Mouse and action zone interaction for scrollbars depend on `v2d->vert`
and `v2d->hor`, which are updated through `view2d_masks`. However,
scrollbar drawing through `UI_view2d_scrollers_draw` calls
`view2d_scrollers_calc`, which pads these sizes further, meaning that
drawn scrollbars are slightly out of sync with their hotspots. This is
noticeable at track edges for shrinking scrollers or when tracks are
opaque. Fix by moving the extra (noticeable) padding code from
`view2d_scrollers_calc` to `view2d_masks`.
Pull Request: https://projects.blender.org/blender/blender/pulls/135021
John Kiril Swenson
d8f3f8cc61