Some regions, like the 3D Viewport Toolbar and Sidebar, can use the "Region Overlap" setting to overlay the working region of the window.
For most themes those regions have transparent backgrounds and we would get frequent complaints that the "invisible" parts could interfere with the working area. For example the mouse would change to a "resize" cursor for the entire vertical height of the area even if the apparent size of that area was much smaller.
This was recently fixed very well so now all mouse action is ignored when over these unused areas. And that works great!
Except I can predict that somebody will now complain about the new behavior if they are to set a non-zero opacity to overlapped regions. For example the image below illustrates the new possibility of not being able to resize at a position that seems logical.
This patch makes it so that the background of areas with "region overlap" have opacity of zero. In the same way that area backgrounds without region overlap are currently forced to 255. That will eliminate this issue altogether and we can just consider "region overlap" to mean areas with transparent backgrounds overlapping other windows.