Bug 1259296 - Scroll snap in the compositor in response to wheel events. r=kats

MozReview-Commit-ID: 9fOlssstgvR

--HG--
extra : source : fbdee753c4618cbc1a7c79e3ec07b7479f93a05d
extra : histedit_source : dd42dfa282ea2e14fe5c9e028acc699746fcf8bc

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -1819,6 +1819,11 @@ nsEventStatus AsyncPanZoomController::OnScrollWheel(const ScrollWheelInput& aEve
   mozilla::Telemetry::Accumulate(mozilla::Telemetry::SCROLL_INPUT_METHODS,
       (uint32_t) ScrollInputMethodForWheelDeltaType(aEvent.mDeltaType));
 
+  // Wheel events from "clicky" mouse wheels trigger scroll snapping to the
+  // next snap point. Check for this, and adjust the delta to take into
+  // account the snap point.
+  bool scrollSnapping = MaybeAdjustDeltaForScrollSnapping(delta, aEvent);
+
   switch (aEvent.mScrollMode) {
     case ScrollWheelInput::SCROLLMODE_INSTANT: {
       ScreenPoint distance = ToScreenCoordinates(
@@ -1850,6 +1855,14 @@ nsEventStatus AsyncPanZoomController::OnScrollWheel(const ScrollWheelInput& aEve
       // update it.
       ReentrantMonitorAutoEnter lock(mMonitor);
 
+      if (scrollSnapping) {
+        // If we're scroll snapping use a smooth scroll animation to get
+        // the desired physics. Note that SmoothScrollTo() will re-use an
+        // existing smooth scroll animation if there is one.
+        CSSPoint snapPoint = mFrameMetrics.GetScrollOffset() + (delta / mFrameMetrics.GetZoom());
+        SmoothScrollTo(snapPoint);
+      } else {
+        // Otherwise, use a wheel scroll animation, also reusing one if possible.
         if (mState != WHEEL_SCROLL) {
           CancelAnimation();
           SetState(WHEEL_SCROLL);
@@ -1868,6 +1881,7 @@ nsEventStatus AsyncPanZoomController::OnScrollWheel(const ScrollWheelInput& aEve
 
         WheelScrollAnimation* animation = mAnimation->AsWheelScrollAnimation();
         animation->Update(aEvent.mTimeStamp, deltaInAppUnits, nsSize(velocity.x, velocity.y));
+      }
       break;
     }
   }
@@ -3937,13 +3951,14 @@ void AsyncPanZoomController::ShareCompositorFrameMetrics() {
   }
 }
 
-void AsyncPanZoomController::ScrollSnapNear(const CSSPoint& aDestination) {
+Maybe<CSSPoint> AsyncPanZoomController::FindSnapPointNear(
+    const CSSPoint& aDestination, nsIScrollableFrame::ScrollUnit aUnit) {
   mMonitor.AssertCurrentThreadIn();
   APZC_LOG("%p scroll snapping near %s\n", this, Stringify(aDestination).c_str());
   CSSRect scrollRange = mFrameMetrics.CalculateScrollRange();
   if (Maybe<nsPoint> snapPoint = ScrollSnapUtils::GetSnapPointForDestination(
           mScrollMetadata.GetSnapInfo(),
-          nsIScrollableFrame::DEVICE_PIXELS,
+          aUnit,
           CSSSize::ToAppUnits(mFrameMetrics.CalculateCompositedSizeInCssPixels()),
           CSSRect::ToAppUnits(scrollRange),
           CSSPoint::ToAppUnits(mFrameMetrics.GetScrollOffset()),
@@ -3953,8 +3968,15 @@ void AsyncPanZoomController::ScrollSnapNear(const CSSPoint& aDestination) {
     // of the scroll frame's scroll range. Clamp it here (this matches the
     // behaviour of the main-thread code path, which clamps it in
     // nsGfxScrollFrame::ScrollTo()).
-    cssSnapPoint = scrollRange.ClampPoint(cssSnapPoint);
-    SmoothScrollTo(cssSnapPoint);
+    return Some(scrollRange.ClampPoint(cssSnapPoint));
+  }
+  return Nothing();
+}
+
+void AsyncPanZoomController::ScrollSnapNear(const CSSPoint& aDestination) {
+  if (Maybe<CSSPoint> snapPoint =
+        FindSnapPointNear(aDestination, nsIScrollableFrame::DEVICE_PIXELS)) {
+    SmoothScrollTo(*snapPoint);
   }
 }
 
@@ -3996,5 +4018,29 @@ void AsyncPanZoomController::ScrollSnapToDestination() {
   }
 }
 
