Added 1 liner to fix shelf background hiding on side shelf alignment.

ash/shelf/shelf_layout_manager.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/shelf/shelf_layout_manager.h"
 
 #include <algorithm>
 #include <cmath>
 #include <vector>
 
@@ skipping 461 matching lines @@
 }
 
 ShelfBackgroundType ShelfLayoutManager::GetShelfBackgroundType() const {
   if (state_.pre_lock_screen_animation_active)
     return SHELF_BACKGROUND_DEFAULT;
 
   // Handle all non active screen states, including OOBE and pre-login.
   if (state_.session_state != session_manager::SessionState::ACTIVE)
     return SHELF_BACKGROUND_OVERLAP;
 
+  // If the app list is active and the shelf is oriented vertically, enable the
+  // shelf background.
+  if (is_app_list_visible_ && !shelf_->IsHorizontalAlignment() &&
+      app_list::features::IsFullscreenAppListEnabled())

[James Cook, 2017/06/21 23:57:33]

IsFullscreenAppListEnabled() runs a non-trivial amount of code. I think you
should either:

* Cache the result here in a local variable, or
* Cache the value in a const member variable for this class (since it won't
change at runtime and this object is torn down between tests)

[newcomer, 2017/06/22 00:16:09]

I picked option 2 because that's what I've done elsewhere!

Done.

+    return SHELF_BACKGROUND_OVERLAP;
+
   // If the app list is active, hide the shelf background to prevent overlap.
   if (is_app_list_visible_ && app_list::features::IsFullscreenAppListEnabled())
     return SHELF_BACKGROUND_DEFAULT;
 
   if (state_.visibility_state != SHELF_AUTO_HIDE &&
       state_.window_state == wm::WORKSPACE_WINDOW_STATE_MAXIMIZED) {
     return SHELF_BACKGROUND_MAXIMIZED;
   }
 
   if (gesture_drag_status_ == GESTURE_DRAG_IN_PROGRESS ||
@@ skipping 671 matching lines @@
       return gesture_drag_amount_ < 0;
     case SHELF_ALIGNMENT_LEFT:
       if (gesture_drag_auto_hide_state_ == SHELF_AUTO_HIDE_SHOWN)
         return gesture_drag_amount_ < 0;
       return gesture_drag_amount_ > 0;
   }
   return false;
 }
 
 }  // namespace ash