At the GTK BOF in Manchester, we discussed the need to have some things to 'show off' our new rendering stack. One suggestion that came up was a blur effect for a translucent overlay over main content, like you see in various OS X apps. API-wise, this could be a child property on GtkOverlay, say 'blur-behind'

We sketched out how this would be implemented using render nodes:

  parent
    overlay
      clip to overlay
        blur
          main content
    clip to ~overlay
       main content

As you can see, this approach requires rendering the main content twice, with inverse clips.