Improve scrolling performance when there are many windowed plugins in a page....

chrome/browser/render_widget_host.cc

 // Copyright (c) 2006-2008 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 "chrome/browser/render_widget_host.h"
 
 #include "base/gfx/gdi_util.h"
 #include "base/message_loop.h"
 #include "chrome/app/chrome_dll_resource.h"
 #include "chrome/browser/render_process_host.h"
@@ skipping 416 matching lines @@
     view_being_painted_ = false;
   }
 
   // Log the time delta for processing a scroll message.
   TimeDelta delta = TimeTicks::Now() - scroll_start;
   UMA_HISTOGRAM_TIMES(L"MPArch.RWH_OnMsgScrollRect", delta);
 }
 
 void RenderWidgetHost::MovePluginWindows(
     const std::vector<WebPluginGeometry>& plugin_window_moves) {
+  if (plugin_window_moves.empty())
+    return;
+
   HDWP defer_window_pos_info =
       ::BeginDeferWindowPos(static_cast<int>(plugin_window_moves.size()));
 
   if (!defer_window_pos_info) {
     NOTREACHED();
     return;
   }
 
   for (size_t i = 0; i < plugin_window_moves.size(); ++i) {
-    unsigned long flags = 0;
+    // Don't invalidate now because that would result in cross process calls
+    // that make scrolling slow.  Instead the window is invalidated
+    // asynchronously by the plugin code.
+    unsigned long flags = SWP_NOREDRAW;
     const WebPluginGeometry& move = plugin_window_moves[i];
 
     if (move.visible)
       flags |= SWP_SHOWWINDOW;
     else
       flags |= SWP_HIDEWINDOW;
 
     HRGN hrgn = ::CreateRectRgn(move.clip_rect.x(),
                                 move.clip_rect.y(),
                                 move.clip_rect.right(),
@@ skipping 379 matching lines @@
       &RenderWidgetHost::CheckRendererIsUnresponsive);
 }
 
 void RenderWidgetHost::RendererExited() {
   BackingStoreManager::RemoveBackingStore(this);
 }
 
 void RenderWidgetHost::SystemThemeChanged() {
   Send(new ViewMsg_ThemeChanged(routing_id_));
 }