Re: master 5b5608c614: Fix glyph skipping optimization when a tab line is enabled

[Eli Zaretskii <eliz@gnu.org>]

> From: Po Lu <luangruo@yahoo.com>
> Cc: emacs-devel@gnu.org
> Date: Mon, 18 Apr 2022 15:53:47 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > OK, and what about the last question I asked above?  If there's no
> > header-line, but there is a tab-line, the code will still compare vpos
> > with 1, although the vpos of the tab-line is now zero, right?
> 
> I see how the code might be made clearer if the assumption that a mode
> line cannot be at vpos 1 isn't made, but can a mode line (the other kind
> of mode_line_p row there is) really be at vpos 1?  Aren't windows
> required to have a body height of at least 1 line?

Maybe not, but why should such subtle assumptions be implicitly
embedded in the code, when you have specific flags to do the job?  A
glyph matrix has both header_line_p and tab_line_p flags, so comparing
to their sum should do the job cleanly, I think.  Or am I missing
something?