I have the following problems with the program in the zip file: When clicking on Teken and then on Rechthoek, "Rechthoek" is printed in the NSLOG part of the IDE but the yellow rectangle is not drawn immediately. It is drawn when: - I go with the mouse cursor on the three "lights" at the left top of the window; - I resize the window; - it loses the focus (due to clicking on a different window for example the Xcode IDE or Google Chrome). The rectangle changes in size when I resize the window, but that has been resolved after making the ZIP file. Why isn't the rectangle displayed immediately? Why only after "touching" the window, resizing it or stealing the focus from it?