Bug 1502059 - Don't always cancel a scroll animation when we have a relative scroll offset update. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D9871

--HG--
extra : source : 2a457617fbf2ca133af18e9972e505f86dd90dd2
extra : amend_source : 119ab67b6c2c9a09a98cfb2f4c4ca20f82c64b9f

gfx/layers/FrameMetrics.h

@@ -276,14 +276,28 @@ public:
     mDoSmoothScroll = aOther.mDoSmoothScroll;
   }
 
-  void ApplyRelativeScrollUpdateFrom(const FrameMetrics& aOther)
+  /**
+   * Applies the relative scroll offset update contained in aOther to the
+   * scroll offset contained in this. The scroll delta is clamped to the
+   * scrollable region.
+   *
+   * @returns The clamped scroll offset delta that was applied
+   */
+  CSSPoint ApplyRelativeScrollUpdateFrom(const FrameMetrics& aOther)
   {
     MOZ_ASSERT(aOther.IsRelative());
+    CSSPoint origin = mScrollOffset;
     CSSPoint delta = (aOther.mScrollOffset - aOther.mBaseScrollOffset);
     ClampAndSetScrollOffset(mScrollOffset + delta);
     mScrollGeneration = aOther.mScrollGeneration;
+    return mScrollOffset - origin;
   }
 
+  /**
+   * Applies the relative scroll offset update contained in aOther to the
+   * smooth scroll destination offset contained in this. The scroll delta is
+   * clamped to the scrollable region.
+   */
   void ApplyRelativeSmoothScrollUpdateFrom(const FrameMetrics& aOther)
   {
     MOZ_ASSERT(aOther.IsRelative());

gfx/layers/apz/src/AsyncPanZoomAnimation.h

@@ -31,6 +31,19 @@ public:
   virtual bool DoSample(FrameMetrics& aFrameMetrics,
                         const TimeDuration& aDelta) = 0;
 
+  /**
+   * Attempt to apply a translation to the animation in response to content
+   * providing a relative scroll offset update.
+   *
+   * @param aShiftDelta the amount to translate the animation in app units
+   * @returns Whether the animation was able to translate. If false, the
+   *    animation must be canceled.
+   */
+  virtual bool ApplyContentShift(const CSSPoint& aShiftDelta)
+  {
+    return false;
+  }
+
   bool Sample(FrameMetrics& aFrameMetrics,
               const TimeDuration& aDelta) {
     // In some situations, particularly when handoff is involved, it's possible

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -4380,6 +4380,7 @@ void AsyncPanZoomController::NotifyLayersUpdated(const ScrollMetadata& aScrollMe
       // becomes incorrect for the purposes of calculating the LD transform. To
       // correct this we need to update mExpectedGeckoMetrics to be the
       // last thing we know was painted by Gecko.
+      Maybe<CSSPoint> relativeDelta;
       if (gfxPrefs::APZRelativeUpdate() && aLayerMetrics.IsRelative()) {
         APZC_LOG("%p relative updating scroll offset from %s by %s\n", this,
           ToString(Metrics().GetScrollOffset()).c_str(),
@@ -4395,7 +4396,7 @@ void AsyncPanZoomController::NotifyLayersUpdated(const ScrollMetadata& aScrollMe
           userAction = true;
         }
 
-        Metrics().ApplyRelativeScrollUpdateFrom(aLayerMetrics);
+        relativeDelta = Some(Metrics().ApplyRelativeScrollUpdateFrom(aLayerMetrics));
       } else {
         APZC_LOG("%p updating scroll offset from %s to %s\n", this,
           ToString(Metrics().GetScrollOffset()).c_str(),
@@ -4408,10 +4409,17 @@ void AsyncPanZoomController::NotifyLayersUpdated(const ScrollMetadata& aScrollMe
       mCompositedScrollOffset = Metrics().GetScrollOffset();
       mExpectedGeckoMetrics = aLayerMetrics;
 
-      // Cancel the animation (which might also trigger a repaint request)
-      // after we update the scroll offset above. Otherwise we can be left
-      // in a state where things are out of sync.
-      CancelAnimation();
+      // If we have applied a relative scroll update and a scroll animation is
+      // happening, attempt to apply a content shift and preserve the
+      // animation.
+      if (!mAnimation ||
+          relativeDelta.isNothing() ||
+          !mAnimation->ApplyContentShift(relativeDelta.value())) {
+        // Cancel the animation (which might also trigger a repaint request)
+        // after we update the scroll offset above. Otherwise we can be left
+        // in a state where things are out of sync.
+        CancelAnimation();
+      }
 
