As shown in the complaint, when a panel region with category tabs is
aligned to the left we are not insetting the position of the scroll
bars like we do if aligned to the right. There are two separate
fixes needed. In ED_region_panels_draw we are just not doing this for
regions on the left. And in UI_view2d_mask_from_win when sizing a
left-algined scrollbar we just use scrollbar width as xmax, when it
should be xmin + width. This means that the scroll bar will not be seen
if the mask xmin is greater than the scrollbar width.
Pull Request: https://projects.blender.org/blender/blender/pulls/141470
f0634f6dfc