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()) {

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

Nit:  make this an else if in the outer block

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

Done.

+      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() &&

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

Nit: doesn't hurt to clear this even if latching is off, right? I'd just add a
DCHECK(wheel_scroll_latching_enabled || !wheel_target_.target) and remove the
enabled check.

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

Done.

+      (event->phase == blink::WebMouseWheelEvent::kPhaseEnded ||
+       event->momentum_phase == blink::WebMouseWheelEvent::kPhaseEnded)) {
+    wheel_target_.target = nullptr;
+  }
 }
 
 void RenderWidgetHostInputEventRouter::RouteGestureEvent(