Bug 1400440 - Apply scroll shift for dynamic toolbar to the composited scroll offset right away. r=kats

MozReview-Commit-ID: 1I7C0lFx4WP

--HG--
extra : rebase_source : 9c6ae39e10a2f9b86d2886140ee2e0d4f5cb4fda

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -2999,10 +2999,14 @@ void AsyncPanZoomController::AdjustScrollForSurfaceShift(const ScreenPoint& aShi
     / mFrameMetrics.GetZoom();
   APZC_LOG("%p adjusting scroll position by %s for surface shift\n",
     this, Stringify(adjustment).c_str());
-  CSSPoint scrollOffset = mFrameMetrics.GetScrollOffset();
-  scrollOffset.y = mY.ClampOriginToScrollableRect(scrollOffset.y + adjustment.y);
-  scrollOffset.x = mX.ClampOriginToScrollableRect(scrollOffset.x + adjustment.x);
-  mFrameMetrics.SetScrollOffset(scrollOffset);
+  CSSRect scrollRange = mFrameMetrics.CalculateScrollRange();
+  // Apply shift to mFrameMetrics.mScrollOffset.
+  mFrameMetrics.SetScrollOffset(scrollRange.ClampPoint(
+      mFrameMetrics.GetScrollOffset() + adjustment));
+  // Apply shift to mCompositedScrollOffset, since the dynamic toolbar expects
+  // the shift to take effect right away, without the usual frame delay.
+  mCompositedScrollOffset = scrollRange.ClampPoint(
+      mCompositedScrollOffset + adjustment);
   RequestContentRepaint();
   UpdateSharedCompositorFrameMetrics();
 }