fix(grid): restore trailing scroll emission so top rows aren't lost after fast scroll

projects/igniteui-angular/grids/grid/src/grid-base.directive.ts

@@ -3698,7 +3698,12 @@ export abstract class IgxGridBaseDirective implements GridType,
                 this.throttleTime$.pipe(
                     take(1),
                     switchMap(time => timer(time, this.throttleScheduler))
-                )
+                ),
+                // `trailing: true` ensures the final settle position of a fast momentum
+                // scroll is processed; otherwise the last scroll events are dropped and the
+                // rows stay frozen at an intermediate startIndex while the scrollbar is at top.
+                // `leading: true` keeps the immediate response on scroll start.
+                { leading: true, trailing: true }
             ),
             destructor
         )

projects/igniteui-angular/grids/grid/src/grid.component.spec.ts

@@ -2161,6 +2161,38 @@ describe('IgxGrid Component Tests #grid', () => {
 
                 expect(grid.headerContainer.state.startIndex).toBeGreaterThan(0);
             });
+
+            it('should settle at the top row when a fast momentum scroll ends at scrollTop 0 (throttle trailing edge)', async () => {
+                const fix = TestBed.createComponent(ZonelessTallGridComponent);
+                fix.detectChanges();
+                await fix.whenStable();
+                const grid = fix.componentInstance.grid;
+                const virtDir = grid.verticalScrollContainer;
+                const scrollEl = virtDir.getScroll();
+                const maxScroll = scrollEl.scrollHeight - scrollEl.clientHeight;
+
+                // Move away from the top so the first rows are virtualized out of view.
+                grid.scrollNotify.next({ target: { scrollTop: maxScroll } });
+                await wait(50);
+                await fix.whenStable();
+                expect(virtDir.state.startIndex).toBeGreaterThan(0);
+
+                // Simulate a fast momentum/inertia scroll back to the top: an intermediate
+                // position lands on the throttle's leading edge and the scrollTop = 0 settle
+                // arrives within the same throttle window, so it can only be delivered on the
+                // trailing edge. Feeding scrollNotify directly (instead of setting scrollTop)
+                // avoids the browser's async native scroll events - which all read the final
+                // scrollTop of 0 - from masking a dropped-trailing regression.
+                grid.scrollNotify.next({ target: { scrollTop: Math.round(maxScroll / 2) } });
+                grid.scrollNotify.next({ target: { scrollTop: 0 } });
+                await wait(50);
+                await fix.whenStable();
+
+                // Without the trailing edge the settle event is dropped and the grid stays
+                // frozen at an intermediate startIndex while the scrollbar sits at the top.
+                expect(virtDir.state.startIndex).toBe(0);
+                expect(grid.gridAPI.get_row_by_index(0)).toBeDefined();
+            });
         });
     });