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diff --git a/dom/animation/CSSTransition.h b/dom/animation/CSSTransition.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_dom_CSSTransition_h
+#define mozilla_dom_CSSTransition_h
+
+#include "mozilla/ComputedTiming.h"
+#include "mozilla/dom/Animation.h"
+#include "mozilla/AnimatedPropertyID.h"
+#include "mozilla/StyleAnimationValue.h"
+#include "AnimationCommon.h"
+
+class nsIGlobalObject;
+
+namespace mozilla {
+namespace dom {
+
+class CSSTransition final : public Animation {
+ public:
+  explicit CSSTransition(nsIGlobalObject* aGlobal,
+                         const AnimatedPropertyID& aProperty)
+      : Animation(aGlobal),
+        mPreviousTransitionPhase(TransitionPhase::Idle),
+        mNeedsNewAnimationIndexWhenRun(false),
+        mTransitionProperty(aProperty) {}
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  CSSTransition* AsCSSTransition() override { return this; }
+  const CSSTransition* AsCSSTransition() const override { return this; }
+
+  // CSSTransition interface
+  void GetTransitionProperty(nsString& aRetVal) const;
+
+  // Animation interface overrides
+  AnimationPlayState PlayStateFromJS() const override;
+  bool PendingFromJS() const override;
+  void PlayFromJS(ErrorResult& aRv) override;
+
+  // A variant of Play() that avoids posting style updates since this method
+  // is expected to be called whilst already updating style.
+  void PlayFromStyle() {
+    ErrorResult rv;
+    PlayNoUpdate(rv, Animation::LimitBehavior::Continue);
+    // play() should not throw when LimitBehavior is Continue
+    MOZ_ASSERT(!rv.Failed(), "Unexpected exception playing transition");
+  }
+
+  void CancelFromStyle(PostRestyleMode aPostRestyle) {
+    // The animation index to use for compositing will be established when
+    // this transition next transitions out of the idle state but we still
+    // update it now so that the sort order of this transition remains
+    // defined until that moment.
+    //
+    // See longer explanation in CSSAnimation::CancelFromStyle.
+    mAnimationIndex = sNextAnimationIndex++;
+    mNeedsNewAnimationIndexWhenRun = true;
+
+    Animation::Cancel(aPostRestyle);
+
+    // It is important we do this *after* calling Cancel().
+    // This is because Cancel() will end up posting a restyle and
+    // that restyle should target the *transitions* level of the cascade.
+    // However, once we clear the owning element, CascadeLevel() will begin
+    // returning CascadeLevel::Animations.
+    mOwningElement = OwningElementRef();
+  }
+
+  void SetEffectFromStyle(KeyframeEffect*);
+
+  void Tick(TickState&) override;
+
+  const AnimatedPropertyID& TransitionProperty() const;
+  AnimationValue ToValue() const;
+
+  bool HasLowerCompositeOrderThan(const CSSTransition& aOther) const;
+  EffectCompositor::CascadeLevel CascadeLevel() const override {
+    return IsTiedToMarkup() ? EffectCompositor::CascadeLevel::Transitions
+                            : EffectCompositor::CascadeLevel::Animations;
+  }
+
+  void SetCreationSequence(uint64_t aIndex) {
+    MOZ_ASSERT(IsTiedToMarkup());
+    mAnimationIndex = aIndex;
+  }
+
+  // Sets the owning element which is used for determining the composite
+  // oder of CSSTransition objects generated from CSS markup.
+  //
+  // @see mOwningElement
+  void SetOwningElement(const OwningElementRef& aElement) {
+    mOwningElement = aElement;
+  }
+  // True for transitions that are generated from CSS markup and continue to
+  // reflect changes to that markup.
+  bool IsTiedToMarkup() const { return mOwningElement.IsSet(); }
+
+  // Return the animation current time based on a given TimeStamp, a given
+  // start time and a given playbackRate on a given timeline.  This is useful
+  // when we estimate the current animated value running on the compositor
+  // because the animation on the compositor may be running ahead while
+  // main-thread is busy.
+  static Nullable<TimeDuration> GetCurrentTimeAt(
+      const AnimationTimeline& aTimeline, const TimeStamp& aBaseTime,
+      const TimeDuration& aStartTime, double aPlaybackRate);
+
+  void MaybeQueueCancelEvent(const StickyTimeDuration& aActiveTime) override {
+    QueueEvents(aActiveTime);
+  }
+
+  // Compute the portion of the *value* space that we should be through
+  // at the current time.  (The input to the transition timing function
+  // has time units, the output has value units.)
