Mouse is not captured when running -game on Linux

Launching a game with the -game command causes the game window to not capture the mouse until the window is clicked.

Regression:
Yes - This behavior does not occur in 4.16.3 (CL 3561208)

1. Open UE4 Editor
2. Create a First Person Template project
3. From the Terminal run the following command (single quotes are intended):

   ./Path/to/UE4Editor 'Path/To/Project.uproject' -game
   

Result:
The mouse is not captured until clicked or (if windowed) it enters the game window. If the mouse overlaps the window when it opens, it must be moved outside the window then re-enter to be captured. In either case, input is not received until the mouse is clicked.

Expected:
Mouse input is captured when the game is launched.