Bug 1502614 - Continue autoscroll animation after any main-thread scroll update (absolute or relative). r=rhunt

This matches the main thread behaviour prior to implementing APZ autoscrolling.

Differential Revision: https://phabricator.services.mozilla.com/D10778

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/src/AsyncPanZoomAnimation.h

@@ -33,14 +33,20 @@ public:
                         const TimeDuration& aDelta) = 0;
 
   /**
-   * Attempt to apply a translation to the animation in response to content
-   * providing a relative scroll offset update.
+   * Attempt to handle a main-thread scroll offset update without cancelling
+   * the animation. This may or may not make sense depending on the type of
+   * the animation and whether the scroll update is relative or absolute.
    *
-   * @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.
+   * If the scroll update is relative, |aRelativeDelta| will contain the
+   * delta of the relative update. If the scroll update is absolute,
+   * |aRelativeDelta| will be Nothing() (the animation can check the APZC's
+   * FrameMetrics for the new absolute scroll offset if it wants to handle
+   * and absolute update).
+   *
+   * Returns whether the animation could handle the scroll update. If the
+   * return value is false, the animation will be cancelled.
    */
-  virtual bool ApplyContentShift(const CSSPoint& aShiftDelta)
+  virtual bool HandleScrollOffsetUpdate(const Maybe<CSSPoint>& aRelativeDelta)
   {
     return false;
   }

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -4409,12 +4409,11 @@ void AsyncPanZoomController::NotifyLayersUpdated(const ScrollMetadata& aScrollMe
       mCompositedScrollOffset = Metrics().GetScrollOffset();
       mExpectedGeckoMetrics = aLayerMetrics;
 
-      // 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 an animation is underway, tell it about the scroll offset update.
+      // Some animations can handle some scroll offset updates and continue
+      // running. Those that can't will return false, and we cancel them.
       if (!mAnimation ||
-          relativeDelta.isNothing() ||
-          !mAnimation->ApplyContentShift(relativeDelta.value())) {
+          !mAnimation->HandleScrollOffsetUpdate(relativeDelta)) {
         // 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.

gfx/layers/apz/src/AutoscrollAnimation.h

@@ -22,10 +22,10 @@ public:
 
   bool DoSample(FrameMetrics& aFrameMetrics, const TimeDuration& aDelta) override;
 
-  bool ApplyContentShift(const CSSPoint& aShiftDelta) override
+  bool HandleScrollOffsetUpdate(const Maybe<CSSPoint>& aRelativeDelta) override
   {
     // Autoscroll works using screen space coordinates, so there's no work we
-    // need to do to handle a content shift
+    // need to do to handle either a relative or an absolute scroll update.
     return true;
   }
 

gfx/layers/apz/src/GenericScrollAnimation.cpp

@@ -111,10 +111,13 @@ GenericScrollAnimation::DoSample(FrameMetrics& aFrameMetrics, const TimeDuration
 }
 
 bool
-GenericScrollAnimation::ApplyContentShift(const CSSPoint& aShiftDelta)
+GenericScrollAnimation::HandleScrollOffsetUpdate(const Maybe<CSSPoint>& aRelativeDelta)
 {
-  mAnimationPhysics->ApplyContentShift(aShiftDelta);
-  return true;
+  if (aRelativeDelta) {
+    mAnimationPhysics->ApplyContentShift(*aRelativeDelta);
+    return true;
+  }
+  return false;
 }
 
 } // namespace layers

gfx/layers/apz/src/GenericScrollAnimation.h

@@ -28,7 +28,7 @@ public:
 
   bool DoSample(FrameMetrics& aFrameMetrics, const TimeDuration& aDelta) override;
 
-  bool ApplyContentShift(const CSSPoint& aShiftDelta) override;
+  bool HandleScrollOffsetUpdate(const Maybe<CSSPoint>& aRelativeDelta) override;
 
   void UpdateDelta(TimeStamp aTime,
                    const nsPoint& aDelta,