So I have the following code:
#region Dropshadow [DllImport(\"Gdi32.dll\", EntryPoint = \"CreateRoundRectRgn\")] private static extern IntPtr CreateRoundRect
I set the bottomHeight to 3, and I found that the border height was included in the form size (the form's size didn't change). So I set a BackgroundImage to this form, and the border was hidden by the image.
bottomHeight
BackgroundImage