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 60 matching lines @@
     window_->RemoveObserver(this);
     window_ = nullptr;
   }
 
   MenuController* menu_controller_;
   SubmenuView* submenu_;
   aura::Window* window_;
 
   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
 
 ////////////////////////////////////////////////////////////////////////////////
 // MenuHost, public:
 
 MenuHost::MenuHost(SubmenuView* submenu)
     : submenu_(submenu),
       destroying_(false),
@@ skipping 27 matching lines @@
   // 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()));
+  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);
   ShowMenuHost(do_capture);
 }
 
 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) {
+#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();
   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