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

[Aaron Jensen]

>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".

While this seems niche, I use a header-line for most things that Emacs
uses a mode line for by default. The mode line is enabled only
occasionally to display things like the number of matches when
searching. It is usually set to `nil', so I see this artifact all the time.


In GNU Emacs 30.0.50 (build 1, aarch64-apple-darwin22.4.0, NS
 appkit-2299.50 Version 13.3.1 (Build 22E261)) of 2023-04-09 built on
 Aarons-Laptop.local
Windowing system distributor 'Apple', version 10.3.2299
System Description:  macOS 13.3.1

Configured using:
 'configure --disable-dependency-tracking --disable-silent-rules
 --enable-locallisppath=/opt/homebrew/share/emacs/site-lisp
 --infodir=/opt/homebrew/Cellar/emacs-plus@30/30.0.50/share/info/emacs
 --prefix=/opt/homebrew/Cellar/emacs-plus@30/30.0.50 --with-xml2
 --with-gnutls --with-native-compilation --without-compress-install
 --without-dbus --without-imagemagick --with-modules --with-rsvg
 --with-ns --disable-ns-self-contained 'CFLAGS=-Os -w -pipe
 -mmacosx-version-min=13
 -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk
 -DFD_SETSIZE=10000 -DDARWIN_UNLIMITED_SELECT'
 'CPPFLAGS=-I/opt/homebrew/opt/zlib/include
 -I/opt/homebrew/opt/jpeg/include -I/opt/homebrew/opt/icu4c/include
 -I/opt/homebrew/opt/openssl@1.1/include -isystem/opt/homebrew/include
 -F/opt/homebrew/Frameworks
 -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk'
 'LDFLAGS=-L/opt/homebrew/opt/zlib/lib -L/opt/homebrew/opt/jpeg/lib
 -L/opt/homebrew/opt/icu4c/lib -L/opt/homebrew/opt/openssl@1.1/lib
 -L/opt/homebrew/lib -F/opt/homebrew/Frameworks
 -Wl,-headerpad_max_install_names
 -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk''

Configured features:
ACL GIF GLIB GMP GNUTLS JPEG JSON LCMS2 LIBXML2 MODULES NATIVE_COMP
NOTIFY KQUEUE NS PDUMPER PNG RSVG SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP XIM ZLIB