[PointerEvent algorithms] Order of boundary events

[mustaqahmed]

When the mouse pointer moves from element A to element B, how does a browser determine the sequence of boundary events (enter, leave, over, out) to fire?

Problem

PointerEvents boundary event (enter, leave, over, out) dispatch is not defined, and the corresponding dispatch mechanism in UIEvents Section 3.4.4. Mouse Event Order is example-based without a precise definition text.

Possible solution

Fix w3c/uievents#383, then override that in PointerEvents into three different methods (one per pointer type) each with a corresponding state for "element under current primary pointer".

[mustaqahmed]

FYI: @garykac @flackr @smaug----

[smaug----]

https://w3c.github.io/uievents/#handle-native-mouse-move-id has some of this defined in an algorithmic way for mouse events.