Affected versions

• 89.0a1(2021-03-25), 88.0b2;

Affected platforms

• Windows 10;

Steps to reproduce

1. Launch Firefox, login to Netflix;
2. Access https://www.netflix.com/browse?jbv=80027042 as example and scroll a bit so the season picker is right at the bottom part of screen;
3. Click on the Seasons button;
4. Scroll up-down;

Expected result

• menu displayed;

Actual result

• horizontal scrollbar triggered on the menu;

Regression range

• prior to this the menu was opened fully, no scrollbars attached or triggered:
• First bad: 2020-02-24;
• Last good: 2020-02-23;
• Pushlog: URL;

Additional notes

• attached screenshot to best illustrate the issue; after triggering the issue scrolled up for visibilty.
• vertical bar triggered as well on other OS's such as macOS 10.13 but that would be useful.

Bug 1617069 maybe?

The scroller has max-height: max(10em, 9.46664px - 2rem); here. So https://hg.mozilla.org/mozilla-central/rev/47d7f75d469ff7c8ae8db10377265da69c66da59 most likely, but in that case it seems expected, before that we were not parsing that declaration.