On OS X 10.8.3 Mountain Lion it’s possible to resize a window by dragging on any of its edges. I notice that ST3033 under certain circumstances registers such action as a click within the program. For example, make the Side Bar visible. Now resizing the window on the left edge can cause the open file to be changed. The drag cursor already appeared, but the actual mouse position is still on the Side Bar. This kind of thing might happen in other situations as well; there just needs to be a check whether the click was the start of a window resize.