When regions are closed, like sidebars and toolbars, we show a little
arrow at the edge. This PR does not change that arrow, nor make any
noticable visual change, but almost doubles the hit size area. Much
easier to hit, especially with a tablet pen. It also changes the mouse
cursor when hovering over it to ones that indicate movement in that one
direction, helping to reinforce that you are over the spot. Otherwise
the mouse cursor looks the same as when over the nearby edge.
Pull Request: https://projects.blender.org/blender/blender/pulls/136334
daa417d304