This broke once before recently and was fixed and is now broken again as of 2019-05-20.

When I expand the attachments pane while viewing a message, instead of the list of attachments fitting in the window all the attachments are on one line and there's a scroll bar below them and I have to scroll right to see them all.

I regressed this:

7:08.18 INFO: Narrowed inbound regression window from [8c2e0c1e, ae087d73] (3 builds) to [42218a90, ae087d73] (2 builds) (~1 steps left)
7:08.18 INFO: No more inbound revisions, bisection finished.
7:08.18 INFO: Last good revision: 42218a9071546f0bbd8e16dbd73df43e537ad998
7:08.18 INFO: First bad revision: ae087d73b3ce70604c356f6043cc8998243bf87f
7:08.18 INFO: Pushlog:
https://hg.mozilla.org/comm-central/pushloghtml?fromchange=42218a9071546f0bbd8e16dbd73df43e537ad998&tochange=ae087d73b3ce70604c356f6043cc8998243bf87f