Steps to reproduce:
- Add a short category or three. (At en.wp, you can add Category:1, Category:2, and Category:3, etc.) The box for adding the next cat should still be on the same (first) line.
- Now try to add a very long category name. If you're on Meta, try Category:Association of Wikipedians - AWWDMBJAWGCAWAIFDSPBATDMTD
The search field pops up, but you can't really see much of it. In particular, unless what you're trying to add can be reliably added from the first and last few characters, then you might add the wrong category.
The category field will be wide enough to see what's in the search list.