Queued wheel events should not use ScopedInputScaleDisabler.

content/browser/frame_host/render_widget_host_view_guest.cc

Index: content/browser/frame_host/render_widget_host_view_guest.cc
diff --git a/content/browser/frame_host/render_widget_host_view_guest.cc b/content/browser/frame_host/render_widget_host_view_guest.cc
index d433e64c164122f7deae254bb6a39da4f45014db..7a120f174318e966af87ae02518243d0cf47c144 100644
--- a/content/browser/frame_host/render_widget_host_view_guest.cc
+++ b/content/browser/frame_host/render_widget_host_view_guest.cc
@@ -555,6 +555,23 @@ void RenderWidgetHostViewGuest::OnHandleInputEvent(
     RenderWidgetHostImpl* embedder,
     int browser_plugin_instance_id,
     const blink::WebInputEvent* event) {
+  // WebMouseWheelEvents go into a queue, and may not be forwarded to the
+  // renderer until after this method goes out of scope. Therefore we need to
+  // explicitly remove the additional device scale factor from the coordinates
+  // before allowing the event to be queued.
+  if (event->type == blink::WebInputEvent::MouseWheel) {

[oshima, 2016/07/19 21:40:48]

you also need to check "IsZoomForDSFEnabled()".

Alternative way is to scale when queued, and then skip scaling for wheel event
in OfferToRenderer, and I have a slight preference over this, although I can be
convinced the other way. WDYT?

+    blink::WebMouseWheelEvent rescaled_event =
+        *static_cast<const blink::WebMouseWheelEvent*>(event);
+    rescaled_event.x /= current_device_scale_factor();
+    rescaled_event.y /= current_device_scale_factor();
+    rescaled_event.deltaX /= current_device_scale_factor();
+    rescaled_event.deltaY /= current_device_scale_factor();
+    rescaled_event.wheelTicksX /= current_device_scale_factor();
+    rescaled_event.wheelTicksY /= current_device_scale_factor();
+    host_->ForwardWheelEvent(rescaled_event);
+    return;
+  }
+
   ScopedInputScaleDisabler disable(host_, current_device_scale_factor());
   if (blink::WebInputEvent::isMouseEventType(event->type)) {
     // The mouse events for BrowserPlugin are modified by all
@@ -578,12 +595,6 @@ void RenderWidgetHostViewGuest::OnHandleInputEvent(
     return;
   }
 
-  if (event->type == blink::WebInputEvent::MouseWheel) {
-    host_->ForwardWheelEvent(
-        *static_cast<const blink::WebMouseWheelEvent*>(event));
-    return;
-  }
-
   if (blink::WebInputEvent::isKeyboardEventType(event->type)) {
     if (!embedder->GetLastKeyboardEvent())
       return;