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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 20 Sep 2021 08:19:16 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: larsi@gnus.org, 50660@debbugs.gnu.org
> 
> I still don't think I understand completely the problem you are
> talking about.  Is the problematic recipe as below?
> 
>   . move the mouse pointer above text with box-face, so it is highlighted
>   . move the text cursor into the highlighted text

Actually, the only recipe in which I can see problems is this:

  . move the text cursor into text that has a face with the box attribute
  . move the mouse pointer into and out of that text

In this scenario, the characters of the text with box-face move
horizontally to account for the box border, as the mouse highlight is
turned on and off, but the character under cursor does NOT move, and
with large enough font this leaves small artifacts (because the
character is not erased correctly).

Is that the problem you are trying to solve, i.e. the problematic
display is limited to the character under cursor when text is
mouse-highlighted and unhighlighted?

Or maybe one needs specially-defined faces to see the problem?  If so,
please show the definitions of those faces, preferably as part of a
complete recipe starting from "emacs -Q".