bug#50660: 28.0.50; Text artifacting when the cursor moves over text under mouse face that originally displayed a box

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: 50660@debbugs.gnu.org
> Date: Sat, 16 Oct 2021 20:12:55 +0800
> 
> The problem occurs when (adjusted_mouse_face)->font has different
> metrics from the font originally used to display each glyph.
> 
> (For instance, if mouse face is `highlight' but the original face was
> `custom-group-tag', which is variable pitch and has a much larger font
> size than `highlight')

That's a separate problem, which exists in the current code as well,
doesn't it?

The only solution to that is to merge the mouse-face with the original
face, ignoring the attributes of mouse-face that affect the text size,
as documented in the ELisp manual.