Example: http://jsbin.com/gapupasahi/

In the above example, the green box has a composited layer with z-index -2, while the blue box has z-index:-1 thus should stack on top of the green box.

In such case, blue box must be composited due to potential overlapping because the green box can be moved by composited scrolling. Our current implementation has an incorrect optimization that ignored the green box if the green box is not clip-escaping (https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/paint/compositing/CompositingRequirementsUpdater.cpp?rcl=c1e683bdef9a76125c2fd4e4e82e100d364f921d&l=374). The blue box can be seen to incorrectly stack below the green box, if they didn't overlap during compositing update but overlapped after scrolling.