Why does a color field turn to opaque white when the scroll bar is active?

When you open a field for editing in SuperCard, the program creates a TextEdit box, and displays it in the area occupied by the field. TextEdit does not support transparent drawing, and instead draws its text on top of a white background. Other programs like HyperCard are able to circumvent this problem by using a separate offscreen buffer for the field, but this scheme requires more memory and often slows down the responsiveness of text editing operations noticeably.