Make shelf item can be dragged when context menu is opened.

ui/views/controls/menu/menu_host.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_host.h"
 
 #include "base/auto_reset.h"
 #include "base/logging.h"
 #include "base/trace_event/trace_event.h"
 #include "build/build_config.h"
@@ skipping 109 matching lines @@
   // revert this change once http://crbug.com/125248 is fixed.
   params.force_software_compositing = true;
 #endif
   Init(params);
 
 #if !defined(OS_MACOSX)
   pre_dispatch_handler_.reset(new internal::PreMenuEventDispatchHandler(
       menu_controller, submenu_, GetNativeView()));
 #endif
 
+  if (parent && !views::MenuController::DoCancel()) {

[xiyuan, 2017/05/15 19:20:52]

nit: Use |menu_controller| instance after making DoCancel non-static.

[minch1, 2017/05/16 22:13:39]

Done.

+    owner_ = parent;
+    TransferEvents(owner_, this);
+  }
   SetContentsView(contents_view);
   ShowMenuHost(do_capture);
 }
 
+void MenuHost::TransferEvents(Widget* source, Widget* target) {
+  ui::GestureRecognizer::Get()->TransferEventsTo(
+      source->GetNativeView(), target->GetNativeView(),
+      ui::GestureRecognizer::ShouldCancelTouches::DontCancel);
+}
+
 bool MenuHost::IsMenuHostVisible() {
   return IsVisible();
 }
 
 void MenuHost::ShowMenuHost(bool do_capture) {
   // Doing a capture may make us get capture lost. Ignore it while we're in the
   // process of showing.
   base::AutoReset<bool> reseter(&ignore_capture_lost_, true);
   ShowInactive();
   if (do_capture) {
     // Cancel existing touches, so we don't miss some touch release/cancel
     // events due to the menu taking capture.
-    ui::GestureRecognizer::Get()->CancelActiveTouchesExcept(nullptr);
+    if (views::MenuController::DoCancel())
+      ui::GestureRecognizer::Get()->CancelActiveTouchesExcept(nullptr);
     native_widget_private()->SetCapture();
   }
 }
 
 void MenuHost::HideMenuHost() {
+  if (owner_ && !views::MenuController::DoCancel()) {
+    TransferEvents(this, owner_);
+    owner_ = NULL;
+    views::MenuController::SetDoCancel(true);

[xiyuan, 2017/05/15 19:20:52]

Is this call necessary?

[minch1, 2017/05/15 22:16:34]

Yes. I think this is necessary. 
1. Press the shelf items. This will set do_calcel_ to false.
2. Touch anywhere else except the shelf.
If we don't set do_cancel_ back to true here. Then, for the LONG_PRESS events
(open context menu) on the screen will do TransferEvents which is not what we
want.

[xiyuan, 2017/05/15 22:41:10]

That would be another reason to make |do_cancel_| non-static.

+  }
   ignore_capture_lost_ = true;
   ReleaseMenuHostCapture();
   Hide();
   ignore_capture_lost_ = false;
 }
 
 void MenuHost::DestroyMenuHost() {
   HideMenuHost();
   destroying_ = true;
   static_cast<MenuHostRootView*>(GetRootView())->ClearSubmenu();
@@ skipping 77 matching lines @@
   }
   menu_controller->OnDragComplete(should_close);
 
   // We may have lost capture in the drag and drop, but are remaining open.
   // Return capture so we get MouseCaptureLost events.
   if (!should_close)
     native_widget_private()->SetCapture();
 }
 
 }  // namespace views