https://bugs.winehq.org/show_bug.cgi?id=35418
--- Comment #13 from Alexandre Julliard julliard@winehq.org --- It's pretty much possibiliy 1. The drawings accumulate in the buffer and are flushed once the app goes idle. This gives faster performance in most cases by suppressing redundant updates.
The problem with things like dragging the scrollbar is that it never truly goes idle. We have heuristics for catching these cases and forcing updates, but they may need some tweaking.