Affected Versions:
Nightly 89.0a1

Tested on:
MacOS 10.15, 11 and M1
Steps to reproduce:

1. Go twitch.com and open any stream
2. Shrink the window size of the browser to create the scrollbars in the recommended channels side-menu (note there is another issue outside apz.overscrolled.enabled, that won't make the list scrollable at all if the window size is not resized to force scrollbars)
3. Make sure the Sign-Up box is at the top of the side-menu (issue not reproducible otherwise)
4. Start to scroll within the recommended channels list and observe the bottom of it

Expected Result:

1. The overscroll effect should start at the bottom of the list, without hindering the view of the last channel
2. (optional) The overscroll-effect should not be applied to scrollable menus/elements inside the page content.

Actual Result:
The last channel from the list is hard to see due to the overscroll effect going over it.

Notes:
Not reproducible with apz.overscroll.enabled False.
Couldn't figure out Safari and Chrome since the Sign-up Box I mentioned is at the bottom of the side menu.

The overscroll effect changes the behavior on twitch in a very odd manner. Upon visiting the site only the area above the video frame has the overscroll effect. Then we have the scrollable recommended channels that also have the overscroll effect.
I would personally prefer to not see the overscroll effect applied to scrollable elements inside the page.

I was able to reproduce, but I see the same behaviour with overscroll disabled.