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

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

Index: third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp
diff --git a/third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp b/third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp
index a2194782061ed57c8b036b8c1d5418d3dfd5a2c3..b03f959c07710fd95e0080875d294e371c433d6c 100644
--- a/third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp
+++ b/third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp
@@ -29,6 +29,7 @@
 #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"
@@ -849,4 +850,60 @@ TEST_F(ScrollingCoordinatorTest, CustomScrollbarShouldTriggerMainThreadScroll) {
       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 custom scrollbar class, the scrollbar is expected to scroll on
+  // impl thread as it is an overlay scrollbar.
+  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);
+}
 }  // namespace blink