Re: How can I force updating mini window's header line when mode line is updated?

[Eli Zaretskii <eliz@gnu.org>]

> From: Amos Bird <amosbird@gmail.com>
> Date: Wed, 25 Apr 2018 20:41:09 +0800
> 
> Dear community,
> 
> I'm trying to setup one mode line per frame. Here is what I've 
> done.
> 
> 1. turn on the head line of mini window by tweaking 
> window_wants_header_line.
> 2. change shrink_mini_window so that it shrinks mini window to two 
> lines instead of one.
> 3. (setq-default header-line-format mode-line-format)

It's great to hear someone hacks the display code to add features.
However, these issues are best discussed on emacs-devel, not here.
Who knows, we could even ask you to contribute the code ;-)

> However the header line doesn't get updated at all. It seems 
> xdisp.c only provides ways to update mode line, and I've reached 
> the limit of my emacs-fu. How can I achieve this without hacking 
> tons of the display logic? Many thanks!

I think you need to modify window_wants_header_line, it currently has
hard-wired knowledge that a minibuffer window will never display a
header line.

Once that is done, the feature should work, because the same triggers
that update the mode line also update the header line, so this:

> It seems xdisp.c only provides ways to update mode line

is inaccurate.  See the part of redisplay_window immediately following
the 'done' label: it calls display_mode_lines, which displays both the
mode line and the header line.