fix(infinitescroll): make sure more data is always loaded if scrolled to top/bottom quickly (#5183)

Author: Leah Bentley

src/features/infinite-scroll/js/infinite-scroll.js

@@ -289,17 +289,28 @@
         }
 
         if (args.y) {
-          var percentage;
-          var targetPercentage = args.grid.options.infiniteScrollRowsFromEnd / args.grid.renderContainers.body.visibleRowCache.length;
-          if (args.grid.scrollDirection === uiGridConstants.scrollDirection.UP ) {
-            percentage = args.y.percentage;
-            if (percentage <= targetPercentage){
-              service.loadData(args.grid);
-            }
-          } else if (args.grid.scrollDirection === uiGridConstants.scrollDirection.DOWN) {
-            percentage = 1 - args.y.percentage;
-            if (percentage <= targetPercentage){
-              service.loadData(args.grid);
+
+          // If the user is scrolling very quickly all the way to the top/bottom, the scroll handler can get confused
+          // about the direction. First we check if they've gone all the way, and data always is loaded in this case.
+          if (args.y.percentage === 0) {
+            args.grid.scrollDirection = uiGridConstants.scrollDirection.UP;
+            service.loadData(args.grid);
+          } else if (args.y.percentage === 1) {
+            args.grid.scrollDirection = uiGridConstants.scrollDirection.DOWN;
+            service.loadData(args.grid);
+          } else { // Scroll position is somewhere in between top/bottom, so determine whether it's far enough to load more data.
+            var percentage;
+            var targetPercentage = args.grid.options.infiniteScrollRowsFromEnd / args.grid.renderContainers.body.visibleRowCache.length;
+            if (args.grid.scrollDirection === uiGridConstants.scrollDirection.UP ) {
+              percentage = args.y.percentage;
+              if (percentage <= targetPercentage){
+                service.loadData(args.grid);
+              }
+            } else if (args.grid.scrollDirection === uiGridConstants.scrollDirection.DOWN) {
+              percentage = 1 - args.y.percentage;
+              if (percentage <= targetPercentage){
+                service.loadData(args.grid);
+              }
             }
           }
         }