[ash-md] Stacks child layers properly for sticky header rows

ash/common/system/tray/tray_details_view.cc

 // Copyright (c) 2012 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 "ash/common/system/tray/tray_details_view.h"
 
 #include "ash/common/ash_view_ids.h"
 #include "ash/common/material_design/material_design_controller.h"
 #include "ash/common/system/tray/fixed_sized_scroll_view.h"
 #include "ash/common/system/tray/system_menu_button.h"
@@ skipping 82 matching lines @@
     views::View::Layout();
     headers_.clear();
     for (int i = 0; i < child_count(); ++i) {
       views::View* view = child_at(i);
       if (view->id() == VIEW_ID_STICKY_HEADER)
         headers_.emplace_back(view);
     }
     PositionHeaderRows();
   }
 
+  void ReorderChildLayers(ui::Layer* parent_layer) override {
+    views::View::ReorderChildLayers(parent_layer);
+    ui::Layer* topmost_layer = TopmostLayer(this, parent_layer);
+    if (!topmost_layer)
+      return;
+
+    // Keep the sticky headers with layers above the rest of the children's
+    // layers. Make sure to avoid changing the stacking order of the layers that
+    // are children of |parent_layer| but do not belong to the same parent view.
+    for (int i = child_count() - 1; i >= 0; --i) {
+      View* child = child_at(i);
+      if (child->id() == VIEW_ID_STICKY_HEADER && child->layer()) {

[sadrul, 2016/12/09 00:55:47]

if child->id() == VIEW_ID_STICKY_HEADER, will child->layer() be ever null?

[varkha, 2016/12/09 02:20:38]

Done.

+        if (child->layer() != topmost_layer)
+          parent_layer->StackAbove(child->layer(), topmost_layer);
+      }
+    }
+  }
+
   void ViewHierarchyChanged(
       const ViewHierarchyChangedDetails& details) override {
     if (!details.is_add && details.parent == this) {
       headers_.erase(std::remove_if(headers_.begin(), headers_.end(),
                                     [details](const Header& header) {
                                       return header.view == details.child;
                                     }),
                      headers_.end());
     } else if (details.is_add && details.parent == this &&
                details.child == child_at(0)) {
@@ skipping 114 matching lines @@
     SkPaint paint;
     gfx::ShadowValues shadow;
     shadow.emplace_back(gfx::Vector2d(0, kShadowOffsetY), kShadowBlur,
                         kSeparatorColor);
     paint.setLooper(gfx::CreateShadowDrawLooperCorrectBlur(shadow));
     paint.setAntiAlias(true);
     canvas->ClipRect(shadowed_area, kDifference_SkClipOp);
     canvas->DrawRect(shadowed_area, paint);
   }
 
+  // Recursively iterates through children to return the topmost child layer.

[sadrul, 2016/12/09 00:55:47]

Just to confirm: 'topmost' == topmost in stacking order?

[varkha, 2016/12/09 02:20:38]

Yes, mentioned this in a comment above TopmostLayer().

+  ui::Layer* TopmostLayer(views::View* view, ui::Layer* parent_layer) {

[sadrul, 2016/12/09 00:55:47]

DCHECK() that parent_layer is not null?

[varkha, 2016/12/09 02:20:38]

Done.

+    if (view->layer() && view->layer() != parent_layer) {

[sadrul, 2016/12/09 00:55:47]

The first view->layer() check isn't necessary.

[varkha, 2016/12/09 02:20:38]

Once we move this (see the next comment) I think we actually need this check.

+      DCHECK_EQ(parent_layer, view->layer()->parent());
+      return view->layer();
+    }

[sadrul, 2016/12/09 00:55:47]

Shouldn't this go after iterating over the children (since the parent is below
the children)?

[varkha, 2016/12/09 02:20:38]

Done. Good catch, just not something that was exercised by the current code.
Since we are recursing first we need to check and only return layers that are
direct children of parent_layer (there may be deeper children that we might
visit first).

+    // Iterate backwards through the children to find the topmost layer.
+    for (int i = view->child_count() - 1; i >= 0; --i) {
+      views::View* child = view->child_at(i);
+      ui::Layer* layer = TopmostLayer(child, parent_layer);
+      if (layer)
+        return layer;
+    }
+    return nullptr;
+  }
+
   views::BoxLayout* box_layout_;
 
   // Header child views that stick to the top of visible viewport when scrolled.
   std::vector<Header> headers_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollContentsView);
 };
 
 // Constants for the title row in material design.
 const int kTitleRowVerticalPadding = 4;
@@ skipping 319 matching lines @@
     if (index < child_count() - 1 && child_at(index + 1) != title_row_)
       scroll_border_->set_visible(true);
     else
       scroll_border_->set_visible(false);
   }
 
   views::View::OnPaintBorder(canvas);
 }
 
 }  // namespace ash