fix(debounceTime): unschedule dangling task on unsubscribe before complete

spec/operators/debounceTime-spec.ts

@@ -1,10 +1,12 @@
 /** @prettier */
 import { expect } from 'chai';
-import { of, Subject } from 'rxjs';
-import { debounceTime, mergeMap } from 'rxjs/operators';
+import { NEVER, of, Subject } from 'rxjs';
+import { AnimationFrameAction } from 'rxjs/internal/scheduler/AnimationFrameAction';
+import { AnimationFrameScheduler } from 'rxjs/internal/scheduler/AnimationFrameScheduler';
+import { debounceTime, mergeMap, startWith } from 'rxjs/operators';
 import { TestScheduler } from 'rxjs/testing';
-import { observableMatcher } from '../helpers/observableMatcher';
 import { VirtualTimeScheduler } from '../../src/internal/scheduler/VirtualTimeScheduler';
+import { observableMatcher } from '../helpers/observableMatcher';
 
 /** @test {debounceTime} */
 describe('debounceTime', () => {
@@ -221,4 +223,21 @@ describe('debounceTime', () => {
 
     expect(results).to.deep.equal([1, 2]);
   });
+
+  it('should unsubscribe from the scheduled debounce action when downstream unsubscribes', () => {
+    const scheduler = new AnimationFrameScheduler(AnimationFrameAction);
+
+    expect(scheduler._scheduled).to.not.exist;
+    expect(scheduler.actions).to.be.empty;
+
+    const subscription = NEVER.pipe(startWith(1), debounceTime(0, scheduler)).subscribe();
+
+    expect(scheduler._scheduled).to.exist;
+    expect(scheduler.actions.length).to.equal(1);
+
+    subscription.unsubscribe();
+
+    expect(scheduler._scheduled).to.not.exist;
+    expect(scheduler.actions).to.be.empty;
+  });
 });

src/internal/operators/debounceTime.ts

@@ -86,6 +86,7 @@ export function debounceTime<T>(dueTime: number, scheduler: SchedulerLike = asyn
       if (now < targetTime) {
         // On that case, re-schedule to the new target
         activeTask = this.schedule(undefined, targetTime - now);
+        subscriber.add(activeTask);
         return;
       }
 
@@ -102,6 +103,7 @@ export function debounceTime<T>(dueTime: number, scheduler: SchedulerLike = asyn
           // Only set up a task if it's not already up
           if (!activeTask) {
             activeTask = scheduler.schedule(emitWhenIdle, dueTime);
+            subscriber.add(activeTask);
           }
         },
         () => {