Forward Compatibility w/ WebKit & Blink

Newer versions of WebKit and Blink will support both `document.body.scrollTop` and `document.documentElement.scrollTop`. Therefore, implementing cross-browser compatibility by summing the two will no longer work.

This changes React to use `getUnboundedScrollPosition` so we get the fix and consistency in one change!

See: https://rniwa.com/2013-10-29/web-compatibility-story-of-scrolltop-and-scrollleft/

src/dom/ViewportMetrics.js

@@ -18,6 +18,8 @@
 
 "use strict";
 
+var getUnboundedScrollPosition = require('getUnboundedScrollPosition');
+
 var ViewportMetrics = {
 
   currentScrollLeft: 0,
@@ -25,10 +27,9 @@ var ViewportMetrics = {
   currentScrollTop: 0,
 
   refreshScrollValues: function() {
-    ViewportMetrics.currentScrollLeft =
-      document.body.scrollLeft + document.documentElement.scrollLeft;
-    ViewportMetrics.currentScrollTop =
-      document.body.scrollTop + document.documentElement.scrollTop;
+    var scrollPosition = getUnboundedScrollPosition(window);
+    ViewportMetrics.currentScrollLeft = scrollPosition.x;
+    ViewportMetrics.currentScrollTop = scrollPosition.y;
   }
 
 };

src/vendor/core/dom/getDocumentScrollElement.js

@@ -0,0 +1,27 @@
+/**
+ * @providesModule getDocumentScrollElement
+ * @typechecks
+ */
+
+"use strict";
+
+// TODO: Replace this with a UserAgent module.
+var isWebkit = navigator.userAgent.indexOf('AppleWebKit') > -1;
+
+/**
+ * Gets the element with the document scroll properties such as `scrollLeft` and
+ * `scrollHeight`. This may differ across different browsers.
+ *
+ * NOTE: The return value can be null if the DOM is not yet ready.
+ *
+ * @param {?DOMDocument} doc Defaults to current document.
+ * @return {?DOMElement}
+ */
+function getDocumentScrollElement(doc) {
+  doc = doc || document;
+  return !isWebkit && doc.compatMode === 'CSS1Compat' ?
+    doc.documentElement :
+    doc.body;
+}
+
+module.exports = getDocumentScrollElement;

src/vendor/core/dom/getUnboundedScrollPosition.js

@@ -0,0 +1,30 @@
+/**
+ * @providesModule getUnboundedScrollPosition
+ * @typechecks
+ */
+
+"use strict";
+
+var getDocumentScrollElement = require('getDocumentScrollElement');
+
+/**
+ * Gets the scroll position of the supplied element or window.
+ *
+ * The return values are unbounded, unlike `getScrollPosition`. This means they
+ * may be negative or exceed the element boundaries (which is possible using
+ * inertial scrolling).
+ *
+ * @param {DOMWindow|DOMElement} scrollable
+ * @return {object} Map with `x` and `y` keys.
+ */
+function getUnboundedScrollPosition(scrollable) {
+  if (scrollable === window) {
+    return getUnboundedScrollPosition(getDocumentScrollElement());
+  }
+  return {
+    x: scrollable.scrollLeft,
+    y: scrollable.scrollTop
+  };
+}
+
+module.exports = getUnboundedScrollPosition;