Need clarification with `MouseEvent::mouseleave`

[yezhizhen]

It would be nice to clarify in current spec what happens when

1. mouse move out of viewport
2. the mouse is static within the viewport, but swap to another window with keyboard shortcut.

For both case 1 & 2, I see

• FireFox not firing mouseleave event.
• Chromium fires mouseleave event

But for mouseout event, the behaviour is consistent as spec is quite clear.

[yezhizhen]

Moved from whatwg/dom#1399
cc @xiaochengh

[masayuki-nakano]

@yezhizhen I saw mouseleave on Firefox at least for the both cases. Do you listen to the event with {capture: true}? mouseenter and mouseleave won't bubble up.

[yezhizhen]

@masayuki-nakano I listen to mouseleave at the document (to reflect leaving viewport). Probably should've clarified it. The following has behaviour differences in the issue description.

document.addEventListener('mouseleave', e => {
        print('Mouseleave the document at ' + new Date().toLocaleTimeString());
    });

[masayuki-nakano]

mouseenter and mouseleave events should be targeted an Element (See "Trusted Targets" in the definitions) and they won't bubble up the tree from the target for the performance. Therefore, if you need to listen the event on the Document or Window, you need to specify {capture: true}. If you see the message in the other browsers, the behavior itself is a bug at least under current spec definition.