fix(infinitescroll): always check on scroll change

Previously infinite scroll would only do checks when it got to
scrollTop it’s never seen before. However, this then requires a reset()
function if the underlying data changes, which would become an API
gotcha that would be a common source of problems for developers. Always
checking on scroll change should have little performance impact since
it was always checking while scrolling down anyway, it just wasn’t
checking when scrolling up. Now it always checks.

ionic/components/infinite-scroll/infinite-scroll.ts

@@ -168,12 +168,6 @@ export class InfiniteScroll {
 
     let d = this._content.getContentDimensions();
 
-    if (d.scrollTop <= this._highestY) {
-      // don't bother if scrollY is less than the highest Y seen
-      return 4;
-    }
-    this._highestY = d.scrollTop;
-
     let reloadY = d.contentHeight;
     if (this._thrPc) {
       reloadY += (reloadY * this._thrPc);
@@ -213,17 +207,15 @@ export class InfiniteScroll {
    * trying to receive new data while scrolling. This method is useful
    * when it is known that there is no more data that can be added, and
    * the infinite scroll is no longer needed.
-   * @param {boolean} shouldEnable  If the infinite scroll should be enabled or not. Setting to `false` will remove scroll event listeners and hide the display.
+   * @param {boolean} shouldEnable  If the infinite scroll should be
+   * enabled or not. Setting to `false` will remove scroll event listeners
+   * and hide the display.
    */
   enable(shouldEnable: boolean) {
     this.state = (shouldEnable ? STATE_ENABLED : STATE_DISABLED);
     this._setListeners(shouldEnable);
   }
 
-  resetHighestY() {
-    this._highestY = 0;
-  }
-
   private _setListeners(shouldListen: boolean) {
     if (this._init) {
       if (shouldListen) {

ionic/components/infinite-scroll/test/infinite-scroll.spec.ts

@@ -11,7 +11,6 @@ describe('Infinite Scroll', () => {
       content.getContentDimensions = function() {
         return { scrollHeight: 1000, scrollTop: 350, contentHeight: 500 };
       };
-      inf._highestY = 0;
       inf.threshold = '100px';
 
       setInfiniteScrollTop(300);
@@ -25,7 +24,6 @@ describe('Infinite Scroll', () => {
       content.getContentDimensions = function() {
         return { scrollHeight: 1000, scrollTop: 500, contentHeight: 500 };
       };
-      inf._highestY = 0;
       inf.threshold = '100px';
 
       setInfiniteScrollTop(300);
@@ -34,18 +32,6 @@ describe('Infinite Scroll', () => {
       expect(result).toEqual(5);
     });
 
-    it('should not continue if the scrolltop is <= the highest Y', () => {
-      inf._highestY = 100;
-      setInfiniteScrollTop(50);
-      setInfiniteScrollHeight(100);
-      content.getContentDimensions = function() {
-        return { scrollTop: 50 };
-      };
-
-      var result = inf._onScroll(scrollEv());
-      expect(result).toEqual(4);
-    });
-
     it('should not run if there is not infinite element height', () => {
       setInfiniteScrollTop(0);
       var result = inf._onScroll(scrollEv());