Re-target wheel events only when a new scroll sequence has started.

third_party/WebKit/Source/core/input/MouseWheelEventManager.cpp

Index: third_party/WebKit/Source/core/input/MouseWheelEventManager.cpp
diff --git a/third_party/WebKit/Source/core/input/MouseWheelEventManager.cpp b/third_party/WebKit/Source/core/input/MouseWheelEventManager.cpp
index e830907851620108275d6043da95b7b8d3ed2e5f..b8c8a1f74724505821125a327486260c49ddf4ea 100644
--- a/third_party/WebKit/Source/core/input/MouseWheelEventManager.cpp
+++ b/third_party/WebKit/Source/core/input/MouseWheelEventManager.cpp
@@ -18,54 +18,111 @@
 namespace blink {
 MouseWheelEventManager::MouseWheelEventManager(LocalFrame& frame,
                                                ScrollManager& scroll_manager)
-    : frame_(frame), scroll_manager_(scroll_manager) {}
+    : frame_(frame), wheel_target_(nullptr), scroll_manager_(scroll_manager) {}
 
 DEFINE_TRACE(MouseWheelEventManager) {
   visitor->Trace(frame_);
+  visitor->Trace(wheel_target_);
   visitor->Trace(scroll_manager_);
 }
 
+void MouseWheelEventManager::Clear() {
+  wheel_target_ = nullptr;
+}
+
 WebInputEventResult MouseWheelEventManager::HandleWheelEvent(
     const WebMouseWheelEvent& event) {
+  bool wheel_scroll_latching =
+      RuntimeEnabledFeatures::TouchpadAndWheelScrollLatchingEnabled();
+
+  if (wheel_scroll_latching) {
+    const int kWheelEventPhaseNoEventMask =
+        WebMouseWheelEvent::kPhaseEnded | WebMouseWheelEvent::kPhaseCancelled |
+        WebMouseWheelEvent::kPhaseMayBegin;

[bokan, 2017/06/15 15:47:18]

Do we really need to clear the target on MayBegin? If we get one it should
always be after a PhaseEnded or PhaseCancelled and before a Begin, right? In
that case, I'd rather we DCHECK on MayBegin that wheel_target_ == nullptr.

[sahel, 2017/06/16 14:56:20]

Done.

+
+    if ((event.phase & kWheelEventPhaseNoEventMask) ||
+        (event.momentum_phase & kWheelEventPhaseNoEventMask)) {
+      // Filter wheel events with zero deltas and reset the wheel_target_ node.
+      DCHECK(!event.delta_x && !event.delta_y);
+      wheel_target_ = nullptr;
+      return WebInputEventResult::kNotHandled;
+    }
+
+    if (event.phase == WebMouseWheelEvent::kPhaseBegan) {
+      // Find and save the wheel_target_, this target will be used for the rest
+      // of the current scrolling sequence.
+      DCHECK(!wheel_target_);
+      Document* doc = frame_->GetDocument();
+
+      if (doc->GetLayoutViewItem().IsNull())
+        return WebInputEventResult::kNotHandled;
+
+      LocalFrameView* view = frame_->View();
+      if (!view)
+        return WebInputEventResult::kNotHandled;

[bokan, 2017/06/15 15:47:18]

Add the early out at the top of the method if there's no Document, FrameView, or
LayoutView.

[sahel, 2017/06/16 14:56:20]

Done.

