bug#73129: 31.0.50; buffer-text-pixel-size vs. newlines

[David Ponce via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hello,

Its seems that the function `buffer-text-pixel-size' is not working
as described when there are newlines in the buffer.  Here is a quick
illustration (on my laptop (frame-char-width) returns 12 pixels):

;; As expected.
(with-temp-buffer
   (insert "abcdef")
   (car (buffer-text-pixel-size nil nil t)))
72

;; I suppose it is as expected because newline is not displayed.
(with-temp-buffer
   (insert "\n")
   (car (buffer-text-pixel-size nil nil t)))
0

;; ?
(with-temp-buffer
   (insert "abcd\nef")
   (car (buffer-text-pixel-size nil nil t)))
48

The doc string of `buffer-text-pixel-size' mentions:

"Return size of whole text of BUFFER-OR-NAME in WINDOW."

So ,I expect that the last example will return 72px (6 x 12px) + 0px
for the newline.  But the result is 48px, which means that all the
text after the first newline is not counted.

This also impacts "string-pixel-width" which is supposed to return
the pixel size of the passed string, not part of it:

(string-pixel-width "abcdef") => 72
(string-pixel-width "\n") => 0
(string-pixel-width "abcd\nef") => 48
(string-pixel-width "abcd\nef\ngh") => 48

At least this limitation should be mentioned in the doc string?

A possible fix for `string-pixel-width' could be to remove all
the newlines from the passed string before to call
`buffer-text-pixel-size'?  Something like this:

diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el
index 4cdb065feeb..c68b3b37f9b 100644
--- a/lisp/emacs-lisp/subr-x.el
+++ b/lisp/emacs-lisp/subr-x.el
@@ -406,6 +406,11 @@ string-pixel-width
                          face-remapping-alist))
          (kill-local-variable 'face-remapping-alist))
        (insert string)
+      ;; Remove newlines.
+      (forward-line 0)
+      (while (not (bobp))
+        (delete-char -1)
+        (forward-line 0))
        ;; Prefer `remove-text-properties' to `propertize' to avoid
        ;; creating a new string on each call.
        (remove-text-properties


Thanks!

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
  3.24.43, cairo version 1.18.0)
Repository revision: 2ce0d397b12cafcb3e1ac5630bc3fbca61bd6b87
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 40 (KDE Plasma)

Configured using:
  'configure --with-x-toolkit=gtk3 --with-cairo-xcb
  --with-native-compilation=no
  PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/usr/lib/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

Important settings:
   value of $LC_TIME: fr_FR.utf8
   value of $LANG: fr_FR.UTF-8
   locale-coding-system: utf-8-unix