fix(grid): reset scrollAmount if bar hides #6627

Author: hanastasov

projects/igniteui-angular/src/lib/directives/for-of/for_of.directive.ts

@@ -1170,15 +1170,18 @@ export class IgxForOfDirective<T> implements OnInit, OnChanges, DoCheck, OnDestr
             this.scrollComponent.size = totalWidth;
             if (totalWidth <= parseInt(this.igxForContainerSize, 10)) {
                 this.scrollPosition = 0;
+                // Need to reset the scrollAmount value here, because horizontalScrollBar is hidden, therefore
+                // onScroll event handler for VirtualHelperBaseDirective will not be called
+                this.scrollComponent.scrollAmount = 0;
             }
         }
         if (this.igxForScrollOrientation === 'vertical') {
             this.scrollComponent.nativeElement.style.height = parseInt(this.igxForContainerSize, 10) + 'px';
             this.scrollComponent.size = this._calcHeight();
             if ( this.scrollComponent.size <= parseInt(this.igxForContainerSize, 10)) {
                 this.scrollPosition = 0;
-                // Need to reset the scrollAmount value here, because
-                // Firefox will not fire the scrollComponent scroll event handler
+                // Need to reset the scrollAmount value here, because verticalScrollBar is hidden, therefore
+                // onScroll event handler for VirtualHelperBaseDirective will not be called
                 this.scrollComponent.scrollAmount = 0;
             }
         }