Unwanted popup scrollbars in Firefox

[ghostwords]

From #1516 (comment).

[lemnis]

I'm unable to reproduce this issue, could you give me some extra info (which site, used OS, etc.) ?

[ghostwords]

Firefox 55.0.2 on Linux. Can reproduce by opening Firefox with a fresh profile, installing Privacy Badger from master, visiting cnn.com (or any site with enough third-party domains) and opening the popup.

[ghostwords]

I can't reproduce on pages with many third-party domains anymore, but I can reproduce in the popup on whitelisted sites (screenshot below) and also on special browser pages (in the #1737 branch).

[lemnis]

I tested before in macOS and in elementary os (fork of ubuntu) and couldn't reproduce it.

It can occur when firefox finished its height calculation before it stop rendering. I only saw this occur when trying the fork in #1445 with flexbox, but I believe it can also occur with calc() and
display: grid;.
Maybe I should create a bug report at bugzilla for above issue, but I didn't bother till now. It's a pretty rare case.

A possible solution could be found while cleaning up popup.css, every time I look at I see css that could be coded neater.