Obtaining 2560 x 1600 on a 3840 x 2160 with surrounding bands When you pair a MacBook that supports only up to 2560 x 1600 on an external display with a Dell 28" 3840 x 2160 monitor, is it possible to request precisely the resolution 2560 x 1600 on the monitor with empty bands on the four sides, or are you stuck with (the awful) 2x scaled 1920 x 1080?

I see that there is yet another hidden feature. If you click Option in the display settings, you will see additional modes. But it seems that if this method will show you any additional modes, you already need to have the monitor in question connected just to run the experiment.
You cannot request this from the computer, as it would have to support the full resolution in order to do that. This is a feature that would have to be provided by the monitor.

You’ll have to check the manual for your monitor to see if it can do this. Most monitors cannot.

On the other hand you’re not stuck with that one specific setting. You can choose to use the full 2560x1600 resolution, but it will typically fill up the entire display.