If you run ./mach run --release -b and click the transparent rects demo, all of the rects congregate in the top left corner. If you run ./mach run --release https://cdn.rawgit.com/servo/servo.org/gh-pages/bhtml-newtab/webrender-demos/transparent_rects.html it works fine.

The constellation creates the new pipeline in handle_script_loaded_url_in_iframe_msg with a null window size, which gets filled in later when the iframe element is reflowed and sends a message to the constellation. If the iframe's content loads before this occurs (such as when using file:// URLs), the script will run before a resize message is received, so Window::GetInnerWidth will return 0 (similar to, but not exactly #15688).