STR: 1. Load the attachment (derived from markus' test case in bug 1425565) 2. Scroll all the way down and click to ensure a main-thread repaint happens 3. Scroll to the top (without moving mouse or doing anything that might trigger a repaint) Expected: Green square in top-left, blue bar in the middle Actual: Green square in the top-left gets clipped away. Once a repaint happens it comes back. Since the main-thread repaint fixes the visual issue, presumably this is a bug in how AsyncCompositionManager is translating clips for async scrolling. But who knows, maybe the layer tree or display list is wrong.