Following on from bug 42949, there should be a way to write a layout test that can verify that the correct invalidation is generated after scrolling a page containing fixed position elements. The repaint test framework is not a great option since the actual repainting done is fairly platform specific. For example, some platforms will coalesce invalidations into a smaller set of invalidations. Or, if scrolling overlaps with invalidation, some platforms may combine those operations in distinct ways. Instead, it seems like it would be useful to have a way to log HostWindow methods like invalidateContentsAndWindow, invalidateContentsForSlowScroll, and scroll.