Angular Jest Test Directive with @HostLinener('window:scroll')

Problem: I want to test a directive which is supposed to change an element's styling after an 'window:scroll' event but don't know how to trigger the event

The Directive:

@Directive({
    selector: '[directiveSelector]',
})
export class Directive {
    private breakpointOffset: number;
    private fixed = false;

    constructor(
        private readonly elementRef: ElementRef,
    ) {}

    ngAfterViewInit() {
            //fix the element when scrollTop reaches past element's offsetTop
            this.breakpointOffset = this.elementRef.nativeElement.offsetTop;
    }

    @HostListener('window:scroll')
    addScrollTrigger() {
        if (!this.fixed && window.pageYOffset > this.breakpointOffset) {
            this.elementRef.nativeElement.setAttribute(
                'style',
                'position: fixed;
            );
            this.fixed = true;
        }

        if (this.fixed && window.pageYOffset <= this.breakpointOffset) {
            this.elementRef.nativeElement.setAttribute('style', '');
            this.fixed = false;
        }
    }
}

What I tried:

@Component({
    template: ` <div #container directive></div> `,
})
class TestComponent {
    @ViewChild('container') container: ElementRef;
}

describe('Directive', () => {
    let fixture: ComponentFixture<TestComponent>;
    let component: TestComponent;

    beforeEach(async(() => {
        TestBed.configureTestingModule({
            declarations: [TestComponent, Directive],
        }).compileComponents();

        fixture = TestBed.createComponent(TestComponent);
        component = fixture.componentInstance;
    }));

    describe('addScrollTrigger', () => {
        it('sets fixed style', () => {
            //Trigger scroll event?

            expect(component.container.nativeElement.styles).toEqual(....);
        });
    });
});

Within my test I tried to create a component with a template that has a div with that directive. What I am not able to do is trigger the scroll-event because I have to direct access to the directive and can not call the "addScrollTrigger"-function, can I?