Roll third_party/polymer.

third_party/polymer/v1_0/components-chromium/iron-scroll-target-behavior/iron-scroll-target-behavior-extracted.js

 /**
    * `Polymer.IronScrollTargetBehavior` allows an element to respond to scroll events from a
    * designated scroll target.
    *
    * Elements that consume this behavior can override the `_scrollHandler`
    * method to add logic on the scroll event.
    *
    * @demo demo/scrolling-region.html Scrolling Region
    * @demo demo/document.html Document Element
    * @polymerBehavior
    */
   Polymer.IronScrollTargetBehavior = {
 
     properties: {
 
       /**
        * Specifies the element that will handle the scroll event
-       * on the behalf of the current element. This is typically a reference to an `Element`,
+       * on the behalf of the current element. This is typically a reference to an element,
        * but there are a few more posibilities:
        *
        * ### Elements id
        *
        *```html
-       * <div id="scrollable-element" style="overflow-y: auto;">
+       * <div id="scrollable-element" style="overflow: auto;">
        *  <x-element scroll-target="scrollable-element">
-       *    Content
+       *    <!-- Content-->
        *  </x-element>
        * </div>
        *```
-       * In this case, `scrollTarget` will point to the outer div element. Alternatively,
-       * you can set the property programatically:
+       * In this case, the `scrollTarget` will point to the outer div element. 
+       *
+       * ### Document scrolling
+       *
+       * For document scrolling, you can use the reserved word `document`:
+       *
+       *```html
+       * <x-element scroll-target="document">
+       *   <!-- Content -->
+       * </x-element>
+       *```
+       *
+       * ### Elements reference
        *
        *```js
        * appHeader.scrollTarget = document.querySelector('#scrollable-element');
        *```
        * 
        * @type {HTMLElement}
        */
       scrollTarget: {
         type: HTMLElement,
         value: function() {
           return this._defaultScrollTarget;
         }
       }
     },
 
     observers: [
       '_scrollTargetChanged(scrollTarget, isAttached)'
     ],
 
     _scrollTargetChanged: function(scrollTarget, isAttached) {
-      // Remove lister to the current scroll target
+      var eventTarget;
+
       if (this._oldScrollTarget) {
-        if (this._oldScrollTarget === this._doc) {
-          window.removeEventListener('scroll', this._boundScrollHandler);
-        } else if (this._oldScrollTarget.removeEventListener) {
-          this._oldScrollTarget.removeEventListener('scroll', this._boundScrollHandler);
-        }
+        eventTarget = this._oldScrollTarget === this._doc ? window : this._oldScrollTarget;
+        eventTarget.removeEventListener('scroll', this._boundScrollHandler);
         this._oldScrollTarget = null;
       }
-      if (isAttached) {
-        // Support element id references
-        if (typeof scrollTarget === 'string') {
 
-          var host = this.domHost;
-          this.scrollTarget = host && host.$ ? host.$[scrollTarget] : 
-              Polymer.dom(this.ownerDocument).querySelector('#' + scrollTarget);
+      if (!isAttached) {
+        return;
+      }
+      // Support element id references
+      if (scrollTarget === 'document') {
 
-        } else if (this._scrollHandler) {
+        this.scrollTarget = this._doc;
 
-          this._boundScrollHandler = this._boundScrollHandler || this._scrollHandler.bind(this);
-          // Add a new listener
-          if (scrollTarget === this._doc) {
-            window.addEventListener('scroll', this._boundScrollHandler);
-            if (this._scrollTop !== 0 || this._scrollLeft !== 0) {
-              this._scrollHandler();
-            }
-          } else if (scrollTarget && scrollTarget.addEventListener) {
-            scrollTarget.addEventListener('scroll', this._boundScrollHandler);
-          }
-          this._oldScrollTarget = scrollTarget;
-        }
+      } else if (typeof scrollTarget === 'string') {
+
+        this.scrollTarget = this.domHost ? this.domHost.$[scrollTarget] :
+            Polymer.dom(this.ownerDocument).querySelector('#' + scrollTarget);
+
+      } else if (this._isValidScrollTarget()) {
+
+        eventTarget = scrollTarget === this._doc ? window : scrollTarget;
+        this._boundScrollHandler = this._boundScrollHandler || this._scrollHandler.bind(this);
+        this._oldScrollTarget = scrollTarget;
+
+        eventTarget.addEventListener('scroll', this._boundScrollHandler);
       }
     },
 
     /**
-     * Runs on every scroll event. Consumer of this behavior may want to override this method.
+     * Runs on every scroll event. Consumer of this behavior may override this method.
      *
      * @protected
      */
     _scrollHandler: function scrollHandler() {},
 
     /**
      * The default scroll target. Consumers of this behavior may want to customize
      * the default scroll target.
      *
      * @type {Element}
@@ skipping 103 matching lines @@
 
     /**
      * Returns true if the scroll target is a valid HTMLElement.
      *
      * @return {boolean}
      */
     _isValidScrollTarget: function() {
       return this.scrollTarget instanceof HTMLElement;
     }
   };