On startup, workspaces are initialized according to the num-workspaces
preference. However when using dynamic workspaces, the actual number
of workspaces in use might be greater than the preference (when
replacing the window manager), forcing windows on those workspaces
to the first workspace.
To fix, ignore the preference completely when using dynamic workspaces
and try to restore the previous number of workspaces (as read from
_NET_NUMBER_OF_DESKTOPS).

https://bugzilla.gnome.org/show_bug.cgi?id=685439