GSB uses delta_hints to calculate scrolling chain.

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 2775 matching lines @@
 
   active_tree_->SetCurrentlyScrollingNode(scrolling_node);
   // TODO(majidvp): get rid of wheel_scrolling_ and set is_direct_manipulation
   // in input_handler_proxy instead.
   wheel_scrolling_ = IsWheelBasedScroll(type);
   scroll_state->set_is_direct_manipulation(!wheel_scrolling_);
   // Invoke |DistributeScrollDelta| even with zero delta and velocity to ensure
   // scroll customization callbacks are invoked.
   DistributeScrollDelta(scroll_state);
 
+  // If the CurrentlyScrollingNode doesn't exist after distributing scroll
+  // delta, no scroller can scroll in the given delta hint direction(s).
+  if (!active_tree_->CurrentlyScrollingNode()) {
+    scroll_status.thread = InputHandler::SCROLL_IGNORED;
+    scroll_status.main_thread_scrolling_reasons =
+        MainThreadScrollingReason::kNotScrollingOnMain;
+    return scroll_status;
+  }
+
   client_->RenewTreePriority();
   RecordCompositorSlowScrollMetric(type, CC_THREAD);
 
   UpdateScrollSourceInfo(wheel_scrolling_);
 
   return scroll_status;
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::RootScrollBegin(
     ScrollState* scroll_state,
@@ skipping 107 matching lines @@
   }
 
   // If |first_scrolling_layer_or_drawn_scrollbar| is not scrollable, it must
   // be a drawn scrollbar. These hit tests require falling back to main-thread
   // scrolling.
   DCHECK(first_scrolling_layer_or_drawn_scrollbar->IsDrawnScrollbar());
   return false;
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimatedBegin(
-    const gfx::Point& viewport_point) {
+    ScrollState* scroll_state) {
   InputHandler::ScrollStatus scroll_status;
   scroll_status.main_thread_scrolling_reasons =
       MainThreadScrollingReason::kNotScrollingOnMain;
   ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
   ScrollNode* scroll_node = scroll_tree.CurrentlyScrollingNode();
   if (scroll_node) {
     gfx::Vector2dF delta;
 
     if (ScrollAnimationUpdateTarget(scroll_node, delta, base::TimeDelta())) {
       scroll_status.thread = SCROLL_ON_IMPL_THREAD;
     } else {
       scroll_status.thread = SCROLL_IGNORED;
       scroll_status.main_thread_scrolling_reasons =
           MainThreadScrollingReason::kNotScrollable;
     }
     return scroll_status;
   }
-  ScrollStateData scroll_state_data;
-  scroll_state_data.position_x = viewport_point.x();
-  scroll_state_data.position_y = viewport_point.y();
-  ScrollState scroll_state(scroll_state_data);
 
   // ScrollAnimated is used for animated wheel scrolls. We find the first layer
   // that can scroll and set up an animation of its scroll offset. Note that
   // this does not currently go through the scroll customization machinery
   // that ScrollBy uses for non-animated wheel scrolls.
-  scroll_status = ScrollBegin(&scroll_state, WHEEL);
+  scroll_status = ScrollBegin(scroll_state, WHEEL);
   if (scroll_status.thread == SCROLL_ON_IMPL_THREAD) {
     scroll_animating_latched_node_id_ = ScrollTree::kInvalidNodeId;
     ScrollStateData scroll_state_end_data;
     scroll_state_end_data.is_ending = true;
     ScrollState scroll_state_end(scroll_state_end_data);
     ScrollEnd(&scroll_state_end);
   }
   return scroll_status;
 }
 
@@ skipping 344 matching lines @@
 }
 
 void LayerTreeHostImpl::DistributeScrollDelta(ScrollState* scroll_state) {
   // TODO(majidvp): in Blink we compute scroll chain only at scroll begin which
   // is not the case here. We eventually want to have the same behaviour on both
   // sides but it may become a non issue if we get rid of scroll chaining (see
   // crbug.com/526462)
   std::list<ScrollNode*> current_scroll_chain;
   ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
   ScrollNode* scroll_node = scroll_tree.CurrentlyScrollingNode();
-  ScrollNode* viewport_scroll_node = OuterViewportScrollNode();
+  ScrollNode* viewport_scroll_node =
+      viewport()->MainScrollLayer()
+          ? scroll_tree.Node(viewport()->MainScrollLayer()->scroll_tree_index())
+          : nullptr;
   if (scroll_node) {
     // TODO(bokan): The loop checks for a null parent but don't we still want to
     // distribute to the root scroll node?
     for (; scroll_tree.parent(scroll_node);
          scroll_node = scroll_tree.parent(scroll_node)) {
       if (scroll_node == viewport_scroll_node) {
         // Don't chain scrolls past the outer viewport scroll layer. Once we
         // reach that, we should scroll the viewport which is represented by the
         // main viewport scroll layer.
         DCHECK(viewport_scroll_node);
         current_scroll_chain.push_front(viewport_scroll_node);
         break;
       }
 
       if (!scroll_node->scrollable)
         continue;
 