+  double CurrentValuePortion() const;
+
+  const AnimationValue& StartForReversingTest() const {
+    return mStartForReversingTest;
+  }
+  double ReversePortion() const { return mReversePortion; }
+
+  void SetReverseParameters(AnimationValue&& aStartForReversingTest,
+                            double aReversePortion) {
+    mStartForReversingTest = std::move(aStartForReversingTest);
+    mReversePortion = aReversePortion;
+  }
+
+  struct ReplacedTransitionProperties {
+    TimeDuration mStartTime;
+    double mPlaybackRate;
+    TimingParams mTiming;
+    Maybe<StyleComputedTimingFunction> mTimingFunction;
+    AnimationValue mFromValue, mToValue;
+  };
+  void SetReplacedTransition(
+      ReplacedTransitionProperties&& aReplacedTransition) {
+    mReplacedTransition.emplace(std::move(aReplacedTransition));
+  }
+
+  // For a new transition interrupting an existing transition on the
+  // compositor, update the start value to match the value of the replaced
+  // transitions at the current time.
+  void UpdateStartValueFromReplacedTransition();
+
+ protected:
+  virtual ~CSSTransition() {
+    MOZ_ASSERT(!mOwningElement.IsSet(),
+               "Owning element should be cleared "
+               "before a CSS transition is destroyed");
+  }
+
+  // Animation overrides
+  void UpdateTiming(SeekFlag aSeekFlag,
+                    SyncNotifyFlag aSyncNotifyFlag) override;
+
+  void QueueEvents(const StickyTimeDuration& activeTime = StickyTimeDuration());
+
+  enum class TransitionPhase;
+
+  // The (pseudo-)element whose computed transition-property refers to this
+  // transition (if any).
+  //
+  // This is used for determining the relative composite order of transitions
+  // generated from CSS markup.
+  //
+  // Typically this will be the same as the target element of the keyframe
+  // effect associated with this transition. However, it can differ in the
+  // following circumstances:
+  //
+  // a) If script removes or replaces the effect of this transition,
+  // b) If this transition is cancelled (e.g. by updating the
+  //    transition-property or removing the owning element from the document),
+  // c) If this object is generated from script using the CSSTransition
+  //    constructor.
+  //
+  // For (b) and (c) the owning element will return !IsSet().
+  OwningElementRef mOwningElement;
+
+  // The 'transition phase' used to determine which transition events need
+  // to be queued on this tick.
+  // See: https://drafts.csswg.org/css-transitions-2/#transition-phase
+  enum class TransitionPhase {
+    Idle = static_cast<int>(ComputedTiming::AnimationPhase::Idle),
+    Before = static_cast<int>(ComputedTiming::AnimationPhase::Before),
+    Active = static_cast<int>(ComputedTiming::AnimationPhase::Active),
+    After = static_cast<int>(ComputedTiming::AnimationPhase::After),
+    Pending
+  };
+  TransitionPhase mPreviousTransitionPhase;
+
+  // When true, indicates that when this transition next leaves the idle state,
+  // its animation index should be updated.
+  bool mNeedsNewAnimationIndexWhenRun;
+
+  // Store the transition property and to-value here since we need that
+  // information in order to determine if there is an existing transition
+  // for a given style change. We can't store that information on the
+  // effect however since it can be replaced using the Web Animations API.
+  AnimatedPropertyID mTransitionProperty;
+  AnimationValue mTransitionToValue;
+
+  // This is the start value to be used for a check for whether a
+  // transition is being reversed.  Normally the same as
+  // mEffect->mProperties[0].mSegments[0].mFromValue, except when this
+  // transition started as the reversal of another in-progress transition
+  // or when the effect has been mutated using the Web Animations API.
+  //
+  // Needed so we can handle two reverses in a row.
+  AnimationValue mStartForReversingTest;
+
+  // Likewise, the portion (in value space) of the "full" reversed
+  // transition that we're actually covering.  For example, if a :hover
+  // effect has a transition that moves the element 10px to the right
+  // (by changing 'left' from 0px to 10px), and the mouse moves in to
+  // the element (starting the transition) but then moves out after the
+  // transition has advanced 4px, the second transition (from 10px/4px
+  // to 0px) will have mReversePortion of 0.4.  (If the mouse then moves
+  // in again when the transition is back to 2px, the mReversePortion
+  // for the third transition (from 0px/2px to 10px) will be 0.8.
+  double mReversePortion = 1.0;
+
+  Maybe<ReplacedTransitionProperties> mReplacedTransition;
+};
+
+}  // namespace dom
+
+}  // namespace mozilla
+
+#endif  // mozilla_dom_CSSTransition_h