fix ResizeGrip usage by setting the WindowChrome ResizeGripDirection

MahApps.Metro/Behaviours/BorderlessWindowBehavior.cs

@@ -314,6 +314,8 @@ private void AssociatedObject_Loaded(object sender, RoutedEventArgs e)
             window.SetIsHitTestVisibleInChromeProperty<ContentPresenter>("PART_LeftWindowCommands");
             window.SetIsHitTestVisibleInChromeProperty<ContentPresenter>("PART_RightWindowCommands");
             window.SetIsHitTestVisibleInChromeProperty<ContentControl>("PART_WindowButtonCommands");
+
+            window.SetWindowChromeResizeGripDirection("WindowResizeGrip", ResizeGripDirection.BottomRight);
         }
 
         [Obsolete(@"This property will be deleted in the next release. You should use BorderThickness=""0"" and a GlowBrush=""Black"" properties in your Window to get a drop shadow around it.")]

MahApps.Metro/Controls/MetroWindow.cs

@@ -977,11 +977,25 @@ protected void TitleBarMouseUp(object sender, MouseButtonEventArgs e)
             }
         }
 
+        /// <summary>
+        /// Gets the template child with the given name.
+        /// </summary>
+        /// <typeparam name="T">The interface type inheirted from DependencyObject.</typeparam>
+        /// <param name="name">The name of the template child.</param>
         internal T GetPart<T>(string name) where T : DependencyObject
         {
             return GetTemplateChild(name) as T;
         }
 
+        /// <summary>
+        /// Gets the template child with the given name.
+        /// </summary>
+        /// <param name="name">The name of the template child.</param>
+        internal DependencyObject GetPart(string name)
+        {
+            return GetTemplateChild(name);
+        }
+
         private static void ShowSystemMenuPhysicalCoordinates(Window window, Point physicalScreenLocation)
         {
             if (window == null) return;

MahApps.Metro/Controls/MetroWindowHelpers.cs

@@ -17,8 +17,8 @@ internal static class MetroWindowHelpers
         /// <summary>
         /// Sets the IsHitTestVisibleInChromeProperty to a MetroWindow template child
         /// </summary>
-        /// <param name="window">The MetroWindow</param>
-        /// <param name="name">The name of the template child</param>
+        /// <param name="window">The MetroWindow.</param>
+        /// <param name="name">The name of the template child.</param>
         public static void SetIsHitTestVisibleInChromeProperty<T>(this MetroWindow window, string name) where T : DependencyObject
         {
             if (window == null)
@@ -32,6 +32,25 @@ public static void SetIsHitTestVisibleInChromeProperty<T>(this MetroWindow windo
             }
         }
 
+        /// <summary>
+        /// Sets the WindowChrome ResizeGripDirection to a MetroWindow template child.
+        /// </summary>
+        /// <param name="window">The MetroWindow.</param>
+        /// <param name="name">The name of the template child.</param>
+        /// <param name="direction">The direction.</param>
+        public static void SetWindowChromeResizeGripDirection(this MetroWindow window, string name, ResizeGripDirection direction)
+        {
+            if (window == null)
+            {
+                return;
+            }
+            var inputElement = window.GetPart(name) as IInputElement;
+            if (inputElement != null)
+            {
+                WindowChrome.SetResizeGripDirection(inputElement, direction);
+            }
+        }
+
         /// <summary>
         /// Adapts the WindowCommands to the theme of the first opened, topmost &amp;&amp; (top || right || left) flyout
         /// </summary>