It is the flag of a mouse buttonCenter event

Namespace:  GLSV
Assembly:  GLSV (in GLSV.dll) Version: 1.2009.6.26 (1.2009.06.26)

Syntax

C#
public bool ButtonCenter { get; set; }
Visual Basic (Declaration)
Public Property ButtonCenter As Boolean
Visual C++
public:
property bool ButtonCenter {
	bool get ();
	void set (bool value);
}

See Also