xfwm4 4.20.0 fails to set _NET_DESKTOP_LAYOUT X property, breaking grid workspace navigation

Steps to reproduce:

1. Configure workspace_layout via xfconf: xfconf-query -c xfwm4 -p /general/workspace_layout -s "2x4"
2. Set desktop_layout: xfconf-query -c xfwm4 -p /general/desktop_layout -n -t int -s 0 -t int -s 2 -t int -s 4
3. Try using Ctrl+Alt+Up/Down to navigate workspaces

Expected: Workspace grid navigation works Actual: Up/Down navigation doesn't work, only Left/Right

Root cause: xfwm4 stores the layout in xfconf but fails to write _NET_DESKTOP_LAYOUT to the X root window property. The property remains unset, causing workspaceGetPosition/workspaceGetNumber to fail.

Workaround: Manually set the property with: xprop -root -f _NET_DESKTOP_LAYOUT 32c -set _NET_DESKTOP_LAYOUT 0,4,2,0

Version: xfwm4 4.20.0-2