We should add a test that simulates user input and has a scroll-driven main thread animation, and that verifies that composited frames have the two in sync.

Also one that has a compositor animation rather than user input. Same thing - needs to verify that composites keep the compositor animation in sync with main-thread animations.

cc Hiro, this seems relevant to bug 1571758, maybe with your test-writing skills we can accomplish this as part of that work :)

I uploaded a mochitest for scroll linked effect. Initially I wrote it for bug 1571758, but it can't be now landed along with other patches for bug 1571758 since the test sometimes fails with the patches because the patches are not going to fix scroll linked effect perfectly.

The mochitest landed in bug 1754129. The mochitest tries to make sure a scroll linked effect scroll position matches async scroll position. So I would be a test of this bug's intention.