[Fixed Position] Keep elements in view

Prevent elements from being positioned at negative x/y
locations in a fixed position view, WTD-882.

platform/features/layout/src/elements/AccessorMutator.js

@@ -8,14 +8,26 @@ define(
         /**
          * Utility function for creating getter-setter functions,
          * since these are frequently useful for element proxies.
+         *
+         * An optional third argument may be supplied in order to
+         * constrain or modify arguments when using as a setter;
+         * this argument is a function which takes two arguments
+         * (the current value for the property, and the requested
+         * new value.) This is useful when values need to be kept
+         * in certain ranges; specifically, to keep x/y positions
+         * non-negative in a fixed position view.
+         *
          * @constructor
          * @param {Object} object the object to get/set values upon
          * @param {string} key the property to get/set
+         * @param {function} [updater] function used to process updates
          */
-        function AccessorMutator(object, key) {
+        function AccessorMutator(object, key, updater) {
             return function (value) {
                 if (arguments.length > 0) {
-                    object[key] = value;
+                    object[key] = updater ?
+                            updater(value, object[key]) :
+                            value;
                 }
                 return object[key];
             };