Usually, all you really have to do is change something in your World subclass (like hit "enter", then, hit "backspace" to counteract). That should get it to try compiling again.

Recompiling after code change does work: that's not the problem. The problem is that after all classes have been compiled after editing code, it is not possible to run the application from the main Greenfoot window: "Execute", "Pause" and "Reset" buttons are grayed (disabled) I've found a workaround: apprently Greenfoot configuration files that Greenfoot creates at first execution and modifies after configuration changes were corrupted. I had a 'greenfoot' folder into my personal folder, with files regularly updated by Greenfoot. I've deleted it and apprently everything works fine again now (at least I hope it will last). Just strange thing: Greenfoot did not create again a 'greenfoot' folder with configuration files into my personnal folder... But instead it seems it now changes configuration settings into 'bluej' configuration folder (I use the last portable version of Greenfoot)