private void AddLastToActiveList(IDockContent content)
{
IDockContent last = this.LastActiveContent;
if (last == content)
return;
DockContentHandler handler = content.DockHandler;
if (this.IsInActiveList(content))
this.RemoveFromActiveList(content);
handler.PreviousActive = last;
handler.NextActive = null;
this.LastActiveContent = content;
if (last != null)
last.DockHandler.NextActive = this.LastActiveContent;
}