This is in Firefox 56. I don't know if it was there in previous Firefox versions because I used to have Chrome as my default browser, which doesn't show the horizontal scrollbar.
It's even worse in Edge:
Thanks for bringing this problem to our attention. I was able to recreate this in my Jira Cloud instance using my Firefox 55.0.3 version. As such I created a new bug for this problem over at https://jira.atlassian.com/browse/JRACLOUD-67703
I would recommend watching that page for updates.
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