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

ui/views/controls/menu/menu_host.cc

Index: ui/views/controls/menu/menu_host.cc
diff --git a/ui/views/controls/menu/menu_host.cc b/ui/views/controls/menu/menu_host.cc
index c6750a6e0cd51be18d9470a45969bed6b27c0e4c..2cc0bfef29f80edf52bde137bf9693caa7b1dafa 100644
--- a/ui/views/controls/menu/menu_host.cc
+++ b/ui/views/controls/menu/menu_host.cc
@@ -98,7 +98,8 @@ 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 =
@@ -127,28 +128,44 @@ void MenuHost::InitMenuHost(Widget* parent,
       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();