This, for me, critical feature is missing, i would appreciate if you implement it in future updates. Keep up the work guys, it’s an awesome game 
Adding -windowed -noborder to the games launch options works for the moment, give it a whirl.
If you’re unaware of how to add this: right click on black mesa in your steam games list, click properties and the bottom of the list then click the Set Launch Options button which will bring up box to add the lines too.
ah, it was -noborder, i tried -nowindowborder earlier, thanks