Thread:Extension talk:Lingo/Glossary entries in titles and sub-titles/reply

It is indeed a CSS issue. Vector and Monobook skin (and probably others) set overflow:hidden for headlines. I do not know, why they would do this, it seems unnecessary to me.

To fix it find the CSS rules governing h1, h2, ... h6 in your skins CSS file and comment out or delete the overflow:hidden rule. For Monobook this is main.css around line 112, for Vector it is screen.css around line 728.