This is not a SystemMenu-exclusive bug. It happens in both UI sets:
Problem: Cannot run game while script is opened in the editor
Expected behavior: We used to be able to run the game while a script was open in the editor and our view would shift to the game, and when we stopped the test game it would take us back into the script editor. Really useful for rapid debugging as you don’t have to keep clicking in and out of the script editor to test.