Re: Emphasizing the top of the frame

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 26 Oct 2016 10:10:32 +0200
> From: martin rudalics <rudalics@gmx.at>
> CC: emacs-devel@gnu.org
> 
>  >> The relevant places to look are xdisp.c, dispnew.c, and dispextern.h.
>  >> IOW, the Emacs display engine.  It currently _knows_ that the
>  >> minibuffer is at bottom and that the mode line is the last screen line
>  >> of any window.
>  >
>  > That was somewhat inaccurate: the fact that the minibuffer window is
>  > at bottom is coded in frame.c:make_frame, where the frame's window
>  > tree is set such that the minibuffer window is on the 'next' pointer
>  > from the frame's root window.  Switching the order shouldn't be hard,
>  > but I'd like Martin's opinion on how many things this could break.
> 
> Does the display engine anywhere rely on that order?

Not that I've seen (by looking at all users of FRAME_MINIBUF_WINDOW),
but I could miss something.  And there is init_xdisp, which you
mentioned, that hardcodes the top coordinate of the minibuffer window.
But see below.

> The frame/window code doesn't and conceptually should even tolerate
> the next window of the root window being nil all the time.

But doesn't the frame's window tree have to start with the root
window?  If it does, then how do we make the minibuffer window be its
"prev"?

> Note, however, that enlarging the minibuffer window currently tries to
> "adjust the trailing edge" of the root window (see that comment in
> ‘window--resize-root-window-vertically’
> 
>        ;; When shrinking the root window, emulate an edge drag in order
>        ;; to not resize other windows if we can avoid it (Bug#12419).
> 
> and the bug mentioned there for why that is important).

And that is at least one implicit dependency on this in the display
engine.  I wonder how many more are there.

IME, eventually there's no other way but to make the changes in the
places you find, fire up Emacs, see what stops working, and debug and
fix each feature that becomes broken.

We probably should have a variable that tells where the minubuffer
window appears, and code which needs to know needs to examine its
value.

Thanks.