Affected versions

• 88.0a1(2021-03-18), 87.0;

Affected platforms

• Win10, macOS 10.15;

Steps to reproduce

• Facebook account with multiple friends requiered;

1. Launch Firefox, have a Facebook page created(takes 1-2 minutes);
2. Access the created Facebook page, click on the [ ... ] button;
3. Click on the Invite Friends option;
4. On either the Not Invited or Invited section scroll up/down to see the friends listed;

Expected result

• scroll-bar thumb indicator is properly displayed;

Actual result

• scroll-bar thumb is truncated after scrolling a couple of times and the bar is resized;

Regression range

• First bad: 2018-09-26
• Last good: 2018-09-25
• Pushlog: URL

Additional notes

• attached recording to best illustrate the issue; on some occasions only part of the scroll-thumb gets truncated(unable to trigger it at the time of recording/reporting).

It's hard to tell from the gif, but it looks like we are loading more content so the scroll thumb naturally gets smaller because there is more content to scroll?

Yes, it does get smaller.
Issue is that it jumps out of view or in some cases it gets truncated on the opposing side of scroll (scrolling down - top part of scroll-thumb is not rounded anymore, only after scrolling a bit back).