I am positive that it should be the above css. I have download the whole website and search this css, and found it only appears under flarum-ext-keywords/resources/forum.less
And after I delete the above css, the overflow issue went away.
If you want to take a look over at my flarum site, please let me know. I am not sure how to reproduce the issue on flarum.it. I can definitely change it back for you to take a look. ๐