Sometimes, when launching Black Mesa: BLue Shift the game opens normally for a few seconds, then it minimizes in the taskbar and can't be maximized. Choosing the Direct3D 9 fallback solves the issue (it still starts minimized, but can be maximized again) ONCE. when closing the game and starting it again with Direct3D 9 it won't work either anymore, you then have to swith back to Default, try to launch it, and launch again with Direct3D 9. Sometimes it starts correctly with the Default launch option.