Smooth scroll kicks in, despite grabbing and holding scrollbar with mouse or holding arrow keys for scrolling navigation

[Martin-Pitt]

Could resolve this via mousedown/mouseup and cancelling smooth scroll while mouse is held (e.g. user is directly interacting with page, probably very bad time to snap).

Also noticed this happens with keyboard navigation. Not as badly, but if you hold down right arrow key for example on coordinates.html, it will jump once and then start snapping back but then keyboard repeats kick in and you scroll right until as normal until you let go. So might need to check for keydown/keyup and arrow keys and if default functionality has not been suppressed.

[Martin-Pitt]

#7 for some attempt in implementation of cancellable smooth scroll.

Need some global checks on touch/mouse states to go with it.

[Martin-Pitt]

No one is assigned to this by the way, I haven't been actively working on this issue since I already did a PR per #7