Re: MPS: face-cache

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  emacs-devel@gnu.org
> Date: Tue, 30 Apr 2024 08:04:17 +0200
> 
> I think it would suffice if we only trace the windows mactrices + check
> that the glyph pool of the frame is not made a root, because it need not
> be a root.
> 
> Reason is that the window matrices on tty frames are sub-allocated from
> the pool in such a way that all matrices taken together form the frame
> matrix. So, when we trace window matrices, all windows taken together
> trace the frame matrices.
> 
> WDYT Eli?

You are basically correct, but there are two exceptions to the "window
matrices sub-allocate from the frame matrix" paradigm: the menu bar
and the tab bar.  These two don't have any corresponding windows in
the TTY frame case, each one of them is a single glyph row that is
updated directly in the frame glyph matrix.  So we need to do
something special about these two, at least.

As another "interesting" anomaly, the implementation of TTY menus
writes directly into the frame glyph matrix, see
xdisp.c:display_tty_menu_item.  I wonder whether there's something we
need to do about that as well, since the menu text comes from Lisp
strings.