MacViews a11y: Support the "Show menu" action in Textfield and Combobox.

ui/views/controls/combobox/combobox.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/combobox/combobox.h"
 
 #include <stddef.h>
 
 #include <utility>
 
 #include "base/logging.h"
 #include "base/macros.h"
 #include "build/build_config.h"
+#include "ui/accessibility/ax_action_data.h"
 #include "ui/accessibility/ax_node_data.h"
 #include "ui/base/default_style.h"
 #include "ui/base/ime/input_method.h"
 #include "ui/base/material_design/material_design_controller.h"
 #include "ui/base/models/combobox_model_observer.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/events/event.h"
 #include "ui/gfx/animation/throb_animation.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/color_palette.h"
@@ skipping 735 matching lines @@
   node_data->SetName(accessible_name_);
   node_data->SetValue(model_->GetItemAt(selected_index_));
   if (enabled()) {
     node_data->AddIntAttribute(ui::AX_ATTR_DEFAULT_ACTION_VERB,
                                ui::AX_DEFAULT_ACTION_VERB_OPEN);
   }
   node_data->AddIntAttribute(ui::AX_ATTR_POS_IN_SET, selected_index_);
   node_data->AddIntAttribute(ui::AX_ATTR_SET_SIZE, model_->GetItemCount());
 }
 
+bool Combobox::HandleAccessibleAction(const ui::AXActionData& action_data) {
+  // The action handling in View would generate a mouse event, but mouse events
+  // for Combobox are handled by |arrow_button_|. Handle the action explicitly.

[msw, 2017/06/29 19:26:12]

nit: It's not clear from this comment why allowing |arrow_button_| to handle the
default generated mouse event would be problematic. Can you rephrase this
comment to explain why we handle the action explicitly? ie. "Handle the default
action explicitly by showing the menu, otherwise |arrow_button_| would handle
the event by <doing xyz>."

[tapted, 2017/06/30 06:04:47]

The problem is that |arrow_button_| isn't exposed to a11y -- it's an
implementation detail, so hidden from the a11y tree.

Updated the comment:

  // The action handling in View would generate a mouse event and send it to
  // |this|. However, mouse events for Combobox are handled by |arrow_button_|,
  // which is hidden from the a11y tree (so can't expose actions). Rather than
  // forwarding AX_ACTION_DO_DEFAULT to View and then forwarding the mouse event
  // it generates to |arrow_button_| to have it forward back to |this| (as its
  // ButtonListener), just handle the action explicitly here and bypass View.

+  if (enabled() && action_data.action == ui::AX_ACTION_DO_DEFAULT) {

[msw, 2017/06/29 19:26:12]

Do we need some logic for handling the event when the menu is already showing?

[tapted, 2017/06/30 06:04:47]

I don't think so. I am not a VoiceOver expert but it seems to be impossible to
enter a situation where this is needed. Even with voiceover navigation set to
"Mouse pointer -> follows-mouse-cursor" it's impossible for the  voiceover
cursor to "escape" a menu while it's still open -- the menu must be dismissed
first (and so there is no way to perform an action on something other than a
menu item while any menu is showing).

+    ShowDropDownMenu(ui::MENU_SOURCE_KEYBOARD);
+    return true;
+  }
+  return View::HandleAccessibleAction(action_data);
+}
+
 void Combobox::ButtonPressed(Button* sender, const ui::Event& event) {
   if (!enabled())
     return;
 
   if (!UseMd())
     RequestFocus();
 
   if (sender == text_button_) {
     OnPerformAction();
   } else {
@@ skipping 235 matching lines @@
   constexpr int kMdPaddingWidth = 8;
   constexpr int kNormalPaddingWidth = 7;
   int arrow_pad = UseMd() ? kMdPaddingWidth : kNormalPaddingWidth;
   int padding = style_ == STYLE_NORMAL
                     ? arrow_pad * 2
                     : kActionLeftPadding + kActionRightPadding;
   return ArrowSize().width() + padding;
 }
 
 }  // namespace views