The border is flat.
Syntax
EdgeEffects Flat
Requirements
Namespace: System.Windows.Forms.VisualStyles
Assembly: System.Windows.Forms (in System.Windows.Forms.dll)
Assembly Versions: 2.0.0.0
Since: .NET 2.0
Assembly: System.Windows.Forms (in System.Windows.Forms.dll)
Assembly Versions: 2.0.0.0
Since: .NET 2.0