Another possibility (relatively common for d3d version < 10) for Bug 57503 is, if the child window is actually present, already offscreen but is getting correctly clipped, is that maybe it uses d3d rendering in exclusive fullscreen mode which is supposed to draw on top of everything (including even clipped child windows or sibling top level windows above in Z order) and while clipping child windows may be ecen correct from the point of view of Vulkan rendering that started behaving worse with d3d. The same happened when GL child window clipping was introduced (and those regressions are still there). Getting rid of child window clipping without implementing exclusive fullscreen, while helps in some cases, breaks other and in the view of d3d exclusive fullscreen support doesn't fix the problem completely as doesn't help against hindering by siblings top level windows (we have some bugs with such cases too).