@Super_Hippo, your scenario does not have the source published, so all that is possible is to see the result -- not what may be causing it. As I have not ever had any issues similar to what you all are experiencing, I am wondering if it could be related to the operating systems we are using.

Okay. I downloaded it and checked it out. I seems to be okay on the download. Maybe it is the particular version of greenfoot you are using.

You have some transparency within the world background along the edges. You can probably avoid this by not using the 'clear' method on the background image. Greenfoot will automatically tile any image smaller than the size of the world when set to the background.

As far as those troubled classes (like the Bullet class), copy the text of the JAVA files onto your clipboard (even if you have to open the file outside of greenfoot and copy them) and then, in greenfoot, delete the class and add a new one to copy the text back into (replacing all code in the new class). You also had copied some files into the scenario folder that do not belong there. The only file types you should have are 'java', 'class' and 'ctxt'; these should be accompanied with one 'project GREENFOOT file' and one 'README Text document'. The only folders you should have are the 'images' and 'sounds' folders. Everything else should be removed.

It appears you are calling a method from within itself which is not wise without proper precautions in making sure you stop calling it at some point. You are calling the 'atWorldEdge' method inside the 'atWorldEdge' method in the Invader class. In your case, you just should not be doing that. You may be able to call the 'isAtEdge' method inside the 'atWorldEdge' method instead.

@olenz, the source for this scenario was not provided for download. It is, however, viewable while running the scenario. Did you try using a different browser (maybe IE)?

I took a quick glance at what you tried here. You are implementing two different behaviors for the world (scrolling and full-screen). However, the scrolling implementation was not intended for a full-screen world (and, of course, the full-screen implementation was not intended for a scrolling world). I am not saying that you cannot have both -- just that modification are required to allow them both to work together. It might be a bit difficult to come up with the proper modifications required; especially since you are not the author of either of those support classes.