Custom Form without border gets distorted when going out of windows screen boundary?

Hey all,

in Winforms you have the option to create Forms without any border.
To make use of this, you need a background picture to fill the application. Optional, to remove the background of such pictures, you can set the color of the picture to make transparent.
I've created some of such applications that work fine.

I noticed, that when you move the applications outside of the screen and drag them back in, it will take flickering white blocks with it in my case. It seems like Windows draws the white background outside of the screen?
When you drag it back in and it has to be removed, it's only removed when you don't move anymore.
So until then Transparency is fighting the white color...
I even tried to create my applications with transparent png images, and set transparency as the color to remove. I hoped this would work and be the solution, but: I got no white blocks anymore which is good, but after moving it out of screen and drag it back, my application itself is flickering with another background color: the transparency! So when moving around after dragging it back on the screen my application is partly disappearing like the flickering light of a lantern in the night.

Does anyone have an idea why it behaves like this when you use a picture as form and remove the background color?
Is there any solution for it?

Tags: C#, Windows Forms, Visual Studio 2010