Clarify `mousemove` event target if preceding `mouseout`, `mouseleave`, `mouseover` or `mouseenter` event listener removes the `mouseover` target

[masayuki-nakano]

For example if website does:

foo.addEventListener("mouseover", event => event.target.remove());

Then, where mousemove event should be targeted?

Firefox started targeting the closest connected ancestor of the removed target since 135.0.1 or later (not yet fixed the schedule, see bug 1944191. Before that, Firefox dispatched mousemove event on the Document node.

Chrome does same thing if enabling the new experimental behavior of mouse/pointer boundary events. However, Chrome with disabling the feature dispatches mousemove on the removed target.

According to the test result, Safari does not dispatch mousemove event or dispatches on the root element or dispatching to the Document or the Window.

[masayuki-nakano]

I believe that mousemove should be fired on the connected closest ancestor of the removed target. I don't think browsers should do hittest again after dispatching mouse boundary events from both logical and performance points of view.