Following bug 88888, we don't walk the non-self-painting subtree as an optimization. We could go one step further and don't collect them altogether. That should save some memory and remove some of the explicit checks that we have during painting. This also means that the optimization from bug 88888 would apply more broadly - it is an all-or-nothing optimization - as we will only walk the needed layer subtree. One of the points to be careful with is that the compositing code requires to know if there are any non-composited visible descendant. Because non-self-painting layers are normal flow and cannot be composited, this is just a matter of storing when we skip one in our parent.