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 267 matching lines @@
     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 {
-    target = FindEventTarget(
-        root_view,
-        gfx::Point(event->PositionInWidget().x, event->PositionInWidget().y),
-        &transformed_point);
+    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->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);
+  if (root_view->wheel_scroll_latching_enabled() &&
+      (event->phase == blink::WebMouseWheelEvent::kPhaseEnded ||
+       event->momentum_phase == blink::WebMouseWheelEvent::kPhaseEnded)) {
+    wheel_target_.target = nullptr;

[tdresser, 2017/06/09 18:13:11]

This isn't needed, is it?

[sahel, 2017/06/13 15:43:20]

By resetting the target on end events we won't send the rest of the wheel events
to a stale target if the event with phaseBegan gets missed for any reason. I
agree that it shouldn't happen normally, so if you think it is not needed I can
get rid of it.

[tdresser, 2017/06/13 15:48:50]

Maybe leave it, and add a DCHECK that the target isn't null for wheel events
other than phase begin?

[sahel, 2017/06/14 19:09:43]

We agreed to leave it as it is since sometimes FindEventTarget fails to find any
targets.

+  }
 }
 
 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