Align the base of the shelf's context menu to the top edge of the shelf.

ui/views/controls/menu/menu_controller.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 "ui/views/controls/menu/menu_controller.h"
 
 #include "base/i18n/case_conversion.h"
 #include "base/i18n/rtl.h"
 #include "base/macros.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 1932 matching lines @@
     // simply opening the menu (bug 142992).
     if (menu_config.offset_context_menus && state_.context_menu)
       x += 1;
 
     y = state_.initial_bounds.bottom();
     if (state_.anchor == MENU_ANCHOR_TOPRIGHT) {
       x = x + state_.initial_bounds.width() - pref.width();
       if (menu_config.offset_context_menus && state_.context_menu)
         x -= 1;
     } else if (state_.anchor == MENU_ANCHOR_BOTTOMCENTER) {
-      x = x - (pref.width() - state_.initial_bounds.width()) / 2;
+      x += (state_.initial_bounds.width() - pref.width()) / 2;
       if (pref.height() >
           state_.initial_bounds.y() + kCenteredContextMenuYOffset) {
-        // Menu does not fit above the anchor. We move it to below.
+        // Place the menu below if it does not fit above.
         y = state_.initial_bounds.y() - kCenteredContextMenuYOffset;
       } else {
         y = std::max(0, state_.initial_bounds.y() - pref.height()) +
             kCenteredContextMenuYOffset;
       }
+    } else if (state_.anchor == MENU_ANCHOR_FIXED_BOTTOMCENTER) {
+      x += (state_.initial_bounds.width() - pref.width()) / 2;
+    } else if (state_.anchor == MENU_ANCHOR_FIXED_SIDECENTER) {
+      y += (state_.initial_bounds.height() - pref.height()) / 2;
     }
 
     if (!state_.monitor_bounds.IsEmpty() &&
         y + pref.height() > state_.monitor_bounds.bottom()) {
       // The menu doesn't fit fully below the button on the screen. The menu
       // position with respect to the bounds will be preserved if it has
       // already been drawn. When the requested positioning is below the bounds
       // it will shrink the menu to make it fit below.
       // If the requested positioning is best fit, it will first try to fit the
       // menu below. If that does not fit it will try to place it above. If
@@ skipping 738 matching lines @@
   if (hot_button_)
     hot_button_->SetHotTracked(false);
   hot_button_ = hot_button;
   if (hot_button) {
     hot_button->SetHotTracked(true);
     hot_button->NotifyAccessibilityEvent(ui::AX_EVENT_SELECTION, true);
   }
 }
 
 }  // namespace views