Consider scroll responsiveness

[bokand]

This proposal seems to be mainly concerned with limiting resource size; have you considered adding a mode/policy to force all scrolling on the compositor thread?

For example: today, a positioned scroller with a clip will prevent compositing so all scrolls will be only as responsive and smooth as the main thread allows. We could have a flag/FeaturePolicy that forces compositing in all cases to ensure responsiveness at the cost of correctness.

We had this experience with a local team that was struggling with performance issues on a low-end device. Figuring out why scrolling is slow required knowing the browser internals and it was very easy for regressions to slip in. It'd be much easier for developers if they could guarantee fast scrolling and then have to notice that "hey, background-attachment: fixed isn't working".