Instead of keep writing in comments of a scenario (see here: http://www.greenfoot.org/scenarios/14841), we continue in this thread.
As written there, the problem is that objects seem to be painted outside the world and aren't removed anymore. For example this scenario SPIKXX let me see the blue squares outside the usual visible world. This problem also happens to me in many other scenarios. If you can't see it, here are three screenshots to see how it happens.
https://gyazo.com/e7a99c0a775c0fe3737e3fd3b76654f0
https://gyazo.com/78467b91933580442788376bd6ccd628
https://gyazo.com/394abc85b82185f5dccdc43b2c1da3b8
I just updated the scenario to make sure the background image doesn't have any transparent areas. (It was just 'fill()' after creating the image, now I set it to black color '0,0,0'.) It didn't change anything.
When resetting the scenario, the area outside the world stays as blue as it was.
I will publish the source for the scenario in a minute if you want to take a look.

