1336516 - Use ActiveScrolledRoot for non-APZ, too

Status: NEW

(Core :: Web Painting, defect, P3)

Description

[Markus Stange [:mstange]]

At the moment (after bug 1298218), only active scroll frames with a display port create new ActiveScrolledRoot objects. It would be nice to create ActiveScrolledRoots for active scroll frames *without* display ports as well. Then the display item clip chain will become an actual clip chain in non-APZ mode, too, and we can make smarter layerization decisions based on it.
We just need to make sure that we're not painting more than what's in the viewport. We could ensure that using a viewport-sized "displayport" clip.

The code we have in FrameLayerBuilder which detects scrolled clips on fixed background images and tries to make smarter layerization decisions for them for the non-APZ case is a bit brittle and currently backfires in the APZ case. Being able to detect scrolled clips based on the clip chain and the ASR of an item would make this code less brittle.