Created attachment 32377 [details]
Origin outside visibile area from the top.
Using glxgears as benchmark..yes I know it is not really.
In non compositing environment, when glxgears window origin is moved out of current desktop(left and up coordinates), rendering stops (shows last rendered frame, clipped with the window border of-course) frames go high ~4500 as of my hardware RS780(full window visible ~ 1900FPS).
On the other hand, when the glxgears window is under other window, entirely, frames drop down to ~100.
When simultaneously origin outside desktop and covered by window entirely FPS are ~100 too.
In compositing environment, when moving window with GL context, window moves (content black), but GL rendering context stays(and continue to render) on the old place, above any windows. When I finish the move, then it moves in to GL window as usual. When the window origin is moved beyond desktop dimensions, window content stays black.
Now I saw, that on the attachment besides glxgears, on unrefreshed area there is a screen from game Nexuiz, I tested 2 hours ago. Maybe this can help too.