Fixing exceptions in Drag LayoutAnchoreables to DocumentPane

#71 (comment)

source/Components/AvalonDock/Layout/LayoutAnchorable.cs

@@ -323,7 +323,7 @@ public void Show()
 			{
 				var previousContainerAsLayoutGroup = PreviousContainer as ILayoutGroup;
 				if (PreviousContainerIndex < previousContainerAsLayoutGroup.ChildrenCount)
-					previousContainerAsLayoutGroup.InsertChildAt(PreviousContainerIndex, this);
+					previousContainerAsLayoutGroup.InsertChildAt((PreviousContainerIndex < 0 ? 0 : PreviousContainerIndex), this);
 				else
 					previousContainerAsLayoutGroup.InsertChildAt(previousContainerAsLayoutGroup.ChildrenCount, this);
 

source/Components/AvalonDock/Layout/LayoutContent.cs

@@ -693,18 +693,25 @@ internal void CloseInternal()
 			var root = Root;
 			var parentAsContainer = Parent;
 
+			// Determine if this pane is the only DocumentPane present in the layout of the main window (not floating)
+			// then it will be skipped for removal in GarbageCollection (even if it is empty)
+			bool isParentRemoved = Root.Manager.Layout.IsParentRemoved(Parent);
+
 			if (PreviousContainer == null)
 			{
 				var parentAsGroup = Parent as ILayoutGroup;
 				PreviousContainer = parentAsContainer;
 				PreviousContainerIndex = parentAsGroup.IndexOfChild(this);
 
-				if (parentAsGroup is ILayoutPaneSerializable layoutPaneSerializable)
+				if (isParentRemoved == false && parentAsGroup is ILayoutPaneSerializable layoutPaneSerializable)
 				{
 					PreviousContainerId = layoutPaneSerializable.Id;
-					// This parentAsGroup will be removed in the GarbageCollection below
+
+					// Check whether this parentAsGroup will be removed in the GarbageCollection below
 					if (parentAsGroup.Children.Count() == 1 && parentAsGroup.Parent != null && Root.Manager != null)
 					{
+						// move LayoutContent up in the layout tree by setting PreviousContainer to it's grandparent
+						// (a LayoutPanel) by assumption that the parent will be garbage collected below.
 						Parent = Root.Manager.Layout;
 						PreviousContainer = parentAsGroup.Parent;
 						PreviousContainerIndex = -1;

source/Components/AvalonDock/Layout/LayoutRoot.cs

@@ -376,9 +376,7 @@ public void CollectGarbage()
 					}
 
 					//...if this pane is the only documentpane present in the layout of the main window (not floating) then skip it
-					if (emptyPane is LayoutDocumentPane &&
-						 emptyPane.FindParent<LayoutDocumentFloatingWindow>() == null &&
-						 this.Descendents().OfType<LayoutDocumentPane>().Count(c => c != emptyPane && c.FindParent<LayoutDocumentFloatingWindow>() == null) == 0)
+					if (IsParentRemoved(emptyPane))
 						continue;
 
 					//...if this empty pane is not referenced by anyone, then remove it from its parent container
@@ -620,6 +618,24 @@ void IXmlSerializable.WriteXml(XmlWriter writer)
 			}
 			writer.WriteEndElement();
 		}
+
+		/// <summary>
+		/// Determine if the parent pane is the only DocumentPane present in the layout of the main window (not floating)
+		/// then it will be skipped for removal in GarbageCollection (even if it is empty).
+		/// 
+		/// Otherwise, an empty pane will be removed in the LayoutRoot.GarbageCollection() which means that PreviousContainer
+		/// must be adjusted before items are removed from their parent pane.
+		/// </summary>
+		/// <param name="parent"></param>
+		/// <returns></returns>
+		internal bool IsParentRemoved(ILayoutContainer parent)
+		{
+			if (parent == null) return false;
+
+			return (parent is LayoutDocumentPane &&
+					parent.FindParent<LayoutDocumentFloatingWindow>() == null &&
+						this.Descendents().OfType<LayoutDocumentPane>().Count(c => c != parent && c.FindParent<LayoutDocumentFloatingWindow>() == null) == 0);
+		}
 		#endregion IXmlSerializable interface members
 
 		#endregion Public Methods