Another solution is to use AutoHotKey and the following script:
---
SetTitleMatchMode, 2
Loop
{
if GetKeyState("Control") && GetKeyState("Space")
break
WinActivate, Digital Combat Simulator
Sleep, 200 ; Adjust the sleep time (in milliseconds) as needed
}
---
Note 2: if you use this or the solution above (VR Focus Guard), it is best to run DCS with FullScreen* unchecked, or else you may see screen stuttering.
Note 3: The script is attached as a script file and an EXE, but you must install AutoHotKey on your PC.
Note 4: To stop the script loop, press the Ctrl and Space keys at the same time (or right-click the H icon on the taskbar and exit or pause the script.)
Beau1969's post in DCS not remaining as active window was marked as the answer