fix(module:back-top): fix more reliable scrolling listener (#7208)

- close #7199

Author: cipchk

components/back-top/back-top.component.ts

@@ -24,7 +24,7 @@ import {
   ViewEncapsulation
 } from '@angular/core';
 import { fromEvent, Subject } from 'rxjs';
-import { takeUntil, throttleTime } from 'rxjs/operators';
+import { debounceTime, takeUntil } from 'rxjs/operators';
 
 import { fadeMotion } from 'ng-zorro-antd/core/animation';
 import { NzConfigKey, NzConfigService, WithConfig } from 'ng-zorro-antd/core/config';
@@ -127,7 +127,7 @@ export class NzBackTopComponent implements OnInit, OnDestroy, OnChanges {
     this.handleScroll();
     this.zone.runOutsideAngular(() => {
       fromEvent(this.getTarget(), 'scroll')
-        .pipe(throttleTime(50), takeUntil(this.scrollListenerDestroy$))
+        .pipe(debounceTime(50), takeUntil(this.scrollListenerDestroy$))
         .subscribe(() => this.handleScroll());
     });
   }

components/back-top/back-top.spec.ts

@@ -162,10 +162,10 @@ describe('Component:nz-back-top', () => {
     }));
 
     it('element scroll shows the button', fakeAsync(() => {
-      const throttleTime = 50;
+      const time = 50;
 
       componentObject.scrollTo(fakeTarget, defaultVisibilityHeight + 1);
-      tick(throttleTime + 1);
+      tick(time + 1);
       fixture.detectChanges();
 
       expect(componentObject.backTopButton() === null).toBe(false);
@@ -174,10 +174,10 @@ describe('Component:nz-back-top', () => {
     it('element (use string id) scroll shows the button', fakeAsync(() => {
       component.nzTarget = '#fakeTarget';
 
-      const throttleTime = 50;
+      const time = 50;
 
       componentObject.scrollTo(fakeTarget, defaultVisibilityHeight + 1);
-      tick(throttleTime + 1);
+      tick(time + 1);
       fixture.detectChanges();
 
       expect(componentObject.backTopButton() === null).toBe(false);