Regression from 7ec7dac860 (I assume). While the block active state is
relevant during drawing to avoid glitches, it should be ignored during
handling. Explained further in comment.
When the tool settings or asset shelf header would draw with some
transparency, the separator line between regions would visibly overlap
the background behind buttons, which looked unpolished. Ensure there is
no overlap (but also no gap, which became visible after accounting for
the separator line in the background drawing).
After changing tools (and possibly other context), the buttons section
background drawing would be broken for one redraw, so the full region
would be drawn with a background, rather than just behind the button
boundaries.
Draw the background of the asset shelf header fully transparent, with an opaque
background with rounded corners behind sections containing buttons. This
reduces the visual space consumed by the asset shelf, and makes the header
follow a tabbed folder metaphor better. Also, this works much better with our
click-through feature, where transparent parts of regions without buttons are
passed through the region under it (we might want to consider unifying code
here a bit).
The edge to drag for region resizing respects the transparent sections.
When there is little space between sections, the sections get merged so that
there are no small gaps in the bar.
Part of #107881.
----
Note that the core of this is implemented in a generic way, so this can be
reused for other regions.
Pull Request: https://projects.blender.org/blender/blender/pulls/112241
