Fix window activation.

Windows/Perspex.Win32/Interop/UnmanagedMethods.cs

@@ -190,6 +190,13 @@ namespace Perspex.Win32.Interop
             SM_SYSTEMDOCKED = 0x2004,
         }
 
+        public enum WindowActivate
+        {
+            WA_INACTIVE,
+            WA_ACTIVE,
+            WA_CLICKACTIVE,
+        }
+
         [Flags]
         public enum WindowStyles : uint
         {

Windows/Perspex.Win32/WindowImpl.cs

@@ -163,17 +163,20 @@ namespace Perspex.Win32
             switch ((UnmanagedMethods.WindowsMessage)msg)
             {
                 case UnmanagedMethods.WindowsMessage.WM_ACTIVATE:
-                    switch ((int)lParam & 0xffff)
+                    var wa = (UnmanagedMethods.WindowActivate)((int)wParam & 0xffff);
+                    
+                    switch (wa)
                     {
-                        case 1:
-                        case 2:
+                        case UnmanagedMethods.WindowActivate.WA_ACTIVE:
+                        case UnmanagedMethods.WindowActivate.WA_CLICKACTIVE:
                             if (this.Activated != null)
                             {
                                 this.Activated();
                             }
 
                             break;
-                        case 0:
+
+                        case UnmanagedMethods.WindowActivate.WA_INACTIVE:
                             if (this.Deactivated != null)
                             {
                                 this.Deactivated();