Mac: Coalesce scroll events.

chrome/browser/renderer_host/render_widget_host.h

 // Copyright (c) 2009 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.
 
 #ifndef CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_H_
 #define CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_H_
 
 #include <deque>
 
 #include "app/gfx/native_widget_types.h"
@@ skipping 493 matching lines @@
   gfx::Size in_flight_size_;
 
   // True if a mouse move event was sent to the render view and we are waiting
   // for a corresponding ViewHostMsg_HandleInputEvent_ACK message.
   bool mouse_move_pending_;
 
   // The next mouse move event to send (only non-null while mouse_move_pending_
   // is true).
   scoped_ptr<WebKit::WebMouseEvent> next_mouse_move_;
 
+  // (Similar to |mouse_move_pending_|.) True if a mouse wheel event was sent
+  // and we are waiting for a corresponding ack.
+  bool mouse_wheel_pending_;
+
+  typedef std::deque<WebKit::WebMouseWheelEvent> WheelEventQueue;
+
+  // (Similar to |next_mouse_move_|.) The next mouse wheel events to send.
+  // Unlike mouse moves, mouse wheel events received while one is pending are
+  // coalesced (by accumulating deltas) if they match the previous event in
+  // modifiers. On the Mac, in particular, mouse wheel events are received at a
+  // high rate; not waiting for the ack results in jankiness, and using the same
+  // mechanism as for mouse moves (just dropping old events when multiple ones
+  // would be queued) results in very slow scrolling.
+  WheelEventQueue coalesced_mouse_wheel_events_;
+
   // The time when an input event was sent to the RenderWidget.
   base::TimeTicks input_event_start_time_;
 
   // If true, then we should repaint when restoring even if we have a
   // backingstore.  This flag is set to true if we receive a paint message
   // while is_hidden_ to true.  Even though we tell the render widget to hide
   // itself, a paint message could already be in flight at that point.
   bool needs_repainting_on_restore_;
 
   // This is true if the renderer is currently unresponsive.
@@ skipping 48 matching lines @@
   // but the Char event generated by alt-2 may also activate a HTML element
   // if its accesskey happens to be "2", then the user may get confused when
   // switching back to the original tab, because the content may already be
   // changed.
   bool suppress_next_char_events_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderWidgetHost);
 };
 
 #endif  // CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_H_