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..adfe0fc9e02813254145f9892326d2217bb58315 100644
--- a/ui/views/controls/menu/menu_host.cc
+++ b/ui/views/controls/menu/menu_host.cc
@@ -78,6 +78,12 @@ class PreMenuEventDispatchHandler : public ui::EventHandler,
 
   DISALLOW_COPY_AND_ASSIGN(PreMenuEventDispatchHandler);
 };
+
+void TransferGesture(Widget* source, Widget* target) {
+  ui::GestureRecognizer::Get()->TransferEventsTo(
+      source->GetNativeView(), target->GetNativeView(),
+      ui::GestureRecognizer::ShouldCancelTouches::DontCancel);
+}
 #endif  // OS_MACOSX
 
 }  // namespace internal
@@ -125,6 +131,7 @@ void MenuHost::InitMenuHost(Widget* parent,
 #if !defined(OS_MACOSX)
   pre_dispatch_handler_.reset(new internal::PreMenuEventDispatchHandler(
       menu_controller, submenu_, GetNativeView()));
+  owner_ = parent;

[sadrul, 2017/08/30 20:31:39]

What if |owner_| is destroyed after the menu comes up? If we expect it won't
happen, can we validate that (e.g. add some DCHECK()s somewhere)?

[minch1, 2017/08/31 23:21:02]

Added DCHECK(owner()) when we need to send events back in
MenuController::OnGestureEvent. In HideMenuHost checked the owner_ before
transfer events back to the owner.

[sadrul, 2017/09/07 19:58:28]

If |owner_| is destroyed, the DCHECK() won't hit though, right? So that DCHECK()
is not really verifying that we are using an existing owner.

[minch1, 2017/09/07 20:54:55]

Do you mean that |parent| is destroyed after assigned to |owner_| here? But in
HideMenuHost we may still transfer events back to a not exist |parent|? 

 #endif
 
   SetContentsView(contents_view);
@@ -141,14 +148,36 @@ void MenuHost::ShowMenuHost(bool do_capture) {
   base::AutoReset<bool> reseter(&ignore_capture_lost_, true);
   ShowInactive();
   if (do_capture) {
+#if !defined(OS_MACOSX)

[sadrul, 2017/08/30 20:31:39]

Do we need the OS_MACOSX checks?

[minch1, 2017/08/31 23:21:02]

I think so. Since GestureRecognizerImplMac currently is stub implementation of
GestureRecognizer for Mac.

+    MenuController* menu_controller =
+        submenu_->GetMenuItem()->GetMenuController();
+    if (menu_controller && menu_controller->send_gesture_events_to_owner()) {
+      // TransferGesture when owner needs gesture events so that the incoming
+      // touch events after MenuHost is created are properly translated into
+      // gesture events instead of being dropped.
+      internal::TransferGesture(owner_, this);
+    } else {
+      ui::GestureRecognizer::Get()->CancelActiveTouchesExcept(nullptr);
+    }
+#else   // defined(OS_MACOSX)
     // Cancel existing touches, so we don't miss some touch release/cancel
     // events due to the menu taking capture.
     ui::GestureRecognizer::Get()->CancelActiveTouchesExcept(nullptr);
+#endif  // !defined(OS_MACOSX)
     native_widget_private()->SetCapture();
   }
 }

[sadrul, 2017/08/30 20:31:39]

An alternate approach, although probably hackier than this:

  internal::TransferGesture(owner_, this);
  SetCapture();
  internal::TransformGesture(this, owner_);

We shouldn't need to worry about owner_ going away, or transfering gesture
events back to owner_ etc. that way.

[minch1, 2017/08/31 23:21:02]

Do you mean to do all of these in ShowMenuHost? Tried it. But it seems that if
we transfer the events back to owner_ and keep MenuHost set capture at the same
time. Both owner_ and MenuController will not receive the touch and gesture
events. 

 
 void MenuHost::HideMenuHost() {
+#if !defined(OS_MACOSX)
+  MenuController* menu_controller =
+      submenu_->GetMenuItem()->GetMenuController();
+  if (owner_ && menu_controller &&
+      menu_controller->send_gesture_events_to_owner()) {
+    internal::TransferGesture(this, owner_);
+    owner_ = nullptr;
+  }
+#endif  // !defined(OS_MACOSX)
   ignore_capture_lost_ = true;
   ReleaseMenuHostCapture();
   Hide();