Queued wheel events should not use ScopedInputScaleDisabler.

content/browser/frame_host/render_widget_host_view_guest.cc

 // Copyright 2014 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/frame_host/render_widget_host_view_guest.h"
 
 #include <utility>
 
 #include "base/bind_helpers.h"
 #include "base/command_line.h"
@@ skipping 537 matching lines @@
   // GestureScrollBegin/End are always consumed by the guest, so we only
   // forward GestureScrollUpdate.
   if (event.type == blink::WebInputEvent::GestureScrollUpdate && not_consumed)
     guest_->ResendEventToEmbedder(event);
 }
 
 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
     // the CSS transforms applied on the <object> and embedder. As a result of
     // this, the coordinates passed on to the guest renderer are potentially
     // incorrect to determine the position of the context menu(they are not the
     // actual X, Y of the window). As a hack, we report the last location of a
     // right mouse up to the BrowserPluginGuest to inform it of the next
     // potential location for context menu (BUG=470087).
     // TODO(ekaramad): Find a better and more fundamental solution. Could the
     // ContextMenuParams be based on global X, Y?
     const blink::WebMouseEvent& mouse_event =
         static_cast<const blink::WebMouseEvent&>(*event);
     // A MouseDown on the ButtonRight could suggest a ContextMenu.
     if (guest_ && mouse_event.type == blink::WebInputEvent::MouseDown &&
         mouse_event.button == blink::WebPointerProperties::ButtonRight)
       guest_->SetContextMenuPosition(
           gfx::Point(mouse_event.globalX - GetViewBounds().x(),
                      mouse_event.globalY - GetViewBounds().y()));
     host_->ForwardMouseEvent(mouse_event);
     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;
     NativeWebKeyboardEvent keyboard_event(*embedder->GetLastKeyboardEvent());
     host_->ForwardKeyboardEvent(keyboard_event);
     return;
   }
 
   if (blink::WebInputEvent::isTouchEventType(event->type)) {
     if (event->type == blink::WebInputEvent::TouchStart &&
@@ skipping 23 matching lines @@
         gesture_event.data.scrollUpdate.inertialPhase ==
             blink::WebGestureEvent::MomentumPhase) {
       return;
     }
     host_->ForwardGestureEvent(gesture_event);
     return;
   }
 }
 
 }  // namespace content