Make scroll anchoring adjustment when the bounds of a scroller is changed

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 176 matching lines @@
     setHeight(document().getElementById("outerChanger"), 150);
 
     EXPECT_EQ(300, scroller->scrollPosition().y());
     EXPECT_EQ(document().getElementById("innerAnchor")->layoutObject(),
         scrollAnchor(scroller).anchorObject());
     EXPECT_EQ(500, viewport->scrollPosition().y());
     EXPECT_EQ(document().getElementById("outerAnchor")->layoutObject(),
         scrollAnchor(viewport).anchorObject());
 }
 
+// Test that scroll anchoring causes no visible jump when a layout change
+// (such as removal of a DOM element) changes the scroll bounds.
+TEST_F(ScrollAnchorTest, AnchoringWhenContentRemoved)
+{
+    setBodyInnerHTML(
+        "<style>"
+        "    #changer { height: 1500px; }"
+        "    #anchor {"
+        "        width: 150px; height: 1000px; background-color: pink;"
+        "    }"
+        "</style>"
+        "<div id='changer'></div>"
+        "<div id='anchor'></div>");
+
+    ScrollableArea* viewport = layoutViewport();
+    scrollLayoutViewport(DoubleSize(0, 1600));
+
+    setHeight(document().getElementById("changer"), 0);
+
+    EXPECT_EQ(100, viewport->scrollPosition().y());
+    EXPECT_EQ(document().getElementById("anchor")->layoutObject(),
+        scrollAnchor(viewport).anchorObject());
+}
+
+// Test that scroll anchoring causes no visible jump when a layout change
+// (such as removal of a DOM element) changes the scroll bounds of a scrolling
+// div.
+TEST_F(ScrollAnchorTest, AnchoringWhenContentRemovedFromScrollingDiv)
+{
+    setBodyInnerHTML(
+        "<style>"
+        "    #scroller { height: 500px; width: 200px; overflow: scroll; }"
+        "    #changer { height: 1500px; }"
+        "    #anchor {"
+        "        width: 150px; height: 1000px; overflow: scroll;"
+        "    }"
+        "</style>"
+        "<div id='scroller'>"
+        "    <div id='changer'></div>"
+        "    <div id='anchor'></div>"
+        "</div>");
+
+    ScrollableArea* scroller = scrollerForElement(document().getElementById("scroller"));
+
+    document().getElementById("scroller")->setScrollTop(1600);
+
+    setHeight(document().getElementById("changer"), 0);
+
+    EXPECT_EQ(100, scroller->scrollPosition().y());
+    EXPECT_EQ(document().getElementById("anchor")->layoutObject(),
+        scrollAnchor(scroller).anchorObject());
+}
+
 TEST_F(ScrollAnchorTest, FractionalOffsetsAreRoundedBeforeComparing)
 {
     setBodyInnerHTML(
         "<style> body { height: 1000px } </style>"
         "<div id='block1' style='height: 50.4px'>abc</div>"
         "<div id='block2' style='height: 100px'>def</div>");
 
     ScrollableArea* viewport = layoutViewport();
     scrollLayoutViewport(DoubleSize(0, 100));
 
@@ skipping 522 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));
 }
 
 }