I'm on Catalina. I don't understand how everything comes together but after a few weeks intense search I found out that a 1920x1080hidpi is marked as "not usable for the desktop". https://developer.apple.com/documentation/coregraphics/1454928-cgdisplaymodeisusablefordesktopg I also think I read that this is not a fine (1to4?) pixel scaling which is more difficult to handle for developers, maybe causing blurry so imperfection is not wanted. But switchresx can easily add a scaled 3840x2160 (1920x1080hidpi) which works fine on Dell u2515h hdmi and miniDP-DP at MBproEarly2013Retina until I close the lid to clamshell mode, then the Dell went black (not inactive, but by a streamer like splashtop you can see there is no more monitor recognized by the system) So I finally found out that I got to set external to 1440x900 !non hidpi, close the lid, the screen doesn't turn black, now same to do again, than back to the new HiDPI and voila I can open and close the lid like expected even after sleep but not after reboot. If I don't do the 1440 twice, the problem changes to: now is the external monitor black in desktop mode. Which is very confusing. Also that the resolution of one monitor can differ from desktop to clamshell mode and that it seems that clamshell activation goes from black over 1440 to 1920. And I wonder if I can do or read something about the "not usable for desktop"-tag or what exactly happens if I close the lid. Tool to check the given resolution with Tags: https://github.com/p00ya/displaymode