Core Animation Layers do allow you to mark a layer or only a portion of a layer as needing an update.

https://developer.apple.com/documentation/quartzcore/calayer

Taking advantage of this would require deeper changes in Gekko than this initial change.

https://bugzilla.mozilla.org/show_bug.cgi?id=1491456

Perhaps WebRender will do a better job of leveraging the possibilities of the native platform APIs for the various lesser used platforms?

In any event, it's a huge amount of work to land a change which makes such a huge difference in battery life, and that work is appreciated.