`mouse-pos.hover` may or may not be false during touch input
(contrary to mouse input, where false means the cursor left the window)
Detecting a change to false as a leave event, while allowing any
other mouse-pos update to cause an enter event solves this.
I don't like this solution, but anything cleaner would probably require converting chapter indicators into individual elements and than managing/updating their positions and sizes in relation to growing timeline, which seems like a lot more work and code, so this'll have to suffice.
closes#370
