private void UpdateWindowRegion()
{
if (this.DocumentStyle == DocumentStyle.DockingMdi)
this.UpdateWindowRegion_ClipContent();
else if (this.DocumentStyle == DocumentStyle.DockingSdi ||
this.DocumentStyle == DocumentStyle.DockingWindow)
this.UpdateWindowRegion_FullDocumentArea();
else if (this.DocumentStyle == DocumentStyle.SystemMdi)
this.UpdateWindowRegion_EmptyDocumentArea();
}