[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..ae8ff6f34d3cf26988a38cfe987da5a8f19245c0 100644
--- a/ash/common/system/tray/tray_details_view.cc
+++ b/ash/common/system/tray/tray_details_view.cc
@@ -100,6 +100,24 @@ 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.
+    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) {
@@ -234,6 +252,22 @@ class ScrollContentsView : public views::View,
     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.