Affected versions

• 85.0a1 (2020-12-09);

Affected platforms

• macOS 10.15, Windows 10, Ubuntu 20;

Steps to reproduce

1. Launch Firefox, add at least 2 user login-password(s) access about:logins
2. Either zoom in or resize browser so it takes up half of the screen only;
3. Click on the modal (optional click on the Yes, remove these logins...);
4. Press the TAB key to get one of the Cancel/Remove all button in focus;
5. Press the left-right key on keyboard;

Expected result

• focus moves between the buttons;

Actual result

• background content moves left right, focus remains on the same button;

Regression range

• not a regression, visible with addition of the modal;

Additional notes

• attached recording with the issue;
• S4 as suggested severity since it's somewhat of an edge case and the impact is minimal;
• main point would be that at least for as long the focus is on the 2 buttons, the background scrolling to be disabled - making it more of an enhancement.