Re: Redisplay problems?

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan <monnier@iro.umontreal.ca>
> Cc: martin rudalics <rudalics@gmx.at>,  kanru@kanru.info,  christian@defun.dk,  cloos@jhcloos.com,  emacs-devel@gnu.org
> Date: Fri, 21 Mar 2014 09:08:02 -0400
> 
> > If the current glyph matrices are not up to date, marking the frame
> > "garbaged" is the only way to redisplay it.
> 
> No: the next redisplay will update the glyph matrices just fine without
> having to set `garbaged'.

This is true (except that only parts of the matrices might be
recomputed, which might not be what you want), but computing the
desired glyph matrices when the current matrices cannot be used for
comparison is simply useless, because Emacs will be unable to redraw
the modified parts correctly.  For redrawing, Emacs must either (1)
have current matrices it can trust, or (2) assume the current matrices
are useful, and use the desired matrices to redraw everything.  The
latter alternative is what the garbaged flag triggers.

> And the portions of the frame made visible by
> unobscuring/deiconifying will be redrawn in response to the expose
> events that windowing-system will send us.

That is only possible if the current matrices of the frame correctly
reflect what should be on the screen, because expose_frame uses the
current matrices without recomputing them.

> > I guess some more debugging is in order, because I don't understand
> > the problem.
> 
> AFAIK there is no problem (other than people trying to understand
> why/how things work).

If we don't understand how things work, how can we be sure we fixed
the bug?