There was a discussion about the Mixer being an overlay (i.e. pops up over the lyric pane) rather than opening in its own pane (i.e. lyric pane would resize to create space for mixer pane), and (I think) Peter mentioned that he went with the overlay method as the thinking was that the Mixer would only be open for a short period of time to perform a task (e.g. tweak a level, mute a Bus, etc.) and then closed, rather than left open on the screen. Therefore, better suited to an overlay rather than the forcing the app to suddenly resize panes (possibly not ideal if you are mid-song and re-sizing lyric pane to get to mixer).
I think there was then also questions about making the Mixer overlay use full width of screen (i.e. if you have Song List/Playlist/Queue sidebar open, could it 'pop over' that as well. I don't remember where that part of the conversation went, but having just checked behaviour in Build 1920, I'm seeing some new and, well, 'interesting' behaviour... so off to create a new thread 🙂