Related: tdf#155266 force flush after drawing native scrollbars

Related: tdf#155266 force flush after drawing native scrollbars

When scrolling on some Intel Macs either via dragging
the scrollbar thumb or via swiping the trackpad with two
fingers, final repaint of scrollbars doesn't appear to
get flushed to the screen. It appears that scrollbars
aren't updated and repainted until after a batch of
native scroll events have been dispatched. On slower
machines, this lag is long enough that any pending
forced flushes have already been done so when the timer
that repaints scrollbars finally fires, the repainted
scrollbars won't get flushed to the native window until
the next normal flush which may not occur until seconds
later.

Change-Id: Iadef6812cd2495a28347560caae81f604c908b51
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/160440
Tested-by: Jenkins
Reviewed-by: Patrick Luby <plubius@libreoffice.org>