diff --git a/projects/igniteui-angular/grids/grid/src/grid-base.directive.ts b/projects/igniteui-angular/grids/grid/src/grid-base.directive.ts index 04af42f9e8d..456f2214e8b 100644 --- a/projects/igniteui-angular/grids/grid/src/grid-base.directive.ts +++ b/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 ) diff --git a/projects/igniteui-angular/grids/grid/src/grid.component.spec.ts b/projects/igniteui-angular/grids/grid/src/grid.component.spec.ts index f8ce0013662..33381e3ac72 100644 --- a/projects/igniteui-angular/grids/grid/src/grid.component.spec.ts +++ b/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(); + }); }); });