Update ViewerWindow.xaml.cs

Keep window non-focusable. Feature added on this commit:
QL-Win@a7115f6
But removed for some issue discussed here:
QL-Win#644

QuickLook/ViewerWindow.xaml.cs

@@ -18,6 +18,8 @@
 using System;
 using System.Windows;
 using System.Windows.Input;
+using System.Windows.Interop;	
+using System.Windows.Media;
 using System.Windows.Media.Animation;
 using QuickLook.Common.ExtensionMethods;
 using QuickLook.Common.Helpers;
@@ -54,6 +56,12 @@ internal ViewerWindow()
             SizeChanged += SaveWindowSizeOnSizeChanged;
 
             StateChanged += (sender, e) => _ignoreNextWindowSizeChange = true;
+
+            // bring the window to top when users click in the client area.	
+            // the non-client area is handled by the WndProc inside OnSourceInitialized().	
+            // This is buggy for Windows 7 and 8: https://github.com/QL-Win/QuickLook/issues/644#issuecomment-628921704	
+            if (App.IsWin10)	
+                PreviewMouseDown += (sender, e) => this.BringToFront(false);
 
             windowFrameContainer.PreviewMouseMove += ShowWindowCaptionContainer;
 
@@ -97,6 +105,32 @@ internal ViewerWindow()
             buttonShare.Click += (sender, e) => ShareHelper.Share(_path, this);
             buttonOpenWith.Click += (sender, e) => ShareHelper.Share(_path, this, true);
         }
+
+        // bring the window to top when users click in the non-client area.	
+        protected override void OnSourceInitialized(EventArgs e)	
+        {	
+            base.OnSourceInitialized(e);	
+
+            // The non-focusable trick is buggy for Windows 7 and 8	
+            // https://github.com/QL-Win/QuickLook/issues/644#issuecomment-628921704	
+            if (App.IsWin10)	
+            {	
+                this.SetNoactivate();	
+
+                HwndSource.FromHwnd(new WindowInteropHelper(this).Handle)?.AddHook(	
+                    (IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled) =>	
+                    {	
+                        switch (msg)	
+                        {	
+                            case 0x0112: // WM_SYSCOMMAND	
+                                this.BringToFront(false);	
+                                break;	
+                        }	
+
+                        return IntPtr.Zero;	
+                    });	
+            }	
+        }
 
         public override void OnApplyTemplate()
         {
@@ -148,4 +182,4 @@ private void AutoHideCaptionContainer(object sender, EventArgs e)
             hide.Begin();
         }
     }
-}
+}