Ignore layout shift due to viewport size change.

I tested this manually, both by resizing a Chrome window, and by
simulating mobile device rotation in devtools.

Change-Id: I69297b2501827070de4255b152d3099cb91ab11e
Bug: 958832
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1653609
Commit-Queue: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669830}

third_party/blink/renderer/core/frame/local_frame_view.cc

@@ -1221,6 +1221,9 @@ void LocalFrameView::ViewportSizeChanged(bool width_changed,
       frame_->GetDocument()->Lifecycle().LifecyclePostponed())
     return;
 
+  if (frame_->IsMainFrame())
+    jank_tracker_->NotifyViewportSizeChanged();
+
   auto* layout_view = GetLayoutView();
   if (layout_view) {
     // If this is the main frame, we might have got here by hiding/showing the

third_party/blink/renderer/core/layout/jank_tracker.cc

@@ -389,6 +389,11 @@ void JankTracker::NotifyScroll(ScrollType scroll_type) {
   observed_input_or_scroll_ = true;
 }
 
+void JankTracker::NotifyViewportSizeChanged() {
+  // This cancels any previously scheduled task from the same timer.
+  timer_.StartOneShot(kTimerDelay, FROM_HERE);
+}
+
 bool JankTracker::IsActive() {
   // This eliminates noise from the private Page object created by
   // SVGImage::DataChanged.

third_party/blink/renderer/core/layout/jank_tracker.h

@@ -39,6 +39,7 @@ class CORE_EXPORT JankTracker {
   void NotifyPrePaintFinished();
   void NotifyInput(const WebInputEvent&);
   void NotifyScroll(ScrollType);
+  void NotifyViewportSizeChanged();
   bool IsActive();
   double Score() const { return score_; }
   double ScoreWithMoveDistance() const { return score_with_move_distance_; }

third_party/blink/renderer/core/layout/jank_tracker_test.cc

@@ -447,6 +447,41 @@ TEST_F(JankTrackerSimTest, SubframeWeighting) {
   EXPECT_FLOAT_EQ(0.15, jank_tracker.WeightedScore());
 }
 
+TEST_F(JankTrackerSimTest, ViewportSizeChange) {
+  WebView().MainFrameWidget()->Resize(WebSize(800, 600));
+
+  SimRequest main_resource("https://example.com/", "text/html");
+  LoadURL("https://example.com/");
+  main_resource.Complete(R"HTML(
+    <style>
+      body { margin: 0; }
+      .square {
+        display: inline-block;
+        position: relative;
+        width: 300px;
+        height: 300px;
+        background:yellow;
+      }
+    </style>
+    <div class='square'></div>
+    <div class='square'></div>
+  )HTML");
+
+  Compositor().BeginFrame();
+  test::RunPendingTasks();
+
+  // Resize the viewport, making it 400px wide. This should cause the second div
+  // to change position during block layout flow. Since it was the result of a
+  // viewport size change, this position change should not affect the score.
+  WebView().MainFrameWidget()->Resize(WebSize(400, 600));
+
+  Compositor().BeginFrame();
+  test::RunPendingTasks();
+
+  JankTracker& jank_tracker = MainFrame().GetFrameView()->GetJankTracker();
+  EXPECT_FLOAT_EQ(0.0, jank_tracker.Score());
+}
+
 TEST_F(JankTrackerTest, StableCompositingChanges) {
   SetBodyInnerHTML(R"HTML(
     <style>