bug#38181: Actual height of mode-line not taken into account

[Eli Zaretskii]

> Cc: 38181@debbugs.gnu.org
> From: martin rudalics <rudalics@gmx.at>
> Date: Sat, 16 Nov 2019 09:20:27 +0100
> 
>  > I think fit-window-to-buffer relies on window's metrics (like the
>  > number of lines in the text area) to be up to date, and that is only
>  > true after a window was redisplayed once since changing the mode-line
>  > height.  Martin, is this correct?
> 
> Yes.

Then I'm not sure we can fix such use cases at all, without causing
display flickering in much more popular use cases.  Do you have any
ideas for a possible fix?  Could fit-window-to-buffer invoke
'redisplay' internally, perhaps?

> But, as I mentioned earlier, the problem shows up with the
> scroll bar immediately when Jonas' form is evaluated, see the attached
> screenshot.

That just means we (or a Lisp program which makes these mode-line
modifications) need to recreate and redisplay the scroll bars anew in
these cases, right?  It's a separate problem, right?