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

ash/common/system/tray/tray_details_view.cc

Index: ash/common/system/tray/tray_details_view.cc
diff --git a/ash/common/system/tray/tray_details_view.cc b/ash/common/system/tray/tray_details_view.cc
index d48c0e3608800ba71fd7bb2ba4261fda9c035589..b273ff6ebd7ffc713e2c07075bd1109d794881aa 100644
--- a/ash/common/system/tray/tray_details_view.cc
+++ b/ash/common/system/tray/tray_details_view.cc
@@ -100,6 +100,26 @@ class ScrollContentsView : public views::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.
+    // Note: this assumes that all views that have id=VIEW_ID_STICKY_HEADER have
+    // a layer.
+    for (int i = child_count() - 1; i >= 0; --i) {
+      View* child = child_at(i);
+      if (child->id() == VIEW_ID_STICKY_HEADER) {
+        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) {
@@ -234,6 +254,25 @@ class ScrollContentsView : public views::View,
     canvas->DrawRect(shadowed_area, paint);
   }
 
+  // Recursively iterates through children to return the child layer that is
+  // stacked at the top. Only considers layers that are direct |parent_layer|'s
+  // children.
+  ui::Layer* TopmostLayer(views::View* view, ui::Layer* parent_layer) {
+    DCHECK(parent_layer);
+    // 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;
+    }
+    if (view->layer() && view->layer() != parent_layer &&
+        view->layer()->parent() == parent_layer) {
+      return view->layer();
+    }
+    return nullptr;
+  }
+
   views::BoxLayout* box_layout_;
 
   // Header child views that stick to the top of visible viewport when scrolled.