Disable anchoring if ancestor changes padding or margin

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

Index: third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp
diff --git a/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp b/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp
index 84a7c4024be401ca6710f1845a4ee5d9ae25493d..2f64fedaea09b47d616641862722dc27a2398dab 100644
--- a/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp
+++ b/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp
@@ -640,6 +640,35 @@ TEST_F(ScrollAnchorTest, OptOutScrollingDiv)
         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)