Non Composited Scroller In Position Fixed Scrolled Document

This turquoise box is a position: fixed box. The black-and-white scroller beelow has a clip region so should be non-composited. The document has scrolling of its own.

Scroll the document down (nothing will move since this box is position: fixed). Now attempt to scroll over the black-and-white scroller with wheel or touch. If the box scrolls, the test passes. The document must not scroll.