R1:c94bd8e4f2f8

Stop using JX.Scrollbar for main page content

Summary:
Ref T8151. This is option (5). It needs a few adjustments but feels pretty good. Major issues are:

- Without a mouse, the scrollbars overlap by default, so we //must// move the column off the right margin.
- Scrolling sometimes "bleeds" between the chat vs the main frame in a way that's not as discrete as the old framed content, but feels generally reasonable to me.

If we pursue this, I'd plan to make these additional changes:

- Move the…