The old fixed-rect method was very error-prone in corner-cases,
resulting in half-visible (cut off) parameters, incorrectly
sized controls, etc. on various devices, which often made it
impossible to use.
While there are still a few rough edges (scrollbar behavior could
be further improved, though it's already much better than it was before),
this method is much better than the previous one.
Fixes#11592 and related tickets.
Change-Id: I65175f760bda98e42d1fc68ba8e526470bf17c25
Reviewed-on: https://review.haiku-os.org/c/889
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
