fixed a bug of window

src/Shared/HandyControl_Shared/Tools/Extension/ValueExtension.cs

@@ -0,0 +1,9 @@
+using System.Windows;
+
+namespace HandyControl.Tools.Extension
+{
+    public static class ValueExtension
+    {
+        public static Thickness Add(this Thickness a, Thickness b) => new Thickness(a.Left + b.Left, a.Top + b.Top, a.Right + b.Right, a.Bottom + b.Bottom);
+    }
+}