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

content/browser/renderer_host/render_widget_host_input_event_router.cc

Index: content/browser/renderer_host/render_widget_host_input_event_router.cc
diff --git a/content/browser/renderer_host/render_widget_host_input_event_router.cc b/content/browser/renderer_host/render_widget_host_input_event_router.cc
index fe73293b46648e74e62ba09a7c44d459d65bf308..091506016280e2ced7d67c6a169b856f704b10d0 100644
--- a/content/browser/renderer_host/render_widget_host_input_event_router.cc
+++ b/content/browser/renderer_host/render_widget_host_input_event_router.cc
@@ -285,10 +285,32 @@ void RenderWidgetHostInputEventRouter::RouteMouseWheelEvent(
             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)
@@ -296,6 +318,11 @@ void RenderWidgetHostInputEventRouter::RouteMouseWheelEvent(
 
   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(