bug#63186: 30.0.50; Modeline is not fully repainted when being set to nil then back to having content

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Sun, 30 Apr 2023 05:54:50 -0400
> 
> >From emacs -Q:
> 
> In the scratch buffer, eval:
> 
> (setq mode-line-format "foo")
> (setq mode-line-format nil)
> (setq mode-line-format "boo")
> 
> After the last line, only the "b" of the mode line will be rendered.
> Triggering a repaint with M-x or resizing the window causes the entire
> mode line to repaint.
> 
> If the buffer has enough content in it such that there is text where the
> mode line is when it is set to `nil', most of that text will remain
> painted in place when the mode line is set to "boo".

Thanks, should be fixed now on the emacs-29 branch.

(This is a regression from Emacs 28, but I have no idea which change
caused it.)