bug#71835: 30.0.60; Using mode-line-format-right-align in header line results in cutoff text

[noa]

I would expect the header-line-format and mode-line-format to behave the same 
way when using the mode-line-format-right-align construct.  But instead, the 
behaviour is a little strange and appears to rely on the value of 
mode-line-format:

;; This will result in the right-aligned text being cut off.
(setq-default header-line-format '("foo" mode-line-format-right-align "bar"))

;; This will make both header and mode line align as expected.
(setq-default header-line-format '("foo" mode-line-format-right-align "bar"))
(setq-default mode-line-format '("foo" mode-line-format-right-align "bar"))

;; This will result in the right-aligned text being cut off.
(setq-default header-line-format '("foo" mode-line-format-right-align "bar"))
(setq-default mode-line-format nil)

;; This will result in the mode-line-format-right-align construct being ignored:
(setq-default header-line-format '("foo" mode-line-format-right-align "bar"))
(setq-default mode-line-format "baz")

;; This will result in the mode-line-format-right-align construct being ignored:
(setq-default header-line-format '("foo" mode-line-format-right-align "bar"))
(setq-default mode-line-format 'mode-line-modes)

So if the header line is set to use this construct, it only works correctly if 
the mode line is also using the construct.  The right aligned text in the 
header line will also be indented from the right to be in line with whatever is 
at the right of the mode line.

If the mode line is set to nil or a list which does not contain the construct, 
the right aligned portion of the header line will be cut off.

If the mode line is set to a string or a symbol, the right aligned portion of 
the header line is on the left hand side as if the construct were not there at 
all.


In GNU Emacs 30.0.60 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.16.0) of 2024-06-29 built on baby
Repository revision: df0eb5be1eac7763236a99ffeb6e2b3561955bd9
Repository branch: emacs-30
Windowing system distributor 'The X.Org Foundation', version 11.0.12201009
System Description: Debian GNU/Linux 12 (bookworm)

Configured using:
 'configure --with-x-toolkit=gtk3 --with-imagemagick
 --with-tree-sitter --with-xwidgets --with-native-compilation=aot
 --with-mailutils'

Configured features:
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MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND
SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM
XINPUT2 XPM XWIDGETS GTK3 ZLIB

Important settings:
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Major mode: Lisp Interaction

Minor modes in effect:
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  auto-save-visited-mode: t
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Load-path shadows:
/home/noa/.config/emacs/site-lisp/anzu hides 
/home/noa/.config/emacs/elpa/anzu-20240202.447/anzu
/home/noa/.config/emacs/site-lisp/orderless hides 
/home/noa/.config/emacs/elpa/orderless-20240606.1026/orderless
/home/noa/.config/emacs/site-lisp/dash hides 
/home/noa/.config/emacs/elpa/dash-20240510.1327/dash
/home/noa/.config/emacs/elpa/transient-20240623.1341/transient hides 
/usr/local/share/emacs/30.0.60/lisp/transient

Features:
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Memory information:
((conses 16 385689 306172) (symbols 48 24817 5)
 (strings 32 91428 30898) (string-bytes 1 4096818) (vectors 16 46910)
 (vector-slots 8 1292544 256651) (floats 8 352 224)
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-- 
~noa (https://noa.pub)
 • I try to reply to formal emails in three sentences or fewer; excuse my 
brevity.
 • I queue replies and batch send them at intervals; excuse my untimeliness.