+bool AsyncPanZoomController::MaybeAdjustDeltaForScrollSnapping(
+    ParentLayerPoint& aDelta, const ScrollWheelInput& aEvent)
+{
+  // Don't scroll snap for pixel scrolls. This matches the main thread
+  // behaviour in EventStateManager::DoScrollText().
+  if (aEvent.mDeltaType == ScrollWheelInput::SCROLLDELTA_PIXEL) {
+    return false;
+  }
+
+  ReentrantMonitorAutoEnter lock(mMonitor);
+  CSSPoint scrollOffset = mFrameMetrics.GetScrollOffset();
+  CSSToParentLayerScale2D zoom = mFrameMetrics.GetZoom();
+  CSSPoint destination = mFrameMetrics.CalculateScrollRange().ClampPoint(
+      scrollOffset + (aDelta / zoom));
+  nsIScrollableFrame::ScrollUnit unit =
+      ScrollWheelInput::ScrollUnitForDeltaType(aEvent.mDeltaType);
+
+  if (Maybe<CSSPoint> snapPoint = FindSnapPointNear(destination, unit)) {
+    aDelta = (*snapPoint - scrollOffset) * zoom;
+    return true;
+  }
+  return false;
+}
+
 } // namespace layers
 } // namespace mozilla

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -25,6 +25,7 @@
 #include "Layers.h"                     // for Layer::ScrollDirection
 #include "LayersTypes.h"
 #include "mozilla/gfx/Matrix.h"
+#include "nsIScrollableFrame.h"
 #include "nsRegion.h"
 #include "PotentialCheckerboardDurationTracker.h"
 
@@ -622,15 +623,6 @@ protected:
   // Common processing at the end of a touch block.
   void OnTouchEndOrCancel();
 
-  // Snap to a snap position nearby the current scroll position, if appropriate.
-  void ScrollSnap();
-  // Snap to a snap position nearby the destination predicted based on the
-  // current velocity, if appropriate.
-  void ScrollSnapToDestination();
-
-  // Helper function for ScrollSnap() and ScrollSnapToDestination().
-  void ScrollSnapNear(const CSSPoint& aDestination);
-
   uint64_t mLayersId;
   RefPtr<CompositorBridgeParent> mCompositorBridgeParent;
 
@@ -763,7 +755,6 @@ public:
   AsyncTransformComponentMatrix GetCurrentAsyncTransformWithOverscroll(AsyncMode aMode) const;
 
 
-
   /* ===================================================================
    * The functions and members in this section are used to manage
    * the state that tracks what this APZC is doing with the input events.
@@ -1156,6 +1147,35 @@ private:
   // be checkerboarding. Combined with other info, this allows us to meaningfully
   // say how frequently users actually encounter checkerboarding.
   PotentialCheckerboardDurationTracker mPotentialCheckerboardTracker;
+
+
+  /* ===================================================================
+   * The functions in this section are used for CSS scroll snapping.
+   */
+
+  // If |aEvent| should trigger scroll snapping, adjust |aDelta| to reflect
+  // the snapping (that is, make it a delta that will take us to the desired
+  // snap point). Returns true iff. the delta was so adjusted.
+  bool MaybeAdjustDeltaForScrollSnapping(ParentLayerPoint& aDelta,
+                                         const ScrollWheelInput& aEvent);
+
+  // Snap to a snap position nearby the current scroll position, if appropriate.
+  void ScrollSnap();
+
+  // Snap to a snap position nearby the destination predicted based on the
+  // current velocity, if appropriate.
+  void ScrollSnapToDestination();
+
+  // Snap to a snap position nearby the provided destination, if appropriate.
+  void ScrollSnapNear(const CSSPoint& aDestination);
+
+  // Find a snap point near |aDestination| that we should snap to.
+  // Returns the snap point if one was found, or an empty Maybe otherwise.
+  // |aUnit| affects the snapping behaviour (see ScrollSnapUtils::
+  // GetSnapPointForDestination). It should generally be determined by the
+  // type of event that's triggering the scroll.
+  Maybe<CSSPoint> FindSnapPointNear(const CSSPoint& aDestination,
+                                    nsIScrollableFrame::ScrollUnit aUnit);
 };
 
 } // namespace layers

widget/InputData.h

@@ -6,8 +6,9 @@
 #ifndef InputData_h__
 #define InputData_h__
 
-#include "nsIDOMWheelEvent.h"
 #include "nsDebug.h"
+#include "nsIDOMWheelEvent.h"
+#include "nsIScrollableFrame.h"
 #include "nsPoint.h"
 #include "nsTArray.h"
 #include "Units.h"
@@ -577,6 +578,22 @@ public:
     return SCROLLDELTA_LINE;
   }
 
+  static nsIScrollableFrame::ScrollUnit
+  ScrollUnitForDeltaType(ScrollDeltaType aDeltaType)
+  {
+    switch (aDeltaType) {
+    case SCROLLDELTA_LINE:
+      return nsIScrollableFrame::LINES;
+    case SCROLLDELTA_PAGE:
+      return nsIScrollableFrame::PAGES;
+    case SCROLLDELTA_PIXEL:
+      return nsIScrollableFrame::DEVICE_PIXELS;
+    default:
+      MOZ_CRASH();
+    }
+    return nsIScrollableFrame::LINES;
+  }
+
   enum ScrollMode
   {
     SCROLLMODE_INSTANT,