Thank you for your reply and the link. I found out that Adblock completely removes the banner (I have no extra rules or filters enabled). But however trashing user input and displaying a small message for 5 seconds is far beyond bad usability design. I can get really mad about things like that when I loose an hour of work just because someone else has the wrong job.
I submitted a bug report too. Of course no feedback if it was sent or a status link (but a redirection to www.twitch.tv). As a user that means to me that I could have written a letter to my cat instead for the same effect.