"If I stop a project, the Stage goes blank. If I restart it, it takes a little while and in the meantime the stage is blank. So, the first time, I kept clicking the green flag (restarting each time the 'blank time'). I guess an 'hourglass' or similar would help"
It's not clear that something is happening when you click the green flag. Add an indicator of some kind to make this obvious.
Per @sfederici: https://scratch.mit.edu/projects/1043871984/#comments-412442742