Clean theme references overflowing table of contents

[StephDriver]

Clean Article viewed on Chrome, in a wide screen.

It is only that first link, later ones break across lines.

[StephDriver]

This happens more frequently when adjusting the text spacing as per WCAG 1.4.12 as lines get longer and more of them overflow:

It is important for compliance with WCAG 1.4.12 that text does not overflow its containers when adjusted, as described in #4260.

[StephDriver]

This is now marked as a separate issue, ignore

Not just References - other content overflows too. This was noted while testing another fix on my dev install, so this article does not exist on Clean, it was the one flagged up for a different problem in #3911 then downloaded for use as a test subject, but it illustrates the problem.

[StephDriver]

following backlog refinement, this has been split into the problem with references overflowing (this issue) and the problem with iframes (#4304).