I have screen updating problems when the turtle is drawing.

This is also a known problem of the JRE. I will try to deal with it in the future, I might be able to do something about it. For now, there are two solutions:

• to minimize the window and increase its size again.
• To use the JRE 1.4.1_07 proposed on the website. With JRE $>1.5$, it seems better.