Add initial UMA metrics for ScrollAnchoring

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/layout/LayoutBox.h"
 #include "core/layout/LayoutTestHelper.h"
 #include "core/paint/PaintLayerScrollableArea.h"
+#include "platform/testing/HistogramTester.h"
 
 namespace blink {
 
 using Corner = ScrollAnchor::Corner;
 
 class ScrollAnchorTest : public RenderingTest {
 public:
     ScrollAnchorTest() { RuntimeEnabledFeatures::setScrollAnchoringEnabled(true); }
     ~ScrollAnchorTest() { RuntimeEnabledFeatures::setScrollAnchoringEnabled(false); }
 
@@ skipping 56 matching lines @@
 
     EXPECT_EQ(250, viewport->scrollPosition().y());
     EXPECT_EQ(document().getElementById("block2")->layoutObject(),
         scrollAnchor(viewport).anchorObject());
 
     // ScrollableArea::userScroll should clear the anchor.
     viewport->userScroll(ScrollByPrecisePixel, FloatSize(0, 100));
     EXPECT_EQ(nullptr, scrollAnchor(viewport).anchorObject());
 }
 
+TEST_F(ScrollAnchorTest, UMAMetricUpdated)
+{
+    HistogramTester histogramTester;
+    setBodyInnerHTML(
+        "<style> body { height: 1000px } div { height: 100px } </style>"
+        "<div id='block1'>abc</div>"
+        "<div id='block2'>def</div>");
+
+    ScrollableArea* viewport = layoutViewport();
+
+    // Scroll position not adjusted, metric not updated.
+    scrollLayoutViewport(DoubleSize(0, 150));
+    histogramTester.expectTotalCount(
+        "Layout.ScrollAnchor.AdjustedScrollOffset", 0);
+
+    // Height changed, verify metric updated once.
+    setHeight(document().getElementById("block1"), 200);
+    histogramTester.expectUniqueSample(
+        "Layout.ScrollAnchor.AdjustedScrollOffset", 1, 1);
+
+    EXPECT_EQ(250, viewport->scrollPosition().y());
+    EXPECT_EQ(document().getElementById("block2")->layoutObject(),
+        scrollAnchor(viewport).anchorObject());
+}
+
 TEST_F(ScrollAnchorTest, AnchorWithLayerInScrollingDiv)
 {
     setBodyInnerHTML(
         "<style>"
         "    #scroller { overflow: scroll; width: 500px; height: 400px; }"
         "    div { height: 100px }"
         "    #block2 { overflow: hidden }"
         "    #space { height: 1000px; }"
         "</style>"
         "<div id='scroller'><div id='space'>"
@@ skipping 170 matching lines @@
         "<div id=c></div>"
         "<div id=d></div>");
 
     checkCorner("a", Corner::BottomRight, DoublePoint(20,  20),  DoubleSize(+400, +300));
     checkCorner("b", Corner::BottomLeft,  DoublePoint(420, 20),  DoubleSize(-400, +300));
     checkCorner("c", Corner::TopRight,    DoublePoint(20,  320), DoubleSize(+400, -300));
     checkCorner("d", Corner::TopLeft,     DoublePoint(420, 320), DoubleSize(-400, -300));
 }
 
 }