non-portal behavior. Nice!

It looks like React doesn't have this same fix for focus/blur, but I think it ideally would. It's quite useful to be able to treat portals exactly like any other child, which means that a focusOut event caused by moving focus into a portal wouldn't bubble to portal ancestors.

I think the full list of events that need special handling might be all those that have a relatedTarget - e.g these.

Update: I think this latter bit isn't true, since React always uses onFocusIn/onFocusOut under the hood which do bubble to parents. Perhaps onMouseEnter / onMouseLeave are the only case... There are also pointer and drag events to consider.

[harrisonlo]

Just wanted to chime in here to express the need for the native onmouseleave event. React's onMouseLeave currently maps to the native onmouseout event, which behaves differently when hovering through a child element in portal. Something like #10269 seems to solve the problem. I ended up having to access the native onmouseleave via a ref, but that's not ideal.