ScrollbarController cleanup.

This CL optimizes hit testing on pointerdown. Previously, 2 hit tests
were performed for a pointerdown on a composited scrollbar. With this
change, we now do only 1. Please see the bug for more details.

Bug: 40160629
Change-Id: Ia71343f12e56dfe8ad70c9e36208e29b04376868
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5273278
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1257041}

cc/input/input_handler.cc

@@ -547,7 +547,7 @@ InputHandlerPointerResult InputHandler::MouseMoveAt(
 }
 
 PointerResultType InputHandler::HitTest(const gfx::PointF& viewport_point) {
-  return scrollbar_controller_->HitTest(viewport_point);
+  return scrollbar_controller_->HitTest(viewport_point).type;
 }
 
 InputHandlerPointerResult InputHandler::MouseDown(

cc/input/input_handler.h

@@ -58,6 +58,13 @@ enum class ScrollBeginThreadState {
   kMaxValue = kScrollingOnMain,
 };
 
+struct CC_EXPORT PointerHitTestResult {
+  PointerHitTestResult() = default;
+
+  PointerResultType type = PointerResultType::kUnhandled;
+  raw_ptr<const LayerImpl> layer_impl = nullptr;
+};
+
 struct CC_EXPORT InputHandlerPointerResult {
   InputHandlerPointerResult() = default;
   // Tells what type of processing occurred in the input handler as a result of

cc/input/scrollbar_controller.cc

@@ -51,7 +51,7 @@ ScrollbarLayerImplBase* ScrollbarController::ScrollbarLayer() const {
   return nullptr;
 }
 
-PointerResultType ScrollbarController::HitTest(
+PointerHitTestResult ScrollbarController::HitTest(
     const gfx::PointF position_in_widget) const {
   // If a non-custom scrollbar layer was not found, we return early as there is
   // no point in setting additional state in the ScrollbarController. Return an
@@ -59,16 +59,20 @@ PointerResultType ScrollbarController::HitTest(
   // to InputHandlerProxy::RouteToTypeSpecificHandler, the pointer event gets
   // passed on to the main thread.
   const LayerImpl* layer_impl = GetLayerHitByPoint(position_in_widget);
+  PointerHitTestResult result;
+
   if (!(layer_impl && layer_impl->IsScrollbarLayer()))
-    return PointerResultType::kUnhandled;
+    return result;
 
   // If the scrollbar layer has faded out (eg: Overlay scrollbars), don't
   // initiate a scroll.
   const ScrollbarLayerImplBase* scrollbar = ToScrollbarLayer(layer_impl);
   if (scrollbar->OverlayScrollbarOpacity() == 0.f)
-    return PointerResultType::kUnhandled;
+    return result;
 
-  return PointerResultType::kScrollbarScroll;
+  result.type = PointerResultType::kScrollbarScroll;
+  result.layer_impl = layer_impl;
+  return result;
 }
 
 // Performs hit test and prepares scroll deltas that will be used by GSB and
@@ -76,13 +80,15 @@ PointerResultType ScrollbarController::HitTest(
 InputHandlerPointerResult ScrollbarController::HandlePointerDown(
     const gfx::PointF position_in_widget,
     bool jump_key_modifier) {
-  if (HitTest(position_in_widget) != PointerResultType::kScrollbarScroll)
+  PointerHitTestResult hit_test_result = HitTest(position_in_widget);
+  if (hit_test_result.type != PointerResultType::kScrollbarScroll) {
     return InputHandlerPointerResult();
+  }
 
   // TODO(arakeri): GetLayerHitByPoint should ideally be called only once per
   // pointerdown. This needs to be optimized. See crbug.com/1156922.
   const ScrollbarLayerImplBase* scrollbar =
-      ToScrollbarLayer(GetLayerHitByPoint(position_in_widget));
+      ToScrollbarLayer(hit_test_result.layer_impl);
   captured_scrollbar_metadata_ = CapturedScrollbarMetadata();
   captured_scrollbar_metadata_->scroll_element_id =
       scrollbar->scroll_element_id();

cc/input/scrollbar_controller.h

@@ -157,7 +157,7 @@ class CC_EXPORT ScrollbarController {
   ScrollbarLayerImplBase* ScrollbarLayer() const;
   void WillBeginImplFrame();
   void ResetState();
-  PointerResultType HitTest(const gfx::PointF position_in_widget) const;
+  PointerHitTestResult HitTest(const gfx::PointF position_in_widget) const;
 
  private:
   FRIEND_TEST_ALL_PREFIXES(LayerTreeHostImplTest, ThumbDragAfterJumpClick);