Additional group content now hidden when added

DarkUI/Docking/DarkDockGroup.cs

@@ -72,6 +72,8 @@ namespace DarkUI.Docking
 
             if (VisibleContent == null)
                 VisibleContent = dockContent;
+            else
+                dockContent.Visible = false;
 
             var menuItem = new ToolStripMenuItem(dockContent.DockText);
             menuItem.Tag = dockContent;

DarkUI/Docking/DarkDockPanel.cs

@@ -32,6 +32,7 @@ namespace DarkUI.Docking
             get { return _activeContent; }
             internal set
             {
+                // Don't let content visibility changes re-trigger event
                 if (_switchingContent)
                     return;