Dear Maintainer,

GNOME Settings does not reliably apply mirror/duplicate mode for my
internal laptop panel and external monitor, while Mutter's D-Bus API can
apply the same configuration successfully.

System:
- Debian GNU/Linux 13.4 (trixie)
- Package: gnome-control-center 1:48.4-1~deb13u1
- GNOME Shell: 48.7-0+deb13u2
- Mutter common files: 48.7-0+deb13u1
- Session: GNOME on Wayland
- Kernel: 6.12.86+deb13-amd64
- Mutter experimental-features:
  ['scale-monitor-framebuffer', 'xwayland-native-scaling']

Displays:
- Internal display: eDP-1, CSO 0x1413, native/preferred
  1920x1200@60.001, 16:10
- External display: DP-3, Philips PHL 271V8, preferred
  1920x1080@60.000, 16:9

Steps to reproduce:
1. Log in to a GNOME Wayland session.
2. Connect the external Philips 1920x1080 monitor.
3. Open GNOME Settings > Displays.
4. Try to enable mirror/duplicate mode between the internal panel and
   the external monitor.
5. Apply the configuration.

Expected result:
GNOME Settings should apply mirror mode using a common mode supported by
both outputs, for example 1920x1080.

Actual result:
GNOME Settings does not successfully apply the mirror configuration for
me. No useful error is shown in the UI.

Important detail:
Applying the same mirror configuration directly through Mutter's
DisplayConfig D-Bus API succeeds. The working configuration is one
logical monitor containing both outputs:

- eDP-1: 1920x1080@59.934
- DP-3: 1920x1080@60.000
- scale: 1.0
- primary: yes

The command used was:

gdbus call --session \
  --dest org.gnome.Mutter.DisplayConfig \
  --object-path /org/gnome/Mutter/DisplayConfig \
  --method org.gnome.Mutter.DisplayConfig.ApplyMonitorsConfig \
  9 2 \
  "[(0, 0, 1.0, uint32 0, true, [('eDP-1', '1920x1080@59.934', {}), ('DP-3', '1920x1080@60.000', {})])]" \
  "{'layout-mode': <uint32 1>}"

After this, GetCurrentState reports a single logical monitor with both
eDP-1 and DP-3 attached, confirming that mirroring is active:

[(0, 0, 1.0, uint32 0, true,
  [('eDP-1', 'CSO', '0x1413', '0x00000000'),
   ('DP-3', 'PHL', 'PHL 271V8', '0x00007580')],
  {})]

Also, ~/.config/monitors.xml is regenerated correctly and contains the
mirrored configuration. The file is writable by my user and is not
immutable:

- permissions: -rw-rw-r--
- owner/group: jul:jul
- lsattr: --------------e-------

This suggests that Mutter can apply the monitor configuration, but the
GNOME Settings display panel fails or mishandles this case, possibly
because the two displays have different native aspect ratios and
fractional scaling support is enabled.

No relevant mutter, gnome-control-center, displayconfig, monitors.xml, or
dconf error was found in the recent user journal while checking this.