When I test Bug 1684520, I can reproduce the choppy scrolling. So, I set apz.force_disable_desktop_zooming_scrollbars to true, then the scrolling is smooth again as expected.
However, Checkerboarding issue appears when scrolling a long page with keyboard.

Steps to reproduce:

1. set apz.force_disable_desktop_zooming_scrollbars to true and restart browser
2. Open long page (e.g. https://en.wikipedia.org/wiki/Fitts%27s_law )
3. Scroll with arrow key-repeat

Actual results:
Checkerboarding issue appears

Expected results:
No issue.

Regression window:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=5dc0cfd11b2215c75bdd10a395855586ca9b5199&tochange=f0a8a5a5f78baa5e246082b71c2c2f56759e0518

profiler: https://share.firefox.dev/390ztMq

Alice, does it happen for you without webrender?

There is an instance of jank that is 601548 seconds long in the content process in that profile, it seems like it's a mistake as the content process is processing events.

During the checkerboarding in the profile all of the transactions are empty transactions in the gpu process.

(In reply to Timothy Nikkel (:tnikkel) from comment #3)

Alice, does it happen for you without webrender?

Yes, I can reproduce the issue.

• New profile(i.e. WebRender) : No checkerboarding, but choppy scrolling
• WebRender + apz.force_disable_desktop_zooming_scrollbars=true : checkerboarding, but scrolling smooth
• gfx.webrender.force-disabled=true(i.e. Direct3D 11) : No checkerboarding, but choppy scrolling
• gfx.webrender.force-disabled=true(i.e. Direct3D 11) + apz.force_disable_desktop_zooming_scrollbars=true : checkerboarding, but scrolling smooth

There is another known regression from bug 1669861 which I am planning to look into: bug 1682919. It may end up being the same issue.

(Did not mean to assign myself just yet.)

This also appears to be fixed by the patch in bug 1682919.