bug#33363: 26.1; display problem with mode-line turned off on terminal

[Eli Zaretskii <eliz@gnu.org>]

> From: Nick Helm <nick@tenpoint.co.nz>
> Date: Tue, 13 Nov 2018 16:40:23 +1300
> 
> Running Emacs in a terminal and setting mode-line-format to nil can 
> cause incorrect display of buffer contents.

Right.  And it's a very old bug.

> emacs -Q -nw
> M-: (setq mode-line-format nil)
> C-x C-f lisp/org/org.el.gz ;for example
> RET RET RET... DEL DEL DEL... ;add and delete white-space
> 
> Note the second-to-last row in the window, which formerly held the
> mode-line, does not display the buffer contents correctly and any buffer
> line that moves through the last row (when back-spacing) appears blank
> further up the window.

The immediate cause of this is that we miscalculate the number of
lines in the window in this case, and then instruct the terminal to
delete the wrong line.  I'm debating how best to fix this; stay tuned.

> I mainly observe this in emacs-lisp-mode and org-mode. The problem 
> doesn't seem to occur when:
> 
> + in fundamental-mode, c-mode or text-mode, or
> + visual-line-mode is active, or
> + the frame contains more than one window (v or h split), or
> + avoiding display of each line (e.g. C-u 10 RET C-u 10 DEL).

The offending code is a redisplay optimization, and is used only under
very specific conditions.  That's why it doesn't rear its ugly head in
many situations.

Thanks.