Preserve active document (and order of documents) on layout switches.

Based on Andrew Olson's suggestion http://community.sharpdevelop.net/forums/t/13034.aspx.

src/Libraries/AvalonDock/AvalonDock/DockingManager.cs

@@ -294,8 +294,8 @@ namespace AvalonDock
 
             if (ActiveDocument == null)
             {
-                var docToActivate = Documents.OrderBy(d => d.LastActivation).FirstOrDefault();
-                if (docToActivate != null)
+                var docToActivate = Documents.OrderBy(d => d.LastActivation).LastOrDefault();
+                if (docToActivate != null && docToActivate.LastActivation != DateTime.MinValue)
                     docToActivate.Activate();
             }
         }
@@ -3775,7 +3775,8 @@ namespace AvalonDock
                 pane.ToggleAutoHide();
 
             DockableContent[] actualContents = DockableContents.ToArray();
-            DocumentContent[] actualDocuments = Documents.ToArray();
+            // preserve the order of the documents
+            DocumentContent[] actualDocuments = Documents.OrderBy(d => d.ContainerPane != null ? d.ContainerPane.Items.IndexOf(d) : -1).ToArray();
 
 
             //first detach all my actual contents
@@ -3892,16 +3893,8 @@ namespace AvalonDock
             ClearEmptyPanes();
             RefreshContents();
 
-            if (ActiveDocument != null &&
-               (ActiveDocument.ContainerPane == null ||
-               ActiveDocument.ContainerPane.GetManager() != this))
-            {
-                if (Documents.Count > 0)
-                    ActiveDocument = Documents[0];
-                else
-                    ActiveDocument = null;
-            }
-
+            // by setting ActiveDocument to null, we activate the previously activated document (as per LastActivation)
+            ActiveDocument = null;
             ActiveContent = ActiveDocument;
         } 
         #endregion

src/Libraries/AvalonDock/AvalonDock/DocumentContent.cs

@@ -340,6 +340,7 @@ namespace AvalonDock
         /// </summary>
         internal void InternalClose()
         {
+            this.LastActivation = DateTime.MinValue; // make this content ineligible for ActiveDocument selection when ActiveDocument is set to null
             DockingManager manager = Manager;
 
             if (manager != null)

src/Libraries/AvalonDock/AvalonDock/ManagedContent.cs

@@ -559,12 +559,7 @@ namespace AvalonDock
             ((ManagedContent)d).OnIsActiveContentChanged(e);
         }
 
-        DateTime _lastActivation = DateTime.MinValue;
-
-        internal DateTime LastActivation
-        {
-            get { return _lastActivation; }
-        }
+        internal DateTime LastActivation = DateTime.MinValue;
 
         /// <summary>
         /// Provides derived classes an opportunity to handle changes to the IsActiveContent property.
@@ -572,7 +567,7 @@ namespace AvalonDock
         protected virtual void OnIsActiveContentChanged(DependencyPropertyChangedEventArgs e)
         {
             if (IsActiveContent)
-                _lastActivation = DateTime.Now;
+                this.LastActivation = DateTime.UtcNow;
 
             FocusContent();