This appears to be because the existing scroll is on the table body, and the new scroll is on table. If you remove the scroll from the table body, the problem will likely be resolved.

The reason why the scrollbar was on tbody (and the root cause of this bug) is that we want to be able to scroll the body of the table vertically without the header scrolling out of view, i.e. a "sticky header". As far as I know, that can't be achieved without a JS hack like the one employed here, where the header is scrolled programmatically as an effect of the user scrolling the body.

So I think the strategy applied here might not be the right way to achieve a solution. But it would be interesting to see if having vertical scrolling on body and horizontal scrolling on the entire table would be an acceptable solution. I'm not sure how the browsers would handle that, but it's worth a try! 😊

Closing this as stale. This issue does not exist in Gen3.