[cssom-view] It is unclear when offsetX/Y are calculated

https://drafts.csswg.org/cssom-view/#extensions-to-the-mouseevent-interface

The spec says "return the x[y]-coordinate of the position where the event occurred relative to the origin of the padding edge of the target node, ignoring the transforms that apply to the element and its ancestors, and terminate these steps."

blink and webkit cache the value at the first access, Edge seems to cache it when event is created and Gecko calculates it always. My reading is that Gecko does it right, bz' reading is that Edge does it right, but then, blink/webkit model is the fastest.