In my scenario, the blue boxes in the background can move outside the world. Only in the world boundaries, the moving objects are displayed correctly. I don't know how it is done, but I could think of the following: Every time the world gets repainted, the 'image' is cleared and then, the background image and all the objects are painted on it. Outside the world, the 'clearing of the image' does not happen. Usually, this shouldn't be a problem because it isn't visible, but in our case it is visible. So objects which are there (or close to the edges) are painted but not removed and create colored 'lines' when they are moving. It could be a bug on this website. Like a minimum size of the world and if the world is actually smaller, the rest will be filled with something that we can see (and you can't for some reason). I am using Windows and Internet Explorer 11. (Windows 10 for a few days, but it was the same on 8.1. The new browser 'Edge' doesn't work because it doesn't support plugins.)

An extreme example can be seen here: http://www.greenfoot.org/scenarios/12156 I think it has something to do with the size of the world. But as you say, it only happens here on greenfoot.org, not offline in the program.

That's how it looks to me: https://gyazo.com/bb3d1c30bfcdcc64927b5f5833aa29f0 For some reason, the edges changes to an object which moves there, but that always happens to me, not only here at your scenario...

Oh I forgot, the lag on my laptop isn't that bad. It is playable at least. Can't test on other one, but I didn't change anything to reduce it.