Reproduction steps:
1. emacs -Q
2. M-x display-line-numbers-mode RET
3. M-x ruler-mode RET
4. C-x C-+ C-+ C-+ C-+

You should notice that the ruler is misaligned.  I've hotfixed this in
my running Emacs by applying:

modified   lisp/ruler-mode.el
@@ -636,7 +636,7 @@ ruler-mode-ruler
                 ;; FIXME: ruler-mode relies on I being an integer, so
                 ;; the column numbers might be slightly off if the
                 ;; line-number face is customized.
-                (round (line-number-display-width 'columns))
+                (+ (round (line-number-display-width)) 2)
               0))
          (j (ruler-mode-text-scaled-window-hscroll))
          ;; Setup the scrollbar, fringes, and margins areas.

... and re-evaling the ruler-mode-ruler defun.

Per the line-number-display-width doc, when it is called with 'columns,
it returns the number of columns in the frames canonical font size, but
that's not the font size that is used for the fringes: the fringes seem
to use the font the buffer is using, at least after text-scale-mode does
its thing.

(+ ... 2) was not chosen at random, padding is specified as 2 as by the
docstring of line-number-display-width.

Interestingly, changing text-scale-remap-header-line appears to have no
effect, but that variable ought to be accounted for somehow also, I
think?  Though, if the line-numbers and header line always use the same
face, then the above should be correct, I think?

Anyway, thanks in advance, have a lovely day and happy holidays!

(PS: I can confirm this happens on PGTK too also)

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.18.2, Xaw3d scroll bars) of 2024-12-28 built on localhost
Repository revision: 72eb5f8bedbe441f6a3952557b9aa3ef6fd6235d
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12401004
System Description: Gentoo GNU/Linux

Configured using:
 'configure --prefix=/usr --build=x86_64-pc-linux-gnu
 --host=x86_64-pc-linux-gnu --mandir=/usr/share/man
 --infodir=/usr/share/info --datadir=/usr/share --sysconfdir=/etc
 --localstatedir=/var/lib --datarootdir=/usr/share
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 --enable-locallisppath=/etc/emacs:/usr/share/emacs/site-lisp
 --without-compress-install --without-hesiod --without-pop
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 --enable-xattr --with-dbus --with-modules --with-gameuser=:gamestat
 --with-libgmp --with-gpm --with-native-compilation=aot
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 --without-imagemagick --with-dumping=pdumper 'CFLAGS=-freport-bug -O2
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 -Werror=strict-aliasing -fuse-ld=mold -Wl,--undefined-version''

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM HARFBUZZ JPEG LCMS2
LIBOTF LIBSYSTEMD LIBXML2 M17N_FLT MODULES NATIVE_COMP NOTIFY INOTIFY
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TREE_SITTER WEBP X11 XAW3D XDBE XIM XINPUT2 XPM LUCID ZLIB

Important settings:
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  value of $LANG: en_US.utf8
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Major mode: ELisp/l

Minor modes in effect:
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Load-path shadows:
/usr/share/emacs/site-lisp/emacs-eat/eat hides /usr/share/emacs/site-lisp/emacs-eat/term/eat
/usr/share/emacs/site-lisp/which-key/which-key hides /usr/share/emacs/31.0.50/lisp/which-key
/usr/share/emacs/site-lisp/transient/transient hides /usr/share/emacs/31.0.50/lisp/transient
/usr/share/emacs/site-lisp/compat/compat hides /usr/share/emacs/31.0.50/lisp/emacs-lisp/compat

Features:
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-- 
Arsen Arsenović