bug#75291: Redisplay not updating fringe when face filter changes

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 75291@debbugs.gnu.org, mina86@mina86.com
> Date: Fri, 03 Jan 2025 22:10:45 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > From: Daniel Colascione <dancol@dancol.org>
> > Cc: 75291@debbugs.gnu.org,  mina86@mina86.com
> > Date: Fri, 03 Jan 2025 14:46:20 -0500
> > 
> > Eli Zaretskii <eliz@gnu.org> writes:
> > 
> > >> From: Daniel Colascione <dancol@dancol.org>
> > >> Cc: 75291@debbugs.gnu.org,  mina86@mina86.com
> > >> Date: Fri, 03 Jan 2025 12:25:05 -0500
> > >> 
> > >> > I think it depends on whether you use double-buffering (some people
> > >> > don't or cannot) and whether you have the mouse pointer over an Emacs
> > >> > frame.  Also, depending on the GUI toolkit, the decorations might
> > >> > flicker.
> > >> 
> > >> TTY windows don't have fringes, and the most commonly-used window
> > >> systems all do atomic updates nowadays.
> > >
> > > People still report flickering from time to time, so I don't think
> > > this never happens.
> > >
> > >> > So you want to add to display_line code that sets each glyph_row's
> > >> > redraw_fringe_bitmaps_p flag when the fringe face changes?  That could
> > >> > probably work, provided that we disable redisplay optimizations which
> > >> > might avoid calling display_line (you will see that we already disable
> > >> > such optimizations when overlay_arrows_changed_p returns non-zero).
> > >> > We might actually need to disable more of the optimizations, because
> > >> > the overlay-arrow thing doesn't contradict the optimizations that
> > >> > scroll the pixels, something that reaction to changes in the fringe
> > >> > face cannot tolerate.
> > >> 
> > >> That might work, but I don't think we even need anything that
> > >> complicated or low-level.  Not many are using :filtered now, and those
> > >> that do big redraws anyway.  How about this simpler code that gets us
> > >> correctness, albeit more conservatively?
> > >
> > > Doesn't that only support face remapping with :filtered attribute?
> > > What about the more general case where the fringe face is remapped in
> > > a way that's independent of the windows?
> > 
> > That seems to work already. It's only in the fringe that I see problems
> > --- it just doesn't seem worth it to limit the redraw to the fringe.
> 
> Sorry, I don't understand.  I _was_ talking about the fringe face.
> 
> But if redraw_frame solves the issue, and doesn't cause unpleasant or
> expensive redraws, feel free to install on the master branch.  But
> please change this:

Btw: this is _really_ a sledgehammer solution, and it will affect
similar changes in any face, not just the fringe face.  I wonder how
long will it take for complaints to come in.

How about leaving behind a variable exposed to Lisp that could be used
to disable this sledgehammer?  Then we at least could tell people who
don't want this how to disable it.