When I minimize the Black Mesa, I cant go back, so I wanted to know how can I start in window mode.
in the shortcut properties in the target box add -window separated by a space
Options -> Video -> Display mode -> Run in a window -> OK
I guess it’s now only possible using Steam itself, so you can try the above fix or in Steam Library right click Black Mesa choose properties and you should be able to find the command line button.
Anyone know if it works with full screen window mode for multiple monitors?
yes but depending on your GPU configuration it might run really slowly
in order to play BM in triple screen eyefinity is recommended