How can I force windows 10 to behave like I always have my monitor plugged in? It's because I want to be able to play with steam link without needing my monitor turned on.
The reason this happens is because Display Port monitors are uninstalled when you turn off the display, and reinstalled when you turn the monitor on.
There are a few possible solutions that will work.
If your monitor has a power saving feature, disable it. On some monitors this actually stops the entire uninstall process and the monitor is remembered by windows even when turned off.
Don't use Display Port. The blue VGA connector (Sub-D) will definitely work, DVI will only work if you use the analog version (DVI-I), not the digital one (DVI-D). HDMI will also work. When the monitor is connected by the proper connection, it will not get uninstalled when powered off.