If 'pcmanfm --desktop' is opened in a run dialogue or autostarted for the session, pressing, say, 'down arrow' to activate first desktop item fails.
In Icewm pcmanfm desktop crashes and in Xfce it just fails to grab the focus.
If after starting the session or starting pcmanfm desktop we activate the window manager with some keyboard or mouse action (like changing workspaces etc.) then using the arrow keys work properly to select desktop item.
In IceWM I think I got a BT by running gdb in another VT so I could reproduce the bug (if I had run gdb in the same display then WM would be activated and desktop selection would work):
As a comparison, xfdesktop works properly both in Icewm and Xfce.