Merge main and reshape the follow opt-out to the gestures the timeline was
missing: a middle-button press starts the platform autoscroll pan (the only
scroll gesture on wheel-less mice and tablets with a pointer), Shift+Space
scrolls up from the keyboard, and an upward wheel over a nested scroller that
still has room above stays with that scroller instead of releasing the chat.
The grace re-pin timer and scroll-direction tracking are dropped: returning to
within the end band already re-arms follow, and the mode machine is built
without timers on purpose. Pause/Break never move the viewport and are not
gestures.
Closes#1640
Bohdan Triapitsyn
1d1c215230