when play demo in the editor, we should click the play button and reclick the viewport to get into the game. If in the Editor Preferences -> Level Editor -> Play -> CHECK the Game Gets Mouse Control OPTION. We dont need to reclick mouse.
Instead of change default loading map, get into the Editor Preference -> Loading & Saving -> CHECK Load Level at Startup TO Last Opened. This will help you when debuging cpp src and reopen the UE editor, load the test level instead default level.