I can confirm this as well. When xfce4-clipman is running, mousepad won't close after copying text. I have to quit, wait a few seconds, and quit again for the window to close. I get (mousepad:13648): Gtk-CRITICAL **: gtk_main_quit: assertion `main_loops != NUL in the console when I quit for the second time (which works). Moving mousepadrc doesn't correct the problem.