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 80 matching lines @@
       ignore_capture_lost_(false) {
   set_auto_release_capture(false);
 }
 
 MenuHost::~MenuHost() {
 }
 
 void MenuHost::InitMenuHost(Widget* parent,
                             const gfx::Rect& bounds,
                             View* contents_view,
-                            bool do_capture) {
+                            bool do_capture,
+                            bool cancel_active_touches) {

[xiyuan, 2017/05/12 20:24:56]

How about get this from MenuController instead of passing as an arg?

   TRACE_EVENT0("views", "MenuHost::InitMenuHost");
   Widget::InitParams params(Widget::InitParams::TYPE_MENU);
   const MenuController* menu_controller =
       submenu_->GetMenuItem()->GetMenuController();
   const MenuConfig& menu_config = MenuConfig::instance();
   bool rounded_border = menu_controller && menu_config.corner_radius > 0;
   bool bubble_border = submenu_->GetScrollViewContainer() &&
                        submenu_->GetScrollViewContainer()->HasBubbleBorder();
   params.shadow_type = bubble_border ? Widget::InitParams::SHADOW_TYPE_NONE
                                      : Widget::InitParams::SHADOW_TYPE_DROP;
   params.opacity = (bubble_border || rounded_border) ?
       Widget::InitParams::TRANSLUCENT_WINDOW :
       Widget::InitParams::OPAQUE_WINDOW;
   params.parent = parent ? parent->GetNativeView() : NULL;
   params.bounds = bounds;
 #if defined(OS_WIN)
   // On Windows use the software compositor to ensure that we don't block
   // the UI thread blocking issue during command buffer creation. We can
   // 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
 
+  owner_ = parent;
+
+  TransferEvents(owner_, this);

[xiyuan, 2017/05/12 20:24:56]

We probably should only do this for the out-most menu. That is, sub menu such as
shelf position should not do this.

Also, |parent| could be nullptr and we need to handle that case.

+
   SetContentsView(contents_view);
-  ShowMenuHost(do_capture);
+  ShowMenuHost(do_capture, cancel_active_touches);
+}
+
+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) {
+void MenuHost::ShowMenuHost(bool do_capture, bool cancel_active_touches) {
   // 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 (cancel_active_touches) {

[xiyuan, 2017/05/12 20:24:56]

This should probably be in the if (do_capture) down below.

+    ui::GestureRecognizer::Get()->CancelActiveTouchesExcept(nullptr);
+  }
   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);
     native_widget_private()->SetCapture();
   }
 }
 
 void MenuHost::HideMenuHost() {
+  if (owner_) {
+    TransferEvents(this, owner_);

[xiyuan, 2017/05/12 20:24:56]

Similarly, we need to figure out how to do this only for the out-most menu.

+    owner_ = NULL;
+  }
   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