DumpRenderTree's calls -setAutoDisplay:NO on its window, unless there are compositing layers. Historically I think this was done to allow tests to control when painting happens, and to improve performance, but it would be better if DRT behaved more like WebKitTestRunner and had normal, rendering-update-drive repaint behavior. -setAutoDisplay:NO even makes the window never appear on the screen (with --show-webview) so it hinders debugging, and makes it harder to adapt DRT to allow repaint testing via ref tests.