Hi, that is no surprise, each control that gets redrawn by default starts of with clearing the background, then painting the foreground. There is an easy way to make this invisible, it is known as "double buffering" (which means the drawing is made first in an off-screen buffer, then that one is copied over the relevant part of the screen). There is Control.DoubleBuffered starting .NET 2.0; for .NET 1.x you can achieve the same effect using SetStyles on an inherited Control. Lots of articles on CP use these techniques. :)
Luc Pattyn [My Articles]