Incorrect background used for blur when changing workspaces or viewing the app list

[finnmayhew]

The source used for the blur is not what is actually behind windows, but what is on screen (expect for the windows that are on screen). You can see this by slowly sliding between workspaces - the blurred background appears to move twice as fast as the window that is in front of it. You can also see this when opening the app list - the background behind windows on the current workspace includes the app list despite the fact that the app list is at the bottom of the screen, not behind those windows.