I'm going to say no again on this one, because a) this type of feature isn't natively supported by irc and will require network + Bancho reworking and b) while I typically prefer having this feature on personal IM clients, it's a different story when you're talking to various people online. Sometimes I run into conversations where I need to think of what I want to say carefully, and the way it is right now, I can type out bits of my sentences while pretending to be afk/looking at another tab/otherwise not fully focused on constructing my statement. Now this may not exactly apply to all people but, uh... this does:
c) there's no space to put the 'user is typing' message. Unless you shove in a little "User is typing..." line in between your input line and the last line in chat whenever they start typing. But that would look really stupid because it would move all of the chatted text whenever the message appeared. If you don't want the chat to move and simply leave it spaced one line up, that doesn't work either because there's no medium, the chat interface is just one darkened area, and it would be inconsistent with the main chat in public channels. Instead of having the message at the bottom, and having a (...) thing appear on the right side when the person is typing, would also be inconsistent with the style of the interface, and right justified stuff sucks anyways. So the only place left to have the notification is on the chat tab, but the purpose of the chat tab colours is to indicate whether there's a new message, so having it flash to people who start typing would make it look like there's a new message already, plus it's so disconnected from where you're actually looking (the bottom of the chat), that people would never bother looking at it and it would be virtually useless anyways.
Basically, the design of irc simply makes this idea not work.
