property Button.ShowFocusRect as Boolean
Sets or returns a value that determines whether or not the focus rectangle should be shown.

Boolean A boolean expression that indicates whether the control displays the focus rectangle around the button's caption or image.
By default, the ShowFocusRect property is False. Use the UseFocusSkin and FocusSkin properties to assign a new visual appearance to your button when it has the focus. The following screen shot shows how the thin focused rectangle looks like: