I have a Dell Windows 10 Laptop, it's connected to a Phlips QHD display over HDMI.
I also have a windows 10 tower connected to the same monitor over display port.
If I have the laptop closed with the monitor showing the laptop signal and switch to the other input the laptop stops the HDMI signal and won't restart it until I open the laptop screen and manually setup the layout again.
When I open the laptop it then "sees" the HDMI monitor and enables both it and the builtin screen in "extend these displays" mode, if I then close the laptop it turns off both displays, I have to got into settings and select "show only on 2" before closing it.
The tower is worse - when you switch the monitor away it drops the video output and won't restart it, full stop! I tried space bar, mouse, Ctrl-alt-delete etc. but have not been able to recover it without a power off reboot.
How can I get Windows 10 to maintain video output in this situation?