In bug 1669625 picture caching was invalidated basically constantly during scrolling a simple page if the user had changed the app unit per dev pixel ratio via full zoom. And it was a regression that wasn't noticed until a developer happened to see it with the picture caching debug pref enabled.

We should have an automated test that some basic things work with picture caching so they are caught immediately.

This'll probably be a dupe of bug 1670809 depending on what comes out of it.