Mouse does not lock to game viewport on Linux

Playing in standalone or a packaged game on Linux, the mouse can escape the viewport which can cause the game to lose focus or be hidden behind another window when the mouse is clicked.

This is inconsistent with behavior on Windows.
This behavior is the same when "lock mouse to viewport" is set as well.

1. Open UE4Editor
2. Run game in standalone (not in full screen)
3. Quickly move the mouse left and right while click the LMB

Result:
The mouse will exit the game window and clicking will cause it to lose focus

Expected: Movement of the mouse does not cause the game to lose focus.