JankTracker: Early exit for small movements and small sizes.

This change ignores movements of less than 3px, which are small enough
to not be distracting.

It also adds an early exit for objects smaller than the jank region
granularity.  This is a performance optimization, since these objects
will not affect the jank region after snapping.

This should mostly eliminate JankTracker's impact on blink_perf.paint
benchmarks (already reduced by crrev.com/562327).

Bug: 581518,847252
Change-Id: I3979421e7a25654e67d3eeba7a2cd5d217ed3f03
Reviewed-on: https://chromium-review.googlesource.com/1175109
Commit-Queue: Steve Kobes <skobes@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583255}

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

@@ -17,6 +17,7 @@ namespace blink {
 
 static constexpr TimeDelta kTimerDelay = TimeDelta::FromSeconds(3);
 static const float kRegionGranularitySteps = 60.0;
+static const float kMovementThreshold = 3.0;  // CSS pixels.
 
 static FloatPoint LogicalStart(const FloatRect& rect,
                                const LayoutObject& object) {
@@ -41,6 +42,21 @@ static float RegionGranularityScale(const IntRect& viewport) {
          std::min(viewport.Height(), viewport.Width());
 }
 
+static bool EqualWithinMovementThreshold(const FloatPoint& a,
+                                         const FloatPoint& b,
+                                         const LayoutObject& object) {
+  float threshold_physical_px =
+      kMovementThreshold * object.StyleRef().EffectiveZoom();
+  return fabs(a.X() - b.X()) < threshold_physical_px &&
+         fabs(a.Y() - b.Y()) < threshold_physical_px;
+}
+
+static bool SmallerThanRegionGranularity(const LayoutRect& rect,
+                                         float granularity_scale) {
+  return rect.Width().ToFloat() * granularity_scale < 0.5 ||
+         rect.Height().ToFloat() * granularity_scale < 0.5;
+}
+
 JankTracker::JankTracker(LocalFrameView* frame_view)
     : frame_view_(frame_view),
       score_(0.0),
@@ -60,8 +76,16 @@ void JankTracker::NotifyObjectPrePaint(const LayoutObject& object,
   if (old_visual_rect.IsEmpty() || new_visual_rect.IsEmpty())
     return;
 
-  if (LogicalStart(FloatRect(old_visual_rect), object) ==
-      LogicalStart(FloatRect(new_visual_rect), object))
+  if (EqualWithinMovementThreshold(
+          LogicalStart(FloatRect(old_visual_rect), object),
+          LogicalStart(FloatRect(new_visual_rect), object), object))
+    return;
+
+  IntRect viewport = frame_view_->GetScrollableArea()->VisibleContentRect();
+  float scale = RegionGranularityScale(viewport);
+
+  if (SmallerThanRegionGranularity(old_visual_rect, scale) &&
+      SmallerThanRegionGranularity(new_visual_rect, scale))
     return;
 
   const auto* local_transform = painting_layer.GetLayoutObject()
@@ -87,7 +111,6 @@ void JankTracker::NotifyObjectPrePaint(const LayoutObject& object,
   // a better idea of how to aggregate multiple scores for a page.
   // See review thread of http://crrev.com/c/1046155 for more details.
 
-  IntRect viewport = frame_view_->GetScrollableArea()->VisibleContentRect();
   if (!old_visual_rect_abs.Intersects(viewport) &&
       !new_visual_rect_abs.Intersects(viewport))
     return;
@@ -106,7 +129,6 @@ void JankTracker::NotifyObjectPrePaint(const LayoutObject& object,
   IntRect visible_new_visual_rect = RoundedIntRect(new_visual_rect_abs);
   visible_new_visual_rect.Intersect(viewport);
 
-  float scale = RegionGranularityScale(viewport);
   visible_old_visual_rect.Scale(scale);
   visible_new_visual_rect.Scale(scale);
 

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

@@ -79,4 +79,31 @@ TEST_F(JankTrackerTest, RtlDistance) {
   EXPECT_FLOAT_EQ(20.0, GetJankTracker().MaxDistance());
 }
 
+TEST_F(JankTrackerTest, SmallMovementIgnored) {
+  SetBodyInnerHTML(R"HTML(
+    <style>
+      #j { position: relative; width: 300px; height: 100px; }
+    </style>
+    <div id='j'></div>
+  )HTML");
+  GetDocument().getElementById("j")->setAttribute(HTMLNames::styleAttr,
+                                                  AtomicString("top: 2px"));
+  GetFrameView().UpdateAllLifecyclePhases();
+  EXPECT_EQ(0.0, GetJankTracker().Score());
+}
+
+TEST_F(JankTrackerTest, SmallMovementIgnoredWithZoom) {
+  GetDocument().GetFrame()->SetPageZoomFactor(2);
+  SetBodyInnerHTML(R"HTML(
+    <style>
+      #j { position: relative; width: 300px; height: 100px; }
+    </style>
+    <div id='j'></div>
+  )HTML");
+  GetDocument().getElementById("j")->setAttribute(HTMLNames::styleAttr,
+                                                  AtomicString("top: 2px"));
+  GetFrameView().UpdateAllLifecyclePhases();
+  EXPECT_EQ(0.0, GetJankTracker().Score());
+}
+
 }  // namespace blink