Record size of scroller that user scrolls

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 18 matching lines @@
 #include "cc/base/math_util.h"
 #include "cc/benchmarks/benchmark_instrumentation.h"
 #include "cc/debug/rendering_stats_instrumentation.h"
 #include "cc/debug/traced_value.h"
 #include "cc/input/browser_controls_offset_manager.h"
 #include "cc/input/main_thread_scrolling_reason.h"
 #include "cc/input/page_scale_animation.h"
 #include "cc/input/scroll_elasticity_helper.h"
 #include "cc/input/scroll_state.h"
 #include "cc/input/scrollbar_animation_controller.h"
+#include "cc/input/scroller_size.h"
 #include "cc/layers/append_quads_data.h"
 #include "cc/layers/effect_tree_layer_list_iterator.h"
 #include "cc/layers/heads_up_display_layer_impl.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/painted_scrollbar_layer_impl.h"
 #include "cc/layers/render_surface_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
 #include "cc/layers/surface_layer_impl.h"
 #include "cc/layers/viewport.h"
 #include "cc/output/compositor_frame.h"
@@ skipping 2712 matching lines @@
         scroll_status.thread = SCROLL_UNKNOWN;
         scroll_status.main_thread_scrolling_reasons =
             MainThreadScrollingReason::kFailedHitTest;
         return scroll_status;
       }
     }
 
     scrolling_node = FindScrollNodeForDeviceViewportPoint(
         device_viewport_point, type, layer_impl, &scroll_on_main_thread,
         &scroll_status.main_thread_scrolling_reasons);
+    if (!scroll_on_main_thread && scrolling_node &&
+        InnerViewportScrollLayer() && OuterViewportScrollLayer() &&

[flackr, 2017/04/20 07:15:01]

Why do we need to have a inner and outer viewport scroll layer to record
overflow scroller sizes?

[bokan, 2017/04/20 14:34:06]

I think it's meant to null check the conditions below but this would be clearer
and more correct like this:

(!InnerViewportScrollLayer() || scrolling_node->element_id !=
InnerViewportScrollLayer()->element_id())

That said, I *think* this check is equivalent to
!scrolling_node->scrolls_inner_viewport

+        scrolling_node->element_id !=
+            InnerViewportScrollLayer()->element_id() &&
+        scrolling_node->element_id !=
+            OuterViewportScrollLayer()->element_id()) {
+      if (IsWheelBasedScroll(type)) {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Scroll.ScrollerSize.OnScroll_Wheel",
+            scrolling_node->scroll_clip_layer_bounds.GetArea(), 1,
+            kMaxScrollerSize, kScrollerSizeBucketCount);
+      } else {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Scroll.ScrollerSize.OnScroll_Touch",
+            scrolling_node->scroll_clip_layer_bounds.GetArea(), 1,
+            kMaxScrollerSize, kScrollerSizeBucketCount);
+      }
+    }
   }
 
   if (scroll_on_main_thread) {
     RecordCompositorSlowScrollMetric(type, MAIN_THREAD);
 
     scroll_status.thread = SCROLL_ON_MAIN_THREAD;
     return scroll_status;
   } else if (scrolling_node) {
     scroll_affects_scroll_handler_ = active_tree_->have_scroll_event_handlers();
   }
@@ skipping 1496 matching lines @@
 }
 
 void LayerTreeHostImpl::UpdateScrollSourceInfo(bool is_wheel_scroll) {
   if (is_wheel_scroll)
     has_scrolled_by_wheel_ = true;
   else
     has_scrolled_by_touch_ = true;
 }
 
 }  // namespace cc