https://stackoverflow.com/questions/18001317/receiving-keypresses-while-in-background
Later edit: if the window is out of focus, it can't process WM_KEYDOWN messages
even if WM_KILLFOCUS is ignored. Therefore you need to check regularly and separately for GetAsyncKeyState().
-->
GetAsyncKeyState
for
https://docs.microsoft.com/en-us/windows/win32/inputdev/virtual-key-codes