Originally Posted by
Packetdancer
FWIW, it's possible to do a slidecast indicator with stock UI. It's kind of a hackish solution, though.
Namely, float a hotbar somewhere you can see it while looking at your castbar, then put an emote on that hotbar. When you start casting, the emote's icon will grey out, because an active cast blocks you from doing emotes. However, the instant the cast is committed to the server side, you're allowed to emote again and the emote button will no longer be disabled... which is also the moment at which you can slide-cast successfully without interrupting your cast.
Thus by keeping an eye on the emote icon, someone still learning can get a good feel for exactly when they can start a slide-cast.