Steps to reproduce: 1. Load the testcase. 2. Move your mouse over the black rectangle. 3. Watch the blue circle be clipped away when it moves downwards. This clipping happens when the ContainerLayer for the opacity draws its contents to an intermediate surface (which is required for group opacity): The intermediate surface is sized to the visible region of the ContainerLayer, and that visible region was calculated on the main thread by FrameLayerBuilder with the pre-animation child layer transforms in mind. APZ has a similar problem. I think we need to add a pass to LayerManagerComposite that throws away the visible region of each ContainerLayer and recomputes it, respecting the current transforms that have been updated by OMTA and APZ.

This is fixed by the patches in bug 1208829.