Re: Text Properties And Buffer Modification

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 06 Dec 2018 11:10:23 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: monnier@iro.umontreal.ca, emacs-devel@gnu.org
> 
> I tried to find all occurrences of the string MODIFF in xdisp.c and
> look at the context of every occurrence I found.  I then tried to find
> out whether that context provided any clue that the display engine
> would handle a text property differently from an overlay change.  In
> particular I tried to find out whether there were any redisplay
> optimizations where this was the case.  Probably I'm missing something
> but the "you will see" hinted at something quite obvious.  Maybe you
> can name 2 or 3 such occurrences and I will then try to look harder.

The optimization I had in mind is try_window_id.  You will see there
that this optimization is disabled if overlays were changed, but not
if the buffer (including text properties) was changed.  you will also
see there that buffer-modification tick is used there to update the
unchanged text at beginning and end of the buffer when necessary.

The other place where the buffer modification tick is used is in
mark_window_display_accurate_1.  If we somehow disrupt that, we could
have excess re-entries into redisplay, because Emacs might think some
windows are not yet up to date.

There's also a place in redisplay_internal where we set the
update_mode_line flag of the selected window when that window's buffer
was modified, and a similar use in window_buffer_changed.