When there's an extremely long 'word', like a link, the tag excerpt popup does not hyphenate the word and break it to a new line. Instead, it extends outwards, like so:
(link to that particular revision history, though it's observable everywhere that popup is supposed to appear)
I'm aware that tag excerpts are generally not formatted and this well-recieved meta post even tries to make a case for banning hyperlinks in tag excerpts, but I do think it's rather useful to be able to put links anyways. On Physics at least, we have a few tags which have links to meta posts for guidance regarding certain kinds of questions.
I'm on Chrome 71.0.3578.98 for Windows 10; I could reproduce this on Edge and Vivaldi too.