[AvalonDock] Fix "InvalidOperationException: This Visual is not connected to a PresentationSource" when closing a view content by middle-clicking while holding the left mouse button.

src/Libraries/AvalonDock/AvalonDock/ManagedContent.cs

@@ -337,7 +337,9 @@ namespace AvalonDock
 
         protected virtual void OnDragMouseLeave(object sender, MouseEventArgs e)
         {
-            if (!e.Handled && isMouseDown && e.LeftButton == MouseButtonState.Pressed && Manager != null)
+        	// The MouseLeave event can occur if the managed content was closed while the user held the left mouse button pressed.
+        	// In that case, this.IsLoaded will be false and we won't handle the event
+            if (!e.Handled && isMouseDown && e.LeftButton == MouseButtonState.Pressed && Manager != null && this.IsLoaded)
             {
                 if (!IsMouseCaptured)
                 {