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diff --git a/dom/animation/PendingAnimationTracker.h b/dom/animation/PendingAnimationTracker.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_PendingAnimationTracker_h
+#define mozilla_PendingAnimationTracker_h
+
+#include "mozilla/dom/Animation.h"
+#include "mozilla/TypedEnumBits.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsTHashSet.h"
+
+class nsIFrame;
+
+namespace mozilla {
+
+namespace dom {
+class Document;
+}
+
+/**
+ * Handle the pending animations which use document-timeline or null-timeline
+ * while playing or pausing.
+ */
+class PendingAnimationTracker final {
+ public:
+  explicit PendingAnimationTracker(dom::Document* aDocument);
+
+  NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(PendingAnimationTracker)
+  NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS(PendingAnimationTracker)
+
+  void AddPlayPending(dom::Animation& aAnimation) {
+    // We'd like to assert here that IsWaitingToPause(aAnimation) is false but
+    // if |aAnimation| was tracked here as a pause-pending animation when it was
+    // removed from |mDocument|, then re-attached to |mDocument|, and then
+    // played again, we could end up here with IsWaitingToPause returning true.
+    //
+    // However, that should be harmless since all it means is that we'll call
+    // Animation::TriggerOnNextTick or Animation::TriggerNow twice, both of
+    // which will handle the redundant call gracefully.
+    AddPending(aAnimation, mPlayPendingSet);
+    mHasPlayPendingGeometricAnimations = CheckState::Indeterminate;
+  }
+  void RemovePlayPending(dom::Animation& aAnimation) {
+    RemovePending(aAnimation, mPlayPendingSet);
+    mHasPlayPendingGeometricAnimations = CheckState::Indeterminate;
+  }
+  bool IsWaitingToPlay(const dom::Animation& aAnimation) const {
+    return IsWaiting(aAnimation, mPlayPendingSet);
+  }
+
+  void AddPausePending(dom::Animation& aAnimation) {
+    // As with AddPausePending, we'd like to assert that
+    // IsWaitingToPlay(aAnimation) is false but there are some circumstances
+    // where this can be true. Fortunately adding the animation to both pending
+    // sets should be harmless.
+    AddPending(aAnimation, mPausePendingSet);
+  }
+  void RemovePausePending(dom::Animation& aAnimation) {
+    RemovePending(aAnimation, mPausePendingSet);
+  }
+  bool IsWaitingToPause(const dom::Animation& aAnimation) const {
+    return IsWaiting(aAnimation, mPausePendingSet);
+  }
+
+  void TriggerPendingAnimationsOnNextTick(const TimeStamp& aReadyTime);
+  void TriggerPendingAnimationsNow();
+  bool HasPendingAnimations() const {
+    return mPlayPendingSet.Count() > 0 || mPausePendingSet.Count() > 0;
+  }
+
+  /**
+   * Looks amongst the set of play-pending animations, and, if there are
+   * animations that affect geometric properties, notifies all play-pending
+   * animations so that they can be synchronized, if needed.
+   */
+  void MarkAnimationsThatMightNeedSynchronization();
+
+ private:
+  ~PendingAnimationTracker() = default;
+
+  void EnsurePaintIsScheduled();
+
+  using AnimationSet = nsTHashSet<nsRefPtrHashKey<dom::Animation>>;
+
+  void AddPending(dom::Animation& aAnimation, AnimationSet& aSet);
+  void RemovePending(dom::Animation& aAnimation, AnimationSet& aSet);
+  bool IsWaiting(const dom::Animation& aAnimation,
+                 const AnimationSet& aSet) const;
+
+  AnimationSet mPlayPendingSet;
+  AnimationSet mPausePendingSet;
+  RefPtr<dom::Document> mDocument;
+
+ public:
+  enum class CheckState {
+    Indeterminate = 0,
+    Absent = 1 << 0,
+    AnimationsPresent = 1 << 1,
+    TransitionsPresent = 1 << 2,
+  };
+
+ private:
+  CheckState mHasPlayPendingGeometricAnimations = CheckState::Indeterminate;
+};
+
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(PendingAnimationTracker::CheckState)
+
+}  // namespace mozilla
+
+#endif  // mozilla_PendingAnimationTracker_h