Make a hole only as big as is necessary.

chrome/browser/renderer_host/render_widget_host_view_views.cc

 // Copyright (c) 2010 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/renderer_host/render_widget_host_view_views.h"
 
 #include <algorithm>
 #include <string>
 
 #include "app/l10n_util.h"
@@ skipping 337 matching lines @@
 void RenderWidgetHostViewViews::SetBackground(const SkBitmap& background) {
   RenderWidgetHostView::SetBackground(background);
   host_->Send(new ViewMsg_SetBackground(host_->routing_id(), background));
 }
 
 void RenderWidgetHostViewViews::Paint(gfx::Canvas* canvas) {
   if (is_hidden_) {
     return;
   }
 
-  // Paint a "hole" in the canvas so that the render of the web page is on
-  // top of whatever else has already been painted in the views hierarchy.
-  // Later views might still get to paint on top.
-  canvas->FillRectInt(SK_ColorBLACK, 0, 0, kMaxWindowWidth, kMaxWindowHeight,
-                      SkXfermode::kClear_Mode);
-
   // Don't do any painting if the GPU process is rendering directly
   // into the View.
   RenderWidgetHost* render_widget_host = GetRenderWidgetHost();
   if (render_widget_host->is_accelerated_compositing_active()) {
     return;

[sadrul, 2011/01/21 15:56:53]

I do not know if painting the hole is necessary here?

[Alex Nicolaou, 2011/01/21 16:00:16]

Yes it is, that's why it was above this point. Otherwise you're in a race with
the GPU process.

   }
 
   GdkWindow* window = GetInnerNativeView()->window;
   DCHECK(!about_to_validate_and_paint_);
 
   // TODO(anicolao): get the damage somehow
   // invalid_rect_ = damage_rect;
   invalid_rect_ = bounds();
   gfx::Point origin;
   ConvertPointToWidget(this, &origin);
 
   about_to_validate_and_paint_ = true;
   BackingStoreX* backing_store = static_cast<BackingStoreX*>(
       host_->GetBackingStore(true));
   // Calling GetBackingStore maybe have changed |invalid_rect_|...
   about_to_validate_and_paint_ = false;
 
   gfx::Rect paint_rect = gfx::Rect(0, 0, kMaxWindowWidth, kMaxWindowHeight);
   paint_rect = paint_rect.Intersect(invalid_rect_);
+  // Make sure we paint within the bounds.
+  paint_rect = paint_rect.Intersect(bounds());
+
+  // Paint a "hole" in the canvas so that the render of the web page is on
+  // top of whatever else has already been painted in the views hierarchy.
+  // Later views might still get to paint on top.
+  canvas->FillRectInt(SK_ColorBLACK, 0, 0,
+                      paint_rect.width(), paint_rect.height(),
+                      SkXfermode::kClear_Mode);

[Alex Nicolaou, 2011/01/21 16:00:16]

You'll have to move this back up.

If you're really worried about it, it would also be great to address the TODO on
line 370 which is 99% of the inefficiency here.

[sadrul, 2011/01/21 16:36:19]

Moved this back up.

 
   if (backing_store) {
     // Only render the widget if it is attached to a window; there's a short
     // period where this object isn't attached to a window but hasn't been
     // Destroy()ed yet and it receives paint messages...
     if (window) {
       if (!visually_deemphasized_) {
         // In the common case, use XCopyArea. We don't draw more than once, so
         // we don't need to double buffer.
         backing_store->XShowRect(origin,
@@ skipping 368 matching lines @@
 }
 
 // static
 RenderWidgetHostView*
     RenderWidgetHostView::GetRenderWidgetHostViewFromNativeView(
         gfx::NativeView widget) {
   gpointer user_data = g_object_get_data(G_OBJECT(widget),
                                          kRenderWidgetHostViewKey);
   return reinterpret_cast<RenderWidgetHostView*>(user_data);
 }