Hi Sorwen,
I am not compleet sure but I think this is the line where it goes wrong.
_GraphicsPath.AddLine(Rect.Width, CornerSize, (Rect.Width + 40), Rect.Height - CornerSize)
You want to go down, but the X values are diffrent and should be the same.
(Rect.Width + 40) Should be Rect.Width...