-      current_scroll_chain.push_front(scroll_node);
+      if (CanConsumeDelta(scroll_node, *scroll_state))
+        current_scroll_chain.push_front(scroll_node);
     }
   }
+  active_tree_->SetCurrentlyScrollingNode(
+      current_scroll_chain.empty() ? nullptr : current_scroll_chain.back());
   scroll_state->set_scroll_chain_and_layer_tree(current_scroll_chain,
                                                 active_tree());
   scroll_state->DistributeToScrollChainDescendant();
 }
 
+bool LayerTreeHostImpl::CanConsumeDelta(ScrollNode* scroll_node,
+                                        const ScrollState& scroll_state) {
+  gfx::Vector2dF delta_to_scroll;
+  if (scroll_state.is_beginning()) {
+    if (scroll_state.ignore_delta_hints())
+      return true;
+
+    delta_to_scroll = gfx::Vector2dF(scroll_state.delta_x_hint(),
+                                     scroll_state.delta_y_hint());
+  } else {
+    delta_to_scroll =
+        gfx::Vector2dF(scroll_state.delta_x(), scroll_state.delta_y());
+  }
+
+  if (delta_to_scroll == gfx::Vector2dF())
+    return true;
+
+  if (ComputeScrollDelta(scroll_node, delta_to_scroll) != gfx::Vector2dF())
+    return true;
+
+  if (scroll_node->scrolls_inner_viewport) {

[bokan, 2017/06/19 21:43:17]

I don't think you need this special case, at least not here. I think
ComputeScrollDelta does though. It should check if the given node is either
Inner or Outer and compute the amount consumable between the both of them.

[sahel, 2017/06/22 16:34:17]

I don't think ComputeScrollDelta checks for viewport nodes.
(https://cs.chromium.org/chromium/src/cc/trees/layer_tree_host_impl.cc?sq=pack...)

The viewport::ScrollAnimated though handles the special cases and calls this for
inner and outer viewport nodes.
(https://cs.chromium.org/chromium/src/cc/layers/viewport.cc?sq=package:chromiu...).

[bokan, 2017/06/22 21:44:37]

Sorry, I meant that we should add it to ComputeScrollDelta. But on second
thought, looking at viewport::ScrollAnimated, I think I was wrong.

I still think this block is wrong though. Currently, if we hit the inner
viewport, we'll check to see if the outer viewport can scroll the delta and add
the inner viewport if it can.

I think you should be checking if scroll_node == viewport()->MainScrollLayer()
and if so, return the result of a (new) viewport()->CanScrollDelta() which
checks whether you can scroll either inner or outer. Alternatively, if you hit
the viewport()->MainScrollLayer(), shouldn't we just return true? I.e. the
viewport always "consumes" the scroll from latching's perspective.

Aside: if you hit the inner viewport layer, it means you haven't hit the outer
(since we break as soon as we hit it above). This can only happen when
document.rootScroller ships so we don't really have to worry about it today.
When it does though, I think this is still right because in that case we'd only
want to scroll the inner viewport so adding it to the chain is fine there.

[sahel, 2017/06/23 18:22:03]

I deleted this special case, as we discussed always adding the outerviewport if
it exists is enough and if not the inner viewport will be treated like other
scroll nodes.

[bokan, 2017/06/23 18:56:44]

Acknowledged.

+    ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
+    ScrollNode* viewport_scroll_node =
+        viewport()->MainScrollLayer()
+            ? scroll_tree.Node(
+                  viewport()->MainScrollLayer()->scroll_tree_index())
+            : nullptr;
+    if (viewport_scroll_node)
+      return ComputeScrollDelta(viewport_scroll_node, delta_to_scroll) !=
+             gfx::Vector2dF();
+  }
+
+  return false;
+}
+
 InputHandlerScrollResult LayerTreeHostImpl::ScrollBy(
     ScrollState* scroll_state) {
   DCHECK(scroll_state);
 
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBy");
   ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
   ScrollNode* scroll_node = scroll_tree.CurrentlyScrollingNode();
 
   if (!scroll_node)
     return InputHandlerScrollResult();
@@ skipping 961 matching lines @@
 
 void LayerTreeHostImpl::ShowScrollbarsForImplScroll(ElementId element_id) {
   if (!element_id)
     return;
   if (ScrollbarAnimationController* animation_controller =
           ScrollbarAnimationControllerForElementId(element_id))
     animation_controller->DidScrollUpdate();
 }
 
 }  // namespace cc