scroller: Fix CRI if scroll animation is disabled

scroller: Fix CRI if scroll animation is disabled

If the scroll animation is disabled, we ended up with an
immediate call from inside a post-event callback to modify
the canvas geometry which led to feeding events. Since
99d21f6d9c6e65 and 54e5841b2f638 it is basically forbidden
to modify the canvas or feed events from the post-event cb.
This is because feeding events from inside the post-event
callback can break the logical order of operations between
post-event cb and event cb.

Note: This also implements no-animation scrolling for page
scroll, in case scroll animation is disabled (unifying the
code did that).

Fixes T5289 (abort inside E)