Affected versions

• 84.0b7, 85.0b7;

Affected platforms

• Windows 7;

Steps to reproduce

1. Launch Firefox, set the browser to take up half of the available screen width;
2. access this link;
3. Scroll all the way down towards the cookie disclaimer with the middle-mouse scroller;
4. Scroll up/down inside the disclaimer message (might need to adjust brwoser-width so @mediaquerries arrange area as in gif);

Expected result

• scrollbar is contained;

Actual result

• scrollbar is not contained for mouse scroll, going past limits;

Regression range

• last good: 2020-10-05;
• first bad: 2020-10-06;
• pushlog URL;

Additional notes

• reproduced on 2 machines, with clean profiles;
• attached recording to illustrate the issue;
• did not see it trigger on Win10, so it might be limited to version 7.

Attaching about:support content, in case it might be of any relevance.

I can't reproduce your issue on Windows 7 with Beta v85.0b7 or v84.0b7. This issue might be specific to some system characteristics. Can you investigate its regressor on your systems?

Managed to find a promising regression window (updated in the initial comment).
Looking at the pushes there are some potential regressors that according to notes might be relevant:bug 1664804, bug 1669131, bug 1666774, bug 1662017.

Few more notes worth mentioning, noticed in trying to find the range:

• issue can be triggered with a bit more ease when scrolling before the text is fully loaded;
• on some builds the bug manifested partially(half of the thumb being scrolled past and "resetting" after a few seconds 1-3);
• tried to check with a nightly build from 2020-10-07 but on Nightly branch the issue does not appear to manifest, while still being triggered on beta builds. With current Nightly 2021-01-13 it only 1 time it got triggered partially it looking to be intermittent.