Fix for MahApps#2780 - set zorder using owner window (if have one) and SWP TOPMOST_FLAGS

Author: fjlaga

src/MahApps.Metro/MahApps.Metro.Shared/Behaviours/BorderlessWindowBehavior.cs

@@ -368,6 +368,11 @@ private void HandleMaximize()
                     var width = rect.Width;
                     var height = rect.Height;
 
+                    // Z-Order would only get refreshed/reflected if clicking the
+                    // the titlebar (as opposed to other parts of the external
+                    // window) unless I first set the window to HWND_BOTTOM then HWND_TOP before HWND_NOTOPMOST
+                    UnsafeNativeMethods.SetWindowPos(this.handle, Constants.HWND_BOTTOM, left, top, width, height, Constants.TOPMOST_FLAGS);
+                    UnsafeNativeMethods.SetWindowPos(this.handle, Constants.HWND_TOP, left, top, width, height, Constants.TOPMOST_FLAGS);
                     // #2780 Don't blindly set the Z-Order to HWWND_NOTOPMOST. If this window has an owner, set
                     // the Z-Order to be after the owner. This keeps external dialogs appearing correctly above
                     // their owner window even when owner window is maximized and ignoring taskbar.
@@ -376,7 +381,7 @@ private void HandleMaximize()
                     {
                         hwndInsAfter = new WindowInteropHelper(this.AssociatedObject.Owner).Handle;
                     }
-                    UnsafeNativeMethods.SetWindowPos(this.handle, hwndInsAfter, left, top, width, height, 0x0040);
+                    UnsafeNativeMethods.SetWindowPos(this.handle, hwndInsAfter, left, top, width, height, Constants.TOPMOST_FLAGS);
                 }
             }