At least on Chrome Linux. Did not verify other platforms.

What steps will reproduce the problem?
1) Cause chrome to prompt for credentials (ie. NTLM).
2) Attempt to enter a username with accented characters

What is the expected result?

You can either paste or use the compose key to type accented characters. eg the username 'côté'

What happens instead?

You can't paste and the compose key does not make the composed characters. Just displays as if they were normal keystrokes.