Whispering people on other servers.
 

I've been trying to track down this bug for 2 days now, and I can't figure out what's causing it.

When right clicking someone that's on another server to whisper the chatbox sometimes only fills in their name, not their server. So for example "Bob-Nagrand" is saying something in trade chat, I right click, click whisper and my chat box changes as if I've written "/w Bob", NOT "/w Bob-Nagrand".

I haven't found a reliable way to reproduce either, it sometimes happens, and sometimes it doesn't.

Do you use the Chat AddOn Prat?

Even if you don't use Prat, a good first step would be to see if the problem still happens when you disable all your addons. If it doesn't (and it almost certainly won't) then you can start enabling the rest of your addons until the problem happens again. Start with addons that seem chat-related, and enable them one at a time. You can do this in-game now (some people still don't know!) by opening the main menu (with the Escape key) and clicking the AddOns button. You still have to reload the UI, but that's much faster than completely logging out and back in.

I have reproduced the problem with all AddOns disabled and running: