I was updating my Cubase 9 workspaces today and accidentally opened my Project window and MixConsole in tabs.
After trying out a few combinations, I discovered that holding down Alt while opening the MixConsole(s) (and other devices such as the Pool) via the main menu consolidates the project window and opens a new tab.
You can cycle through tabs using cmd + shift + [ or ]
Additionally a new option ‘Close Tab’ appears in the File menu.
I’m on MacOS 10.12.3, I gather it’s a new feature in Sierra, it may be useful for those on single screens.