bug#62606: [PATCH] function to align mode-line elements to right

[hugo@heagren.com]

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I wanted to be able to right-align some elements of the mode-line. I
used `tab-bar-format-align-right' as a base and modified it to work with
the mode-line.

The functin aligns to the right /fringe/ rather than the margin because
some modes (e.g. [visual-fill-column-mode]) mess around the margin size,
but the right-align should be unaffected by this.

Patch attached. I want to know that the code is good before I do NEWS
and docs etc. Obviously it would be useful to document this in info
(elisp) Mode Line Variables and maybe in the `mode-line-format'
docstring.

Blue skies,

Hugo


[visual-fill-column-mode]
<https://codeberg.org/joostkremers/visual-fill-column>

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From 194ead37193a134040ac3f907199cbae2057efbb Mon Sep 17 00:00:00 2001
From: Hugo Heagren <hugo@heagren.com>
Date: Sat, 1 Apr 2023 22:27:25 +0100
Subject: [PATCH] Support right-align in mode-line

* lisp/bindings.el (mode-line-format-right-align):  New function.  If
the symbol `mode-line-format-right-align' appears in
`mode-line-format', then return return a padding string which aligns
everything after that symbol to the right. Padding width is altered
with the display property.
(mode-line-format-right-align):  New variable.  Convenience definition
for including right alignment in `mode-line-format'.
---
 lisp/bindings.el | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/lisp/bindings.el b/lisp/bindings.el
index 34aa8399a96..95a063fbbaf 100644
--- a/lisp/bindings.el
+++ b/lisp/bindings.el
@@ -304,6 +304,35 @@ mode-line-process
 ;;;###autoload
 (put 'mode-line-process 'risky-local-variable t)
 
+(defun mode-line-format-right-align ()
+  "Right-align all following mode-line constructs.
+
+When the symbol `mode-line-format-right-align' appears in
+`mode-line-format', return a string of one space, with a display
+property to make it appear long enough to align anything after
+that symbol to the right of the rendered modeline.
+
+It is important that the symbol `mode-line-format-right-align' be
+included in `mode-line-format' (and not another similar construct
+such as `(:eval (mode-line-format-right-align)'). This is because
+`mode-line-format-right-align' is processed by `format-mode-line'
+as a variable."
+  (let* ((rest (cdr (memq 'mode-line-format-right-align
+			  mode-line-format)))
+	 (rest-str (format-mode-line rest))
+	 (rest-width (string-pixel-width rest-str)))
+    (propertize " " 'display
+		;; The `right' spec doesn't work on TTY frames
+		;; when windows are split horizontally (bug#59620)
+		(if (window-system)
+		    `(space :align-to (- right-fringe (,rest-width)))
+		  `(space :align-to (,(- (window-pixel-width) rest-width)))))))
+
+(defvar mode-line-format-right-align '(:eval (mode-line-format-right-align))
+  "Mode line construct to right align all following constructs.")
+;;;###autoload
+(put 'mode-line-format-right-align 'risky-local-variable t)
+
 (defun bindings--define-key (map key item)
   "Define KEY in keymap MAP according to ITEM from a menu.
 This is like `define-key', but it takes the definition from the
-- 
2.20.1