Disable anchoring if ancestor changes padding or margin

third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp

 // Copyright 2015 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 "core/layout/ScrollAnchor.h"
 
 #include "core/dom/ClientRect.h"
 #include "core/frame/VisualViewport.h"
 #include "core/layout/LayoutBox.h"
 #include "core/layout/LayoutTestHelper.h"
@@ skipping 622 matching lines @@
 
     // Scroll anchoring should not apply within #scroller.
     EXPECT_EQ(100, scroller->scrollPosition().y());
     EXPECT_EQ(nullptr, scrollAnchor(scroller).anchorObject());
     // Scroll anchoring should apply within main frame.
     EXPECT_EQ(250, viewport->scrollPosition().y());
     EXPECT_EQ(document().getElementById("outerAnchor")->layoutObject(),
         scrollAnchor(viewport).anchorObject());
 }
 
+TEST_F(ScrollAnchorTest, ExcludeAnchoringIfAncestorChangesPaddingOrMargin)
+{
+    setBodyInnerHTML(
+        "<style> body { height: 1000px; margin: 0px; } div { height: 100px } </style>"
+        "<div id='block1'>abc</div>"
+        "<div id='block2'>def</div>");
+
+    ScrollableArea* viewport = layoutViewport();
+    scrollLayoutViewport(DoubleSize(0, 150));
+
+    // Padding change on anchor's ancestor shouldn't trigger anchoring.
+    document().body()->setAttribute(HTMLNames::styleAttr,
+        AtomicString(String::format("padding: %dpx", 10)));
+    update();
+    EXPECT_EQ(150, viewport->scrollPosition().y());
+
+    // Margin change on anchor's ancestor shouldn't trigger anchoring.
+    document().body()->setAttribute(HTMLNames::styleAttr,
+        AtomicString(String::format("margin: %dpx", 10)));
+    update();
+    EXPECT_EQ(150, viewport->scrollPosition().y());
+
+    // Padding change on other elements should trigger anchoring.
+    document().getElementById("block1")->setAttribute(HTMLNames::styleAttr,
+        AtomicString(String::format("padding: %dpx", 10)));
+    update();
+    EXPECT_EQ(170, viewport->scrollPosition().y());
+}
+
 class ScrollAnchorCornerTest : public ScrollAnchorTest {
 protected:
     void checkCorner(const AtomicString& id, Corner corner, DoublePoint startPos, DoubleSize expectedAdjustment)
     {
         ScrollableArea* viewport = layoutViewport();
         Element* element = document().getElementById(id);
 
         viewport->setScrollPosition(startPos, UserScroll);
         element->setAttribute(HTMLNames::classAttr, "big");
         update();
@@ skipping 129 matching lines @@
         "<div id=c></div>"
         "<div id=d></div>");
 
     checkCorner("b", Corner::TopRight, DoublePoint(-20,  20),  DoubleSize(0, 0));
     checkCorner("a", Corner::TopRight, DoublePoint(-420, 20),  DoubleSize(400, 0));
     checkCorner("d", Corner::TopRight, DoublePoint(-20,  320), DoubleSize(0, -300));
     checkCorner("c", Corner::TopRight, DoublePoint(-420, 320), DoubleSize(400, -300));
 }
 
 }