c# drawing multiple screens on a form

I'm trying to create a program that draws a rectangle on a specific area, well basically on the entire screen size of the user.

If you have two 1080p monitors the form size should be:
3840x2160

I already have this working and my Form maximizes across both of my screens.

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But now the actual problem, whenever I setup so that my program uses all screens it somehow draws in the incorrect location. If I use a single screen this doesn't happen.
If I click on a certain point 920,590 then the start position of the rectangle will somehow be -920,-590 even though it prints out 920,590 in a messagebox. When I use a single screen it draws the rectangle perfectly fine..

BTW: I have two screens and the rectangle gets drawn on the LEFT screen(negative coordinates) while I click on the RIGHT screen..

Sorry i ment 3840x1080

Fixed it.

Instead of using the Cursor.Position.* on the Mouse_Down event I used MouseEventArgs.*
The Cursor.Position is global while the other is local position on the form.