If the user used the mouse to change window focus (moving into or clicking on a
window), this should be handled differently than focus changes due to keyboard
commands. Specifically, it's inappropriate to discard window enter/leave events
while the mouse is moving. This fixes the bug where a fast mouse motion across
multiple windows resulted in the wrong window keeping focus.
It's also helpful information for contrib modules such as UpdatePointer - it's
supposed to move the mouse pointer only in response to keyboard actions, not if
the user was moving the mouse.