Mark SizeToContent with [Flags].

And explicitly assign values.

src/Avalonia.Controls/Window.cs

@@ -19,27 +19,28 @@ namespace Avalonia.Controls
     /// <summary>
     /// Determines how a <see cref="Window"/> will size itself to fit its content.
     /// </summary>
+    [Flags]
     public enum SizeToContent
     {
         /// <summary>
         /// The window will not automatically size itself to fit its content.
         /// </summary>
-        Manual,
+        Manual = 0,
 
         /// <summary>
         /// The window will size itself horizontally to fit its content.
         /// </summary>
-        Width,
+        Width = 1,
 
         /// <summary>
         /// The window will size itself vertically to fit its content.
         /// </summary>
-        Height,
+        Height = 2,
 
         /// <summary>
         /// The window will size itself horizontally and vertically to fit its content.
         /// </summary>
-        WidthAndHeight,
+        WidthAndHeight = 3,
     }
 
     /// <summary>