Desktop background is black with Nemo icons enabled at X11 depth 30; depth 24 fixes it

Reproduce the issue using /etc/X11/xorg.conf.d/10-bit-color.conf and the org.nemo.desktop show-desktop-icons setting, comparing depth 30 with the default depth 24. Start by tracing Nemo's desktop-icon and wallpaper behavior under the affected X11 session. Done means the selected wallpaper and desktop icons are both visible at depth 30 without the workaround.

Environment

• Linux Mint 22.3 Zena, Cinnamon edition
• Cinnamon 6.6.9
• Nemo 6.6.3+zena
• cinnamon-settings-daemon 6.6.4+zena
• X11, X.Org 1.21.1.11
• NVIDIA GeForce RTX 3080 Lite Hash Rate, NVIDIA driver 595.84
• Display: ASUS PG32UCDM, 3840 × 2160

Problem

With X11 configured for depth 30 (10 bits per color channel), the desktop background is solid black while Nemo desktop icons are enabled. Icons remain visible. Disabling Nemo's desktop icons immediately reveals the selected wallpaper.

Removing the depth-30 override and rebooting fixes the problem with desktop icons enabled. This identifies a configuration-dependent reproduction, but does not establish whether the underlying defect is in Nemo, GTK, Muffin, or the graphics driver.

Steps to reproduce on the affected system

1. Configure /etc/X11/xorg.conf.d/10-bit-color.conf as follows:

   Section "Screen"
       Identifier "Screen0"
       DefaultDepth 30
   EndSection
   

2. Restart the computer and log into Cinnamon's X11 session.

3. Select a built-in Mint wallpaper and enable desktop icons:

   gsettings set org.nemo.desktop show-desktop-icons true
   

4. Observe the black background with visible icons.

5. Disable desktop icons:

   gsettings set org.nemo.desktop show-desktop-icons false
   

6. Observe that the wallpaper becomes visible. Re-enabling icons makes it black again.

The X.Org log confirmed the affected session was running at Depth 30 with framebuffer bpp 32.

Expected behavior

The selected wallpaper and desktop icons should both be visible at depth 30.

Troubleshooting results

• Selecting a built-in wallpaper: still black.
• Logging out and back in: still black.
• Changing the application theme to Mint-Y: still black.
• Testing both values of org.nemo.desktop use-desktop-grid: still black with icons enabled.
• Testing without fractional scaling: did not fix it.
• Disabling desktop icons: wallpaper immediately visible.
• Disabling the depth-30 X.Org override and rebooting: wallpaper and icons work together.
• Wallpaper settings had picture-options='zoom' and picture-opacity=100; the selected image file existed and was readable.

Workaround

Rename the depth-30 override so it no longer ends in .conf, then reboot to return to the default color depth. Alternatively, leave desktop icons disabled to show the wallpaper while keeping depth 30.

Related report

https://github.com/linuxmint/muffin/issues/534 describes a different Cinnamon problem with depth 30 (an unclickable panel). It may be related, but this report concerns the black desktop background when Nemo icons are enabled.