At present, wr::DisplayListBuilder::PushRoundedRect kludges drawing a rounded rectangle by drawing a border so thick that it overlaps itself at the center. WebRender should instead offer a proper rounded rectangle primitive.

At present, PushRoundedRect is not used for background-color with border-radius. It is only used for drawing list-style: disc; markers, and WebRender's caching usually ensures that drawing those borders is just a blit, so the main problem with this implementation is the ugliness and obscurity of the code. (There's also an odd bit of image-loading feedback UI, but this not heavily used.)

But rounded rectangles are popular. For example, bug 1694059 needed some, and introduced a new instance of this 'very thick border' kludge.