Add a UseCounter for position: sticky

This patch adds a UseCounter for position: sticky by tracking when new
StickyConstraints are added. This feat is achieved by removing non-const
access to PaintLayerScrollableArea::GetStickyConstraintsMap and adding
an explicit "AddStickyConstraints" function that increments the counter.

Bug: 1109232
Change-Id: Ic0543c6e3501019ada58aeec2c3885c3854c7897
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317607
Auto-Submit: Philip Rogers <pdr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791493}

third_party/blink/public/mojom/web_feature/web_feature.mojom

@@ -2686,6 +2686,7 @@ enum WebFeature {
   kV8PointerEvent_AzimuthAngle_AttributeGetter = 3351,
   kV8PointerEvent_AltitudeAngle_AttributeGetter = 3352,
   kCrossBrowsingContextGroupMainFrameNulledNonEmptyNameAccessed = 3353,
+  kPositionSticky = 3354,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

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

@@ -1066,7 +1066,7 @@ void LayoutBoxModelObject::UpdateStickyPositionConstraints() const {
   }
   PaintLayerScrollableArea* scrollable_area =
       Layer()->AncestorOverflowLayer()->GetScrollableArea();
-  scrollable_area->GetStickyConstraintsMap().Set(Layer(), constraints);
+  scrollable_area->AddStickyConstraints(Layer(), constraints);
 }
 
 bool LayoutBoxModelObject::IsSlowRepaintConstrainedObject() const {
@@ -1125,19 +1125,20 @@ PhysicalOffset LayoutBoxModelObject::StickyPositionOffset() const {
   if (!ancestor_overflow_layer || !ancestor_overflow_layer->GetScrollableArea())
     return PhysicalOffset();
 
-  StickyConstraintsMap& constraints_map =
-      ancestor_overflow_layer->GetScrollableArea()->GetStickyConstraintsMap();
-  auto it = constraints_map.find(Layer());
-  if (it == constraints_map.end())
+  auto* constraints =
+      ancestor_overflow_layer->GetScrollableArea()->GetStickyConstraints(
+          Layer());
+  if (!constraints)
     return PhysicalOffset();
-  StickyPositionScrollingConstraints* constraints = &it->value;
 
   // The sticky offset is physical, so we can just return the delta computed in
   // absolute coords (though it may be wrong with transforms).
   PhysicalRect constraining_rect = ComputeStickyConstrainingRect();
   constraining_rect.Move(PhysicalOffset::FromFloatPointRound(
       ancestor_overflow_layer->GetScrollableArea()->ScrollPosition()));
-  return constraints->ComputeStickyOffset(constraining_rect, constraints_map);
+  return constraints->ComputeStickyOffset(
+      constraining_rect,
+      ancestor_overflow_layer->GetScrollableArea()->GetStickyConstraintsMap());
 }
 
 PhysicalOffset LayoutBoxModelObject::AdjustedPositionRelativeTo(

third_party/blink/renderer/core/paint/paint_layer_scrollable_area.cc

@@ -2102,6 +2102,21 @@ void PaintLayerScrollableArea::UpdateResizerStyle(
   }
 }
 
+StickyPositionScrollingConstraints*
+PaintLayerScrollableArea::GetStickyConstraints(PaintLayer* layer) {
+  auto it = EnsureRareData().sticky_constraints_map_.find(layer);
+  if (it == EnsureRareData().sticky_constraints_map_.end())
+    return nullptr;
+  return &it->value;
+}
+
+void PaintLayerScrollableArea::AddStickyConstraints(
+    PaintLayer* layer,
+    StickyPositionScrollingConstraints constraints) {
+  UseCounter::Count(GetLayoutBox()->GetDocument(), WebFeature::kPositionSticky);
+  EnsureRareData().sticky_constraints_map_.Set(layer, constraints);
+}
+
 void PaintLayerScrollableArea::InvalidateAllStickyConstraints() {
   if (PaintLayerScrollableAreaRareData* d = RareData()) {
     for (PaintLayer* sticky_layer : d->sticky_constraints_map_.Keys()) {

third_party/blink/renderer/core/paint/paint_layer_scrollable_area.h

@@ -529,9 +529,11 @@ class CORE_EXPORT PaintLayerScrollableArea final
     had_vertical_scrollbar_before_relayout_ = val;
   }
 
-  StickyConstraintsMap& GetStickyConstraintsMap() {
+  const StickyConstraintsMap& GetStickyConstraintsMap() {
     return EnsureRareData().sticky_constraints_map_;
   }
+  StickyPositionScrollingConstraints* GetStickyConstraints(PaintLayer*);
+  void AddStickyConstraints(PaintLayer*, StickyPositionScrollingConstraints);
 
   void InvalidateAllStickyConstraints();
   void InvalidateStickyConstraintsFor(PaintLayer*);

third_party/blink/renderer/core/paint/paint_layer_scrollable_area_test.cc

@@ -1268,6 +1268,25 @@ TEST_P(PaintLayerScrollableAreaTest, CompositedStickyDescendant) {
                                   ->Translation2D());
 }
 
+TEST_P(PaintLayerScrollableAreaTest, StickyPositionUseCounter) {
+  SetBodyInnerHTML(R"HTML(
+    <div style="overflow: scroll; width: 500px; height: 300px;">
+      <div id=test></div>
+      <div id=forcescroll style="width: 10px; height: 700px;"></div>
+    </div>
+  )HTML");
+  EXPECT_FALSE(GetDocument().IsUseCounted(WebFeature::kPositionSticky));
+
+  auto* test = GetElementById("test");
+  test->setAttribute(html_names::kStyleAttr, "position: sticky;");
+  UpdateAllLifecyclePhasesForTest();
+  EXPECT_FALSE(GetDocument().IsUseCounted(WebFeature::kPositionSticky));
+
+  test->setAttribute(html_names::kStyleAttr, "top: 0; position: sticky;");
+  UpdateAllLifecyclePhasesForTest();
+  EXPECT_TRUE(GetDocument().IsUseCounted(WebFeature::kPositionSticky));
+}
+
 // Delayed scroll offset clamping should not crash. https://crbug.com/842495
 TEST_P(PaintLayerScrollableAreaTest, IgnoreDelayedScrollOnDestroyedLayer) {
   SetBodyInnerHTML(R"HTML(

tools/metrics/histograms/enums.xml

@@ -28541,6 +28541,7 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3352" label="V8PointerEvent_AltitudeAngle_AttributeGetter"/>
   <int value="3353"
       label="CrossBrowsingContextGroupMainFrameNulledNonEmptyNameAccessed"/>
+  <int value="3354" label="PositionSticky"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">