+
+      LayoutPoint v_point = view->RootFrameToContents(
+          FlooredIntPoint(event.PositionInRootFrame()));
+
+      HitTestRequest request(HitTestRequest::kReadOnly);
+      HitTestResult result(request, v_point);
+      doc->GetLayoutViewItem().HitTest(result);
+
+      wheel_target_ = result.InnerNode();
+      // Wheel events should not dispatch to text nodes.
+      if (wheel_target_ && wheel_target_->IsTextNode())
+        wheel_target_ = FlatTreeTraversal::Parent(*wheel_target_);
+
+      // If we're over the frame scrollbar, scroll the document.
+      if (!wheel_target_ && result.GetScrollbar())
+        wheel_target_ = doc->documentElement();
+    }
+  } else {  // !wheel_scroll_latching, wheel_target_ will be updated for each
+            // wheel event.
 #if OS(MACOSX)
-  // Filter Mac OS specific phases, usually with a zero-delta.
-  // https://crbug.com/553732
-  // TODO(chongz): EventSender sends events with
-  // |WebMouseWheelEvent::PhaseNone|,
-  // but it shouldn't.
-  const int kWheelEventPhaseNoEventMask = WebMouseWheelEvent::kPhaseEnded |
-                                          WebMouseWheelEvent::kPhaseCancelled |
-                                          WebMouseWheelEvent::kPhaseMayBegin;
-  if ((event.phase & kWheelEventPhaseNoEventMask) ||
-      (event.momentum_phase & kWheelEventPhaseNoEventMask))
-    return WebInputEventResult::kNotHandled;
+    // Filter Mac OS specific phases, usually with a zero-delta.
+    // https://crbug.com/553732
+    // TODO(chongz): EventSender sends events with
+    // |WebMouseWheelEvent::PhaseNone|,
+    // but it shouldn't.
+    const int kWheelEventPhaseNoEventMask =
+        WebMouseWheelEvent::kPhaseEnded | WebMouseWheelEvent::kPhaseCancelled |
+        WebMouseWheelEvent::kPhaseMayBegin;
+    if ((event.phase & kWheelEventPhaseNoEventMask) ||
+        (event.momentum_phase & kWheelEventPhaseNoEventMask))
+      return WebInputEventResult::kNotHandled;
 #endif
-  Document* doc = frame_->GetDocument();
 
-  if (doc->GetLayoutViewItem().IsNull())
-    return WebInputEventResult::kNotHandled;
+    Document* doc = frame_->GetDocument();
 
-  LocalFrameView* view = frame_->View();
-  if (!view)
-    return WebInputEventResult::kNotHandled;
+    if (doc->GetLayoutViewItem().IsNull())
+      return WebInputEventResult::kNotHandled;
 
-  LayoutPoint v_point =
-      view->RootFrameToContents(FlooredIntPoint(event.PositionInRootFrame()));
+    LocalFrameView* view = frame_->View();
+    if (!view)
+      return WebInputEventResult::kNotHandled;
 
-  HitTestRequest request(HitTestRequest::kReadOnly);
-  HitTestResult result(request, v_point);
-  doc->GetLayoutViewItem().HitTest(result);
+    LayoutPoint v_point =

[bokan, 2017/06/15 15:47:17]

Everything from here to end of the block is identical between the two branches.
Please factor it out into a private helper method and use it in both branches.

[sahel, 2017/06/16 14:56:20]

Done.

+        view->RootFrameToContents(FlooredIntPoint(event.PositionInRootFrame()));
 
-  Node* node = result.InnerNode();
-  // Wheel events should not dispatch to text nodes.
-  if (node && node->IsTextNode())
-    node = FlatTreeTraversal::Parent(*node);
+    HitTestRequest request(HitTestRequest::kReadOnly);
+    HitTestResult result(request, v_point);
+    doc->GetLayoutViewItem().HitTest(result);
 
-  // If we're over the frame scrollbar, scroll the document.
-  if (!node && result.GetScrollbar())
-    node = doc->documentElement();
+    Node* node = result.InnerNode();
+    // Wheel events should not dispatch to text nodes.
+    if (node && node->IsTextNode())
+      node = FlatTreeTraversal::Parent(*node);
+
+    // If we're over the frame scrollbar, scroll the document.
+    if (!node && result.GetScrollbar())
+      node = doc->documentElement();
+
+    wheel_target_ = node;
+  }
 
-  LocalFrame* subframe = EventHandlingUtil::SubframeForTargetNode(node);
+  LocalFrame* subframe =
+      EventHandlingUtil::SubframeForTargetNode(wheel_target_.Get());
   if (subframe) {
     WebInputEventResult result =
         subframe->GetEventHandler().HandleWheelEvent(event);
@@ -74,10 +131,11 @@ WebInputEventResult MouseWheelEventManager::HandleWheelEvent(
     return result;
   }
 
-  if (node) {
+  if (wheel_target_) {
     WheelEvent* dom_event =
-        WheelEvent::Create(event, node->GetDocument().domWindow());
-    DispatchEventResult dom_event_result = node->DispatchEvent(dom_event);
+        WheelEvent::Create(event, wheel_target_->GetDocument().domWindow());
+    DispatchEventResult dom_event_result =
+        wheel_target_->DispatchEvent(dom_event);
     if (dom_event_result != DispatchEventResult::kNotCanceled)
       return EventHandlingUtil::ToWebInputEventResult(dom_event_result);
   }