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

content/browser/renderer_host/render_widget_host_input_event_router.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/renderer_host/render_widget_host_input_event_router.h"
 
 #include <vector>
 
 #include "base/metrics/histogram_macros.h"
 #include "cc/quads/surface_draw_quad.h"
@@ skipping 266 matching lines @@
   if (root_view->IsMouseLocked()) {
     target = RenderWidgetHostImpl::From(root_view->GetRenderWidgetHost())
                  ->delegate()
                  ->GetMouseLockWidget()
                  ->GetView();
     if (!root_view->TransformPointToCoordSpaceForView(
             gfx::Point(event->PositionInWidget().x,
                        event->PositionInWidget().y),
             target, &transformed_point))
       return;
-  } else {
+  } else if (root_view->wheel_scroll_latching_enabled()) {
+    if (event->phase == blink::WebMouseWheelEvent::kPhaseBegan) {
+      wheel_target_.target = FindEventTarget(
+          root_view,
+          gfx::Point(event->PositionInWidget().x, event->PositionInWidget().y),
+          &transformed_point);
+      wheel_target_.delta =
+          transformed_point -
+          gfx::Point(event->PositionInWidget().x, event->PositionInWidget().y);
+      target = wheel_target_.target;
+    } else {
+      if (wheel_target_.target) {
+        target = wheel_target_.target;
+        transformed_point = gfx::Point(event->PositionInWidget().x,
+                                       event->PositionInWidget().y) +
+                            wheel_target_.delta;
+      }
+    }
+
+  } else {  // !root_view->IsMouseLocked() &&
+            // !root_view->wheel_scroll_latching_enabled()
     target = FindEventTarget(
         root_view,
         gfx::Point(event->PositionInWidget().x, event->PositionInWidget().y),
         &transformed_point);
   }
 
   if (!target)
     return;
 
   event->SetPositionInWidget(transformed_point.x(), transformed_point.y());
   target->ProcessMouseWheelEvent(*event, latency);
+
+  DCHECK(root_view->wheel_scroll_latching_enabled() || !wheel_target_.target);
+  if (event->phase == blink::WebMouseWheelEvent::kPhaseEnded ||
+      event->momentum_phase == blink::WebMouseWheelEvent::kPhaseEnded) {
+    wheel_target_.target = nullptr;
+  }
 }
 
 void RenderWidgetHostInputEventRouter::RouteGestureEvent(
     RenderWidgetHostViewBase* root_view,
     blink::WebGestureEvent* event,
     const ui::LatencyInfo& latency) {
   if (event->IsTargetViewport()) {
     root_view->ProcessGestureEvent(*event, latency);
     return;
   }
@@ skipping 538 matching lines @@
 std::vector<RenderWidgetHostView*>
 RenderWidgetHostInputEventRouter::GetRenderWidgetHostViewsForTests() const {
   std::vector<RenderWidgetHostView*> hosts;
   for (auto entry : owner_map_)
     hosts.push_back(entry.second);
 
   return hosts;
 }
 
 }  // namespace content