If you are using the latest version or the drivers it is possible to hide the mixer software so it isn't taking up screen space during a busy session. Bringing the mixer window back again is a nice easy process.
Mac:
On MacOS, when the iD mixer application is running, you'll see a small iD icon in the system tray, found in the top right hand corner of the screen. Simply click on this and select Show Mixer.
Windows
On windows, when the iD mixer application is running, you'll see a small iD icon in the system tray, found in the bottom right hand corner of the screen. Simply right-click on this and select Show Mixer.