Is it reproducible with all themes?
On _GTK_FRAME_EXTENTS? No. Probably last work I am doing on compiz is to update gtk-window-decorator for metacity changes. If everyting will go as I am planing then gtk-window-decorator will have following changes:- it will work with metacity 3.20+ only- it will be able to use cairo theme (builtin), metacity theme (metacity-theme-x.xml) and GTK+ theme for window decorations- It might got support for dark theme variant.