Background command gets appended to custom Xsession with no option to disable it

[Slabity]

Issue description

Custom sessions added to xservices.xserver.desktopManager.session will have a command added to them that checks for $HOME/.background-image and applies it as the wallpaper using feh.

This breaks configurations that attempt to set the wallpaper in other ways. For example, my custom session looks like the following:

services.xserver.desktopManager.session = [
    {
        name = "custom";
        start = ''                                                                                                                                  
            # Set background                                                                                                                        
            ${pkgs.feh}/bin/feh --bg-scale ${./background.jpg}                                                                                      
 
            # Load custom Xresources                                                                                                                
            ${pkgs.xlibs.xrdb}/bin/xrdb -load ${./Xresources}                                                                                                                                                                                          
        '';
    }
];

However, the actual Xsession case looks like this:

# Start the desktop manager.                                                                                                                                 
case "$desktopManager" in
  (custom)
  # Set background                                                                                                                                           
/nix/store/mpvyhf3jgakzrdi55pyvkmdyv3rwaix7-feh-2.21/bin/feh --bg-scale /nix/store/q7q5zcmwxklg6wfhfmybkmvb0jczimbj-background.jpg

# Load custom Xresources                                                                                                                                     
/nix/store/namx115zb80hvmcybp6cr85ydqpl6l43-xrdb-1.1.0/bin/xrdb -load /nix/store/4nw6izywdgq20g0cqlzgcasmgcg67705-Xresources

if [ -e $HOME/.background-image ]; then
  /nix/store/mpvyhf3jgakzrdi55pyvkmdyv3rwaix7-feh-2.21/bin/feh --bg-scale  $HOME/.background-image
else
  # Use a solid black background as fallback                                                                                                                 
  /nix/store/7g7cyycaiymfs3ff96dczds2hpn0algg-xsetroot-1.1.0/bin/xsetroot -solid black
fi

This breaks any configuration that attempts to set the wallpaper themselves, or even configurations that expect the background to be already set by the display manager.

I don't mind having an option that sets the wallpaper, but I'd prefer it to be optional (and not enabled by default).

Steps to reproduce

1. Add a custom session to desktopManager.session (or some other method of setting the wallpaper).
2. Background is black (unless you have a .background-image file).

Technical details

Affects the unstable branch.

[srhb]

I'm not sure you're supposed to actually set that option directly, but if you do, try settings bgSupport = true.

[rnhmjoj]

I think you are supposed to set xserver.displayManager.sessionCommands instead. At least that's what I'm currently doing to set a background.
Anyway we should add an option to change .background-image to something else.

[stale]

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