       // Since the scroll offset has changed, we need to recompute the
       // displayport margins and send them to layout. Otherwise there might be

gfx/layers/apz/src/AutoscrollAnimation.h

@@ -22,6 +22,13 @@ public:
 
   bool DoSample(FrameMetrics& aFrameMetrics, const TimeDuration& aDelta) override;
 
+  bool ApplyContentShift(const CSSPoint& aShiftDelta) override
+  {
+    // Autoscroll works using screen space coordinates, so there's no work we
+    // need to do to handle a content shift
+    return true;
+  }
+
   void Cancel(CancelAnimationFlags aFlags) override;
 private:
   AsyncPanZoomController& mApzc;

gfx/layers/apz/src/GenericScrollAnimation.cpp

@@ -109,5 +109,12 @@ GenericScrollAnimation::DoSample(FrameMetrics& aFrameMetrics, const TimeDuration
   return !finished;
 }
 
+bool
+GenericScrollAnimation::ApplyContentShift(const CSSPoint& aShiftDelta)
+{
+  mAnimationPhysics->ApplyContentShift(aShiftDelta);
+  return true;
+}
+
 } // namespace layers
 } // namespace mozilla

gfx/layers/apz/src/GenericScrollAnimation.h

@@ -28,6 +28,8 @@ public:
 
   bool DoSample(FrameMetrics& aFrameMetrics, const TimeDuration& aDelta) override;
 
+  bool ApplyContentShift(const CSSPoint& aShiftDelta) override;
+
   void UpdateDelta(TimeStamp aTime,
                    const nsPoint& aDelta,
                    const nsSize& aCurrentVelocity);

layout/generic/ScrollAnimationBezierPhysics.cpp

@@ -53,6 +53,14 @@ ScrollAnimationBezierPhysics::Update(const TimeStamp& aTime,
   mIsFirstIteration = false;
 }
 
+void
+ScrollAnimationBezierPhysics::ApplyContentShift(const CSSPoint& aShiftDelta)
+{
+  nsPoint shiftDelta = CSSPoint::ToAppUnits(aShiftDelta);
+  mStartPos += shiftDelta;
+  mDestination += shiftDelta;
+}
+
 TimeDuration
 ScrollAnimationBezierPhysics::ComputeDuration(const TimeStamp& aTime)
 {

layout/generic/ScrollAnimationBezierPhysics.h

@@ -35,6 +35,8 @@ public:
               const nsPoint& aDestination,
               const nsSize& aCurrentVelocity) override;
 
+  void ApplyContentShift(const CSSPoint& aShiftDelta) override;
+
   // Get the velocity at a point in time in nscoords/sec.
   nsSize VelocityAt(const TimeStamp& aTime) override;
 

layout/generic/ScrollAnimationMSDPhysics.cpp

@@ -48,6 +48,14 @@ ScrollAnimationMSDPhysics::Update(const TimeStamp& aTime,
   mIsFirstIteration = false;
 }
 
+void
+ScrollAnimationMSDPhysics::ApplyContentShift(const CSSPoint& aShiftDelta)
+{
+  nsPoint shiftDelta = CSSPoint::ToAppUnits(aShiftDelta);
+  mStartPos += shiftDelta;
+  mDestination += shiftDelta;
+}
+
 double
 ScrollAnimationMSDPhysics::ComputeSpringConstant(const TimeStamp& aTime)
 {

layout/generic/ScrollAnimationMSDPhysics.h

@@ -25,6 +25,8 @@ public:
               const nsPoint& aDestination,
               const nsSize& aCurrentVelocity) override;
 
+  void ApplyContentShift(const CSSPoint& aShiftDelta) override;
+
   // Get the velocity at a point in time in nscoords/sec.
   nsSize VelocityAt(const TimeStamp& aTime) override;
 

layout/generic/ScrollAnimationPhysics.h

@@ -19,6 +19,8 @@ public:
                       const nsPoint& aDestination,
                       const nsSize& aCurrentVelocity) = 0;
 
+  virtual void ApplyContentShift(const CSSPoint& aShiftDelta) = 0;
+
   // Get the velocity at a point in time in nscoords/sec.
   virtual nsSize VelocityAt(const TimeStamp& aTime) = 0;