Bug 1383912 - Ensure we always get a composite for the latest async scroll offset. r=kats

This fixes a regression with apz.frame_delay.enabled=true introduced in
bug 1375949.

MozReview-Commit-ID: AIcGA7c2Co0

--HG--
extra : rebase_source : b118a97674cadef1359e7658539e4e0e9cb785b8

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -3238,7 +3238,7 @@ bool AsyncPanZoomController::UpdateAnimation(const TimeStamp& aSampleTime,
   // Sample the composited async transform once per composite. Note that we
   // call this after the |mLastSampleTime == aSampleTime| check, to ensure
   // it's only called once per APZC on each composite.
-  SampleCompositedAsyncTransform();
+  bool needComposite = SampleCompositedAsyncTransform();
 
   TimeDuration sampleTimeDelta = aSampleTime - mLastSampleTime;
   mLastSampleTime = aSampleTime;
@@ -3258,9 +3258,9 @@ bool AsyncPanZoomController::UpdateAnimation(const TimeStamp& aSampleTime,
       RequestContentRepaint();
     }
     UpdateSharedCompositorFrameMetrics();
-    return true;
+    needComposite = true;
   }
-  return false;
+  return needComposite;
 }
 
 AsyncTransformComponentMatrix
@@ -3421,12 +3421,17 @@ AsyncPanZoomController::GetEffectiveZoom(AsyncTransformConsumer aMode) const
   return mFrameMetrics.GetZoom();
 }
 
-void
+bool
 AsyncPanZoomController::SampleCompositedAsyncTransform()
 {
   ReentrantMonitorAutoEnter lock(mMonitor);
-  mCompositedScrollOffset = mFrameMetrics.GetScrollOffset();
-  mCompositedZoom = mFrameMetrics.GetZoom();
+  if (mCompositedScrollOffset != mFrameMetrics.GetScrollOffset() ||
+      mCompositedZoom != mFrameMetrics.GetZoom()) {
+    mCompositedScrollOffset = mFrameMetrics.GetScrollOffset();
+    mCompositedZoom = mFrameMetrics.GetZoom();
+    return true;
+  }
+  return false;
 }
 
 AsyncTransformComponentMatrix

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -850,11 +850,14 @@ private:
    * |GetCurrentAsyncTransform(eForCompositing)| and similar functions reflect
    * the async scroll offset and zoom stored in |mFrameMetrics|.
    *
+   * Returns true if the newly sampled value is different from the previously
+   * sampled value.
+   *
    * (This is only relevant when |gfxPrefs::APZFrameDelayEnabled() == true|.
    * Otherwise, GetCurrentAsyncTransform() always reflects what's stored in
    * |mFrameMetrics| immediately, without any delay.)
    */
-  void SampleCompositedAsyncTransform();
+  bool SampleCompositedAsyncTransform();
 
   /*
    * Helper functions to query the async scroll offset and zoom either