Masks for scrollbars were properly shrunk to match their drawn
representation in #135021, however, click logic was not updated
alongside this change.
For one, `mouse_in_scroller_handle` incorrectly assumed that if the
slider filled up the entire scrollbar extents, we would only be able to
select the bar. This code never ran in practice since `scrollers` were
always smaller than their mask, but now that they're the right size,
it's being run. Fix by removing the case.
It is also guaranteed that the "scroller"/"bubble"/"thumb" is not out of
view of the scrollbar track itself because it is clamped at the end of
`view2d_scrollers_calc` (which always runs before
`mouse_in_scroller_handle`), so this is dead code that can be removed.
Finally, rename `mouse_in_scroller_handle` to `scrollbar_zone_get` to
better represent its function.
Pull Request: https://projects.blender.org/blender/blender/pulls/137437
7999f720c1