Comment 2 for bug 1667690

This bug was totally expected...

I don't think we have the luxury of GL clipping planes in ES. We could however modify the renderer to limit the glViewport to each window's region (which annoyingly also changes the coordinates ands transformations). Some work would be required there, possibly in our scene architecture to expose window vs renderable information, and also would need separate re-implementation in every compositor/renderer. So that's both Mir and QtMir.