Web content usually has two OS compositor slices: A fixed background color, and the scrolled content. The fixed color is usually occluded by the scrolled content. Currently, we allocate memory for it even though it is rarely visible.
The changes planned in bug 1591526 will eliminate any memory from it as long as it stays occluded. However, there are some cases during which it is not 100% occluded:

• when the web page is small
• during checkerboarding
• in the future: during overscroll rubber banding

In those cases where it is (partially) visible, we don't want to use up unnecessary GPU resources to store pixels for it. We should have a special OS surface type that just stores the color and no pixels.

With https://bugzilla.mozilla.org/show_bug.cgi?id=1594305 we no longer allocate memory for occluded tiles.

It's probably still worth doing this, but I think it's a low priority for now - let me know if you come across any cases where it would be a significant memory saving and/or performance win.

Un-assigning for now - not planning to work on this until we are at a point where this is an important memory saving / performance gain on specific pages.