If your mouse-wheel repeat is very frequent (>= 60Hz) or your machine is slow enough that you can receive more than one mouse-wheel event before the associated animation callback for smooth scrolling, the page will jump to 0,0.
I have a one-line fix for this, but seeing if I can write a test to go with it.
Created attachment 442361 [details]
Committed r284778 (243487@main): <https://commits.webkit.org/243487@main>
All reviewed patches have been landed. Closing bug and clearing flags on attachment 442361 [details].