Chat window/movement issues

I'm playing with a keyboard & mouse on PC and the following happens at least a couple of times per play session:

I suddenly stop moving in the middle of a fight, usually in a group, and when I look at my chat window I see that I've been typing in it rather than issuing keyboard commands. ESCaping out of the chat window (which now reads "wwwwwwwwwwwwwwwsssssssssaaaaaaaaaaaaaawwwwwww") allows me to move again, but I'm wondering what exactly I'm doing to prompt the chat window to start recording input? I've seen in the new player guide that Alt+some key command will activate the chat window, and while I do use Alt+mouse buttons to trigger my hotbar actions, I'm generally very careful not to hit other keys at the same time as Alt.

Has anybody else experienced this? What did you do to solve it?