LayoutShiftTracker: Reset post-prepaint state even if we early out.

This patch ensures that we reset some state that we accumulate and
only reset if we actually report a layout shift.

The problem is that the accumulated values can later cause us _not_
to early out from ObjectShifted and report a layout shift where we
should not have reported one.

By resetting the state after every prepaint, we ensure that we reset
the accumulated state.

R=skobes@chromium.org

Change-Id: I2e764d3d847669cbdf06219da183cda02df57684
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2639862
Commit-Queue: vmpstr <vmpstr@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845904}

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

@@ -423,7 +423,7 @@ double LayoutShiftTracker::SubframeWeightingFactor() const {
          main_frame_size.Area();
 }
 
-void LayoutShiftTracker::NotifyPrePaintFinished() {
+void LayoutShiftTracker::NotifyPrePaintFinishedInternal() {
   if (!is_active_)
     return;
   if (region_.IsEmpty())
@@ -465,8 +465,13 @@ void LayoutShiftTracker::NotifyPrePaintFinished() {
 
   if (!region_.IsEmpty())
     SetLayoutShiftRects(region_.GetRects());
-  region_.Reset();
+}
 
+void LayoutShiftTracker::NotifyPrePaintFinished() {
+  NotifyPrePaintFinishedInternal();
+
+  // Reset accumulated state.
+  region_.Reset();
   frame_max_distance_ = 0.0;
   frame_scroll_delta_ = ScrollOffset();
   attributions_.fill(Attribution());

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

@@ -157,6 +157,7 @@ class CORE_EXPORT LayoutShiftTracker final
   void UpdateInputTimestamp(base::TimeTicks timestamp);
   LayoutShift::AttributionList CreateAttributionList() const;
   void SubmitPerformanceEntry(double score_delta, bool input_detected) const;
+  void NotifyPrePaintFinishedInternal();
 
   Member<LocalFrameView> frame_view_;
   bool is_active_;

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

@@ -5,6 +5,7 @@
 #include "third_party/blink/renderer/core/layout/layout_shift_tracker.h"
 
 #include "third_party/blink/public/common/input/web_mouse_event.h"
+#include "third_party/blink/renderer/core/dom/dom_token_list.h"
 #include "third_party/blink/renderer/core/frame/local_dom_window.h"
 #include "third_party/blink/renderer/core/frame/visual_viewport.h"
 #include "third_party/blink/renderer/core/frame/web_local_frame_impl.h"
@@ -623,4 +624,46 @@ TEST_F(LayoutShiftTrackerTest, ClipByVisualViewport) {
                   GetLayoutShiftTracker().Score());
 }
 
+TEST_F(LayoutShiftTrackerTest, ScrollThenCauseScrollAnchoring) {
+  SetBodyInnerHTML(R"HTML(
+    <style>
+      .big {
+        width: 100px;
+        height: 500px;
+        background: blue;
+      }
+      .small {
+        width: 100px;
+        height: 100px;
+        background: green;
+      }
+    </style>
+    <div class=big id=target></div>
+    <div class=big></div>
+    <div class=big></div>
+    <div class=big></div>
+    <div class=big></div>
+    <div class=big></div>
+  )HTML");
+  auto* target_element = GetDocument().getElementById("target");
+
+  // Scroll the window which accumulates a scroll in the layout shift tracker.
+  GetDocument().domWindow()->scrollBy(0, 1000);
+  UpdateAllLifecyclePhasesForTest();
+
+  EXPECT_FLOAT_EQ(0, GetLayoutShiftTracker().Score());
+
+  target_element->classList().Remove("big");
+  target_element->classList().Add("small");
+  UpdateAllLifecyclePhasesForTest();
+
+  EXPECT_FLOAT_EQ(0, GetLayoutShiftTracker().Score());
+
+  target_element->classList().Remove("small");
+  target_element->classList().Add("big");
+  UpdateAllLifecyclePhasesForTest();
+
+  EXPECT_FLOAT_EQ(0, GetLayoutShiftTracker().Score());
+}
+
 }  // namespace blink