To clarify, one of the other concerns about draw occlusion is that without it, performance can suffer a lot. For example, today (pre-MUS) a full-screen window doesn't need to draw the background (possibly expensive if it's a large image) every time something changes in the page. This is doubly important for e.g. full-screen videos.

I seem to remember having lots of performance issues before occlusion culling. I suspect we need it to launch.

So what kind of work would need to be done in SurfaceAggregator? If we skip occluded DrawQuads, then we can skip waiting on sync points, but we will still upload textures to the GPU? Will skipping DrawQuads and sync points give us the wins we want/need?

A possible third component of this bug is to implement some occlusion detection code in the window server. The window server does not use LayerTreeHost/LayerTreeHostImpl. Instead it generates a CompositorFrame of SurfaceDrawQuads directly. Thus, we lose out on all occlusion culling code in cc today in the window server. The window server knows when a SurfaceDrawQuad is entirely occluded and can mark the window as not visible. 

Alternatively this could be the responsibility of the display compositor and CompositorFrameSinks. We should probably stop BeginFrames to windows that are not visible on screen?

This probably also fits in with evicting CompositorFrames: https://bugs.chromium.org/p/chromium/issues/detail?id=646242

Some design work is required here as well. 

Skipping DrawQuads would solve opaque overdraw performance issues (the ones I mentioned in #1), but wouldn't do anything to memory, or anything upstream of the draw (uploads, etc.), which could be handled at a higher level (IsVisible).

BTW, I'm not sure there's a lot of value in skipping the sync token waits (it's an ordering primitive, not a blocking wait), so unless it's trivial I would probably punt. Not waiting will not skip the uploads or other commands necessary to produce the resources for example (it's just that they may happen out-of-order with the draw commands).

I just had a chat with Antoine about copy requests and draw occlusion. We cannot skip DrawQuads if there is a copy request on a surface.