Fixed bug: unmovable window in stacking (non-tiling) window manager.

Tested in the following WM/DE:
* awesome wm
* e16
* e17
* fluxbox
* gnome 3
* i3 wm
* xfce4

--HG--
extra : amend_source : 37c4f132637027f544383d2e05dace15d3ce09a9