Deflake virtual/threaded/fast/scroll-behavior/smooth-scroll/ongoing-smooth-scroll-vertical-rl-ancho…

third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/ongoing-smooth-scroll-vertical-rl-anchors.html

 <script src="../../../resources/testharness.js"></script>
 <script src="../../../resources/testharnessreport.js"></script>
 <style>
   html {
     -webkit-writing-mode: vertical-rl;
   }
   body {
     margin: 0px;
     height: 2000px;
     width: 2000px;
   }
   #changer {
     background-color: #FF7F7F;
-    width: 0px;
+    width: 10px;
   }
   #anchor {
     width: 1500px;
     background-color: #84BE6A;
   }
 </style>
 
 <div id="changer"></div>
 <div id="anchor"></div>
 
 <script>
-  var asyncTest = async_test("Verify smooth scroll interaction with scroll anchroing on page with vertical-lr writing mode");
+  var asyncTest = async_test("Verify smooth scroll interaction with scroll anchoring on page with vertical-lr writing mode");
 
   // The element that will change in height.
   var ch;
+  // Initital scroll position.
+  var initialX = -10;
+  var initialY = 0;
   // Amount to smooth scroll by.
   var userScrollX = -205;
   var userScrollY = 51;
   // Amount to change the height of the element above the viewport.
-  var changerX =  100;
+  var changerX = 100;
   // End position: height of ch + userScroll.
   var endX = -305;
   var endY = 51;
 
   function scrollListener() {
     if (window.scrollX == endX && window.scrollY == endY) {
       asyncTest.done();
       return;
     }
 
     if (ch.style.width != "100")
       ch.style.width = changerX;
   }
 
   window.onload = function() {
     assert_false(!window.internals, 'This test requires internals');
     internals.runtimeFlags.scrollAnchoringEnabled = true;
     // Turn on smooth scrolling.
     internals.settings.setScrollAnimatorEnabled(true);
 
     ch = document.getElementById("changer");
+    window.scrollTo(initialX, initialY);
 
-    // Smooth scroll.
-    eventSender.mouseMoveTo(100, 100);
-    eventSender.continuousMouseScrollBy(-userScrollX, -userScrollY);
+    asyncTest.step(function() {
+      assert_equals(window.scrollX, initialX);
+      assert_equals(window.scrollY, initialY);
 
-    document.addEventListener("scroll", scrollListener);
+      // Smooth scroll.
+      eventSender.mouseMoveTo(100, 100);
+      eventSender.continuousMouseScrollBy(-userScrollX, -userScrollY);
+
+      document.addEventListener("scroll", scrollListener);
+    });
   }
 </script>