Re: Stipples and scroll optimizations

[Eli Zaretskii <eliz@gnu.org>]

> From: Po Lu <luangruo@yahoo.com>
> Cc: emacs-devel@gnu.org
> Date: Sun, 08 May 2022 15:34:17 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > If using scrolling_window in this case causes incorrect display, then
> > scrolling_window should be disabled.  But how would you know, in
> > update_window or its subroutines, that this is the case?
> 
> We could make x_draw_glyph_string (and its counterparts) set a flag
> inside the glyph row if a stipple is encountered, and then test that
> inside scrolling_window.

I think doing this is x_draw_glyph_string is too late, because that is
called after the test for applicability of scrolling_window was
already done, no?

Is the fact that some of the glyphs in a glyph-row use stipple easily
detectable by scanning the row's glyphs?  If so, update_window could
do that, just before it calls scrolling_window, and refrain from doing
so.

Alternatively, we could add a flag to the glyph_row structure, which
will be set the first time we see a face with stipple while producing
glyphs for the row, and then update_window could test that flag.