Add a Rect.Normalize method with tests.

src/Avalonia.Visuals/Rect.cs

@@ -421,11 +421,48 @@ namespace Avalonia
         }
 
         /// <summary>
-        /// Gets the union of two rectangles.
-        /// </summary>
-        /// <param name="rect">The other rectangle.</param>
-        /// <returns>The union.</returns>
-        public Rect Union(Rect rect)
+		/// Normalizes the rectangle so both the <see cref="Width"/> and <see cref="Height"/> are positive, without changing the location of the rectangle
+		/// </summary>
+        /// <returns>Normalized Rect</returns>
+		/// <remarks>
+		/// Empty rect will be return when Rect contains invalid values. Like NaN.
+		/// </remarks>
+		public Rect Normalize()
+        {
+            Rect rect = this;            
+
+            if(double.IsNaN(rect.Right) || double.IsNaN(rect.Bottom) || double.IsNaN(rect.X) || double.IsNaN(rect.Y) || double.IsNaN(Height) || double.IsNaN(Width))
+            {
+                return Rect.Empty;
+            }
+
+            if (rect.Width < 0)
+            {
+                var old = X;
+                var x = X + Width;
+                var width = old - x;
+
+                rect = rect.WithX(x).WithWidth(width);
+            }
+            if (rect.Height < 0)
+            {
+                var old = Y;
+                var y = Y + Height;
+                var height = old - y;
+
+                rect = rect.WithY(y).WithHeight(height);
+            }
+
+            return rect;
+        }
+
+
+            /// <summary>
+            /// Gets the union of two rectangles.
+            /// </summary>
+            /// <param name="rect">The other rectangle.</param>
+            /// <returns>The union.</returns>
+            public Rect Union(Rect rect)
         {
             if (IsEmpty)
             {

tests/Avalonia.Visuals.UnitTests/RectTests.cs

@@ -35,5 +35,21 @@ namespace Avalonia.Visuals.UnitTests
 
             Assert.Equal(new Rect(0, 0, 100, 100), result);
         }
+
+        [Fact]
+        public void Normalize_Should_Reverse_Negative_Size()
+        {
+            var result = new Rect(new Point(100, 100), new Point(0, 0)).Normalize();
+
+            Assert.Equal(new Rect(0, 0, 100, 100), result);
+        }
+
+        [Fact]
+        public void Normalize_Should_Make_Invalid_Rects_Empty()
+        {
+            var result = new Rect(double.NegativeInfinity,double.PositiveInfinity, double.PositiveInfinity, double.PositiveInfinity).Normalize();
+
+            Assert.Equal(Rect.Empty, result);
+        }
     }
 }