fix aurelia#87: Adding items on top when scrolled down is not working…

… correctly

After scrolling down inserting items at the top doesn't work as
expected, as it's only making the top buffer bigger. By triggering the
scrolling mechanism, the appropriate items are rendered.

src/array-virtual-repeat-strategy.js

@@ -239,6 +239,7 @@ export class ArrayVirtualRepeatStrategy extends ArrayRepeatStrategy {
   _handleAddedSplices(repeat: VirtualRepeat, array: Array<any>, splices: any): void {
     let arrayLength = array.length;
     let viewSlot = repeat.viewSlot;
+    let hasInsertedBeforeViewSlot = false;
     for (let i = 0, ii = splices.length; i < ii; ++i) {
       let splice = splices[i];
       let addIndex = splice.index;
@@ -262,12 +263,21 @@ export class ArrayVirtualRepeatStrategy extends ArrayRepeatStrategy {
           }
         } else if (this._isIndexBeforeViewSlot(repeat, viewSlot, addIndex)) {
           repeat._topBufferHeight = repeat._topBufferHeight + repeat.itemHeight;
+          // As we're adding an item before the view slots, this can cause the buffer to run out of rendered items. 
+          // By triggering the scrolling mechanism (after setting the previousFirst & lastRebind) the appropriate items are rendered. 
+          hasInsertedBeforeViewSlot = true;
+          repeat._previousFirst = repeat._previousFirst + 1;
+          repeat._lastRebind = repeat._lastRebind + 1;
         } else if (this._isIndexAfterViewSlot(repeat, viewSlot, addIndex)) {
           repeat._bottomBufferHeight = repeat._bottomBufferHeight + repeat.itemHeight;
           repeat.isLastIndex = false;
         }
       }
     }
     repeat._adjustBufferHeights();
+
+    if (hasInsertedBeforeViewSlot) {
+      repeat._handleScroll();
+    }
   }
 }