bug#64596: 30.0.50; On FIXME: in src/buffer.c:1481 (force-mode-line-update)

[Eli Zaretskii <eliz@gnu.org>]

> From: Ihor Radchenko <yantar92@posteo.net>
> Cc: rudalics@gmx.at, monnier@iro.umontreal.ca, 64596@debbugs.gnu.org
> Date: Sun, 16 Jul 2023 09:41:28 +0000
> 
> The top commentary in xdisp.c defines "display elements" as elements in
> glyph matrix. These include characters (possibly composed) and images.

They include more than that, see enum glyph_type in dispextern.h.

> However, not everything drawn in Emacs frame is represented by display
> elements :
> 
> 1. Title bar is not using glyphs at all
> 2. Same for scroll bars

These two are window-system/toolkit widgets.

> 3. and menu bars

This is either a toolkit widget or an Emacs-produced text; in the
latter case it does use our glyphs.

> 4. and tool bars

Depending on whether the tool bar is "external" or not, this could be
a special kind of window with our glyphs.

> 5. and window boundaries

On TTY frames, these are character glyphs.

> 6. and things like fill-column-indicator (AFAIU)

Those are fringe bitmaps, so they belong to the fringe.

> Further, some display elements are grouped:
> 1. per frame
> 2. per window
> 3. within fringes
> 4. within mode-line
> 5. within buffer text area
> 6. within a line of text
> 7. within header line
> 8. within tab-line

The latter 5 are in display-engine realm, and some of them are only
calculated as part of redisplay, so other code should not try to
handle that.  But yes, we definitely need (and already have) buffer
flags, window flags, and frame flags.

> I think that the flags should provide a way to mark each of the groups
> or individual glyphs for future redisplay.

Nothing knows (or should know) about glyphs except the display code.