[#15] MailWindow: Small quality fixes

Elwig/App.xaml.cs

@@ -252,7 +252,7 @@ namespace Elwig {
         }
 
         public static MailWindow FocusMailWindow(int? year = null) {
-            return FocusWindow<MailWindow>(() => new(year));
+            return FocusWindow<MailWindow>(() => new(year), w => year == null || w.Year == year);
         }
     }
 }