Comment 18 for bug 692395

Ok, since this bug is a low priority on Compiz side I made a workaround for DockbarX. The part of the popup where the preview should be made transparent.

Also there's another bug connected to this one. The size of the preview isn't correct. It seems that compiz uses window sizes without decorations instead of window sizes with decorations somewhere in its calculations because if window decorations with thin frames on left and right side is used, the error in height is twice height of the decoration. In other words I use this formula to compensate for it in DockbarX:
preview_height = preview_height - preview_height * 2 * (window_height - client_window_height) / window_height