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

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

> So I think all that's needed is to adjust the value of
> w->phys_cursor.x, when needed, in draw_phys_cursor_glyph, before
> passing it to draw_glyphs, or perhaps in its callers.  The value of
> w->phys_cursor.x should stay unaltered, but what we pass to
> draw_glyphs should be offset if needed.

Thanks, I'll work on that.

> The glyph row doesn't store any dimensions, it only stores the
> pixel_width of each glyph and the starting X-coordinate of the row.
> So where the mouse face is drawn, we don't need any updating.

> The cursor position of a window, stored in w->phys_cursor, is
> calculated when the cursor is moved.  It is never recalculated
> thereafter, until point is moved again.  In particular, displaying
> mouse-sensitive text in mouse-face just reuses the glyphs already
> produced, it doesn't recalculate them.  But since we always redraw the
> entire sequence of glyphs in the mouse face or in the box face, the
> glyphs gets moved horizontally because we see the first glyph with the
> box and handle that accordingly.  But when we then redraw the cursor,
> we reuse the information in w->phys_cursor, which is slightly off when
> the box attributes of the mouse face is different from that of the
> "regular" face.  This is what causes the cursor glyph be drawn in the
> wrong location.

> The correct place to fix this is therefore somewhere under
> note_mouse_highlight, which is where we handle redrawing of the
> mouse-sensitive face, including the cursor.

Thanks, I've learned a lot.  I hope I haven't been inconveniencing you
in any way.