Only scroll on main if the targeted frames need to scroll on main

third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 11 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "core/page/scrolling/ScrollingCoordinator.h"
 
 #include "core/css/CSSStyleSheet.h"
 #include "core/css/StyleSheetList.h"
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/VisualViewport.h"
+#include "core/html/HTMLIFrameElement.h"
 #include "core/layout/LayoutPart.h"
 #include "core/layout/api/LayoutViewItem.h"
 #include "core/layout/compositing/CompositedLayerMapping.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
 #include "core/page/Page.h"
 #include "platform/geometry/IntPoint.h"
 #include "platform/geometry/IntRect.h"
 #include "platform/graphics/GraphicsLayer.h"
 #include "platform/testing/URLTestHelpers.h"
 #include "public/platform/Platform.h"
@@ skipping 800 matching lines @@
   container->removeAttribute("class");
   forceFullCompositingUpdate();
   scrollbarGraphicsLayer = compositedLayerMapping->layerForVerticalScrollbar();
   ASSERT_FALSE(
       scrollbarGraphicsLayer->platformLayer()->shouldScrollOnMainThread());
   ASSERT_FALSE(
       scrollbarGraphicsLayer->platformLayer()->mainThreadScrollingReasons() &
       MainThreadScrollingReason::kCustomScrollbarScrolling);
 }
 
+TEST_F(ScrollingCoordinatorTest,
+       BackgroundAttachmentFixedShouldTriggerMainThreadScroll) {
+  registerMockedHttpURLLoad("iframe-background-attachment-fixed.html");
+  registerMockedHttpURLLoad("iframe-background-attachment-fixed-inner.html");
+  registerMockedHttpURLLoad("white-1x1.png");
+  navigateTo(m_baseURL + "iframe-background-attachment-fixed.html");
+  forceFullCompositingUpdate();
+
+  Element* iframe = frame()->document()->getElementById("iframe");
+  ASSERT_TRUE(iframe);
+
+  LayoutObject* layoutObject = iframe->layoutObject();
+  ASSERT_TRUE(layoutObject);
+  ASSERT_TRUE(layoutObject->isLayoutPart());
+
+  LayoutPart* layoutPart = toLayoutPart(layoutObject);
+  ASSERT_TRUE(layoutPart);
+  ASSERT_TRUE(layoutPart->widget());
+  ASSERT_TRUE(layoutPart->widget()->isFrameView());
+
+  FrameView* innerFrameView = toFrameView(layoutPart->widget());
+  LayoutViewItem innerLayoutViewItem = innerFrameView->layoutViewItem();
+  ASSERT_FALSE(innerLayoutViewItem.isNull());
+
+  PaintLayerCompositor* innerCompositor = innerLayoutViewItem.compositor();
+  ASSERT_TRUE(innerCompositor->inCompositingMode());
+  ASSERT_TRUE(innerCompositor->scrollLayer());
+
+  GraphicsLayer* scrollLayer = innerCompositor->scrollLayer();
+  ASSERT_EQ(innerFrameView, scrollLayer->getScrollableArea());
+
+  WebLayer* webScrollLayer = scrollLayer->platformLayer();
+  ASSERT_TRUE(webScrollLayer->scrollable());
+  ASSERT_TRUE(webScrollLayer->mainThreadScrollingReasons() &
+              MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects);
+
+  // Remove fixed background-attachment should make the iframe
+  // scroll on cc.
+  auto* iframeDoc = toHTMLIFrameElement(iframe)->contentDocument();
+  iframe = iframeDoc->getElementById("scrollable");
+  ASSERT_TRUE(iframe);
+
+  iframe->removeAttribute("class");
+  forceFullCompositingUpdate();
+
+  layoutObject = iframe->layoutObject();
+  ASSERT_TRUE(layoutObject);
+
+  scrollLayer = layoutObject->frameView()->layerForScrolling();
+  ASSERT_TRUE(scrollLayer);
+
+  webScrollLayer = scrollLayer->platformLayer();
+  ASSERT_TRUE(webScrollLayer->scrollable());
+  ASSERT_FALSE(webScrollLayer->mainThreadScrollingReasons() &
+               MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects);
+
+  // Force main frame to scroll on main thread. All its descendants
+  // should scroll on main thread as well.
+  Element* element = frame()->document()->getElementById("scrollable");
+  element->setAttribute(
+      "style",
+      "background-image: url('white-1x1.png'); background-attachment: fixed;;",

[pdr., 2016/12/19 04:04:28]

Nit: extra semicolon

[yigu, 2016/12/20 00:48:34]

Done.

+      ASSERT_NO_EXCEPTION);
+
+  forceFullCompositingUpdate();
+
+  layoutObject = iframe->layoutObject();
+  ASSERT_TRUE(layoutObject);
+
+  scrollLayer = layoutObject->frameView()->layerForScrolling();
+  ASSERT_TRUE(scrollLayer);
+
+  webScrollLayer = scrollLayer->platformLayer();
+  ASSERT_TRUE(webScrollLayer->scrollable());
+  ASSERT_TRUE(webScrollLayer->mainThreadScrollingReasons() &
+              MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects);
+}
 }  // namespace blink