Use set and clear debouncer upon completion. Fixes #5250.

lib/utils/debounce.js

@@ -47,6 +47,7 @@ export class Debouncer {
     if (this.isActive()) {
       this._asyncModule.cancel(/** @type {number} */(this._timer));
       this._timer = null;
+      debouncerQueue.delete(this);
     }
   }
   /**
@@ -112,3 +113,38 @@ export class Debouncer {
     return debouncer;
   }
 }
+
+let debouncerQueue = new Set();
+
+/**
+ * Adds a `Debouncer` to a list of globally flushable tasks.
+ *
+ * @param {!Debouncer} debouncer Debouncer to enqueue
+ * @return {void}
+ */
+export const enqueueDebouncer = function(debouncer) {
+  if (debouncerQueue.has(debouncer)) {
+    debouncerQueue.delete(debouncer);
+  }
+  debouncerQueue.add(debouncer);
+};
+
+/**
+ * Flushes any enqueued debouncers
+ *
+ * @return {void}
+ */
+export const flushDebouncers = function() {
+  const didFlush = Boolean(debouncerQueue.size);
+  debouncerQueue.forEach(debouncer => {
+    try {
+      debouncer.flush();
+    } catch(e) {
+      setTimeout(() => {
+        throw e;
+      });
+    }
+  });
+  debouncerQueue = new Set();
+  return didFlush;
+};

lib/utils/flush.js

@@ -11,32 +11,8 @@ import './boot.js';
 /* eslint-disable no-unused-vars */
 import { Debouncer } from '../utils/debounce.js';  // used in type annotations
 /* eslint-enable no-unused-vars */
-
-let debouncerQueue = [];
-
-/**
- * Adds a `Debouncer` to a list of globally flushable tasks.
- *
- * @param {!Debouncer} debouncer Debouncer to enqueue
- * @return {void}
- */
-export const enqueueDebouncer = function(debouncer) {
-  debouncerQueue.push(debouncer);
-};
-
-function flushDebouncers() {
-  const didFlush = Boolean(debouncerQueue.length);
-  while (debouncerQueue.length) {
-    try {
-      debouncerQueue.shift().flush();
-    } catch(e) {
-      setTimeout(() => {
-        throw e;
-      });
-    }
-  }
-  return didFlush;
-}
+import { flushDebouncers } from '../utils/debounce.js';  // used in type annotations
+export { enqueueDebouncer } from '../utils/debounce.js';  // used in type annotations
 
 /**
  * Forces several classes of asynchronously queued tasks to flush:

test/unit/debounce.html

@@ -33,6 +33,7 @@
 import { Polymer } from '../../polymer-legacy.js';
 import { Debouncer } from '../../lib/utils/debounce.js';
 import { microTask, timeOut, animationFrame, idlePeriod } from '../../lib/utils/async.js';
+import { enqueueDebouncer, flush } from '../../lib/utils/flush.js';
 Polymer({is: 'x-basic'});
 
 suite('debounce', function() {
@@ -209,6 +210,53 @@
     });
 
   });
+
+  suite('enqueueDebouncer & flush', function() {
+    function testEnqueue(shouldFlush, done) {
+      // Longer-running debouncer
+      const timeoutCallback = sinon.spy(() => actualCallbacks.push(timeoutCallback));
+      enqueueDebouncer(Debouncer.debounce(null, timeOut, timeoutCallback));
+      // Set of short-running debouncers enqueued in the middle of first set
+      const nestedCallbacks = new Array(150).fill().map((_, i) => sinon.spy(() =>
+        actualCallbacks.push(nestedCallbacks[i])));
+      // First set of short-running debouncers
+      const microtaskCallbacks = new Array(150).fill().map((_, i) => sinon.spy(() => {
+        actualCallbacks.push(microtaskCallbacks[i]);
+        if (i === 125) {
+          nestedCallbacks.forEach(cb => 
+            enqueueDebouncer(Debouncer.debounce(null, microTask, cb)));
+        }
+      }));
+      microtaskCallbacks.forEach(cb => 
+        enqueueDebouncer(Debouncer.debounce(null, microTask, cb)));
+      // Expect short before long
+      let expectedCallbacks;
+      const actualCallbacks = [];
+      const verify = () => {
+        actualCallbacks.forEach(cb => assert.isTrue(cb.calledOnce));
+        assert.deepEqual(expectedCallbacks, actualCallbacks);
+        done();
+      };
+      if (shouldFlush) {
+        expectedCallbacks = [timeoutCallback, ...microtaskCallbacks, ...nestedCallbacks];
+        flush();
+        // When flushing, order is order of enqueing
+        verify();
+      } else {
+        expectedCallbacks = [...microtaskCallbacks, ...nestedCallbacks, timeoutCallback];
+        timeOut.run(verify);
+      }
+    }
+
+    test('non-flushed', function(done) {
+      testEnqueue(false, done);
+    });
+
+    test('flushed', function(done) {
+      testEnqueue(true, done);
+    });
+
+  });
 });
 </script>
 </body>