Define behavior for wheel event group when the initial target is mutated

Browsers currently differ on what should happen when the target of a wheel event group is mutated. For example, when the target of a wheel event group is removed:

Blink will retarget once and all following events will be sent to the same element
WebKit will send each following event to the topmost target

The initial work to add wheel event groups to Gecko (bug 1168182) will implement behavior similar to WebKit, but I intend to file a follow-up bug to make Gecko behave like Blink.

w3c / uievents

Define behavior for wheel event group when the initial target is mutated #341