Sets a gdk.Window as composited, or unsets it. Composited windows do not automatically have their contents drawn to the screen. Drawing is redirected to an offscreen buffer and an expose event is emitted on the parent of the composited window. It is the responsibility of the parent’s expose handler to manually merge the off-screen content onto the screen in whatever way it sees fit.
It only makes sense for child windows to be composited; see Window.setOpacity if you need translucent toplevel windows.
An additional effect of this call is that the area of this window is no longer clipped from regions marked for invalidation on its parent. Draws done on the parent window are also no longer clipped by the child.
This call is only supported on some systems (currently, only X11 with new enough Xcomposite and Xdamage extensions). You must call Display.supportsComposite to check if setting a window as composited is supported before attempting to do so.