Game Output Window goes totally unresponsive after the game actually loads

After Minecraft finishes loading and gets to the menu screen, the game output window goes totally unresponsive and frozen without even getting a "(not responding)" in the title bar thing if it was the focused window, or goes blank white if it wasn't.

Curse forces the old launcher (their changelog mentions a compatibility change-appears the new launcher can't handle modpacks), so I'll be using that, most likely. I may never go back to actually using your launcher.