On each startup, I get the following message in .xsession-errors:
"(xfce4-panel:2183): libwindowmenu-CRITICAL **: windowmenu.c:622 (window_menu_plugin_active_window_changed): expression 'WNCK_IS_SCREEN (screen)' failed."

Looks dangerous (however, the window menu works).

(4.8 release and 4.8 today's git)