GNOME remembers the display settings of different monitor combinations!
So, if I just have my laptop’s screen on, it knows to use 4K@ 300%. Since it can’t do different scales for different screens, when I plug my 21" 4K monitor, it knows to display that at 4K@200% but scale the laptop to 2,560×1,440@200% (which keeps things legible). But if I plug in my tiny 1080i Lilliput output monitor, resolutions change to 1,920×1,080@200% on all so I can simulate my presentation environment.
Amazing!