Makes WindowTree::SetFocus(null) work

components/mus/ws/window_tree.cc

 // Copyright 2014 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 "components/mus/ws/window_tree.h"
 
 #include <stddef.h>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 251 matching lines @@
       !access_policy_->CanChangeWindowVisibility(window)) {
     return false;
   }
   Operation op(this, window_server_, OperationType::SET_WINDOW_VISIBILITY);
   window->SetVisible(visible);
   return true;
 }
 
 bool WindowTree::SetFocus(const ClientWindowId& window_id) {
   ServerWindow* window = GetWindowByClientId(window_id);
-  // TODO(beng): consider shifting non-policy drawn check logic to VTH's
-  //             FocusController.
-  // TODO(sky): this doesn't work to clear focus. That is because if window is
-  // null, then |host| is null and we fail.
+  ServerWindow* currently_focused = window_server_->GetFocusedWindow();
+  if (!currently_focused && !window)
+    return false;
+
   Display* display = GetDisplay(window);
-  if (!window || !display || !window->IsDrawn() || !window->can_focus() ||
-      !access_policy_->CanSetFocus(window)) {
+  if (window && (!display || !window->can_focus() || !window->IsDrawn()))
     return false;
-  }
+
+  if (!access_policy_->CanSetFocus(window))
+    return false;
 
   Operation op(this, window_server_, OperationType::SET_FOCUS);
   window_server_->SetFocusedWindow(window);
   return true;
 }
 
 bool WindowTree::Embed(const ClientWindowId& window_id,
                        mojom::WindowTreeClientPtr client) {
   if (!client || !CanEmbed(window_id))
     return false;
@@ skipping 375 matching lines @@
 }
 
 ClientWindowId WindowTree::ClientWindowIdForWindow(
     const ServerWindow* window) const {
   auto iter = window_id_to_client_id_map_.find(window->id());
   DCHECK(iter != window_id_to_client_id_map_.end());
   return iter->second;
 }
 
 bool WindowTree::IsValidIdForNewWindow(const ClientWindowId& id) const {
+  // Reserve 0 to indicate a null window.
   return client_id_to_window_id_map_.count(id) == 0u &&
-         access_policy_->IsValidIdForNewWindow(id);
+         access_policy_->IsValidIdForNewWindow(id) && id != ClientWindowId();
 }
 
 WindowId WindowTree::GenerateNewWindowId() {
   // TODO(sky): deal with wrapping and uniqueness.
   return WindowId(id_, next_window_id_++);
 }
 
 bool WindowTree::CanReorderWindow(const ServerWindow* window,
                                   const ServerWindow* relative_window,
                                   mojom::OrderDirection direction) const {
@@ skipping 680 matching lines @@
 }
 
 bool WindowTree::IsWindowRootOfAnotherTreeForAccessPolicy(
     const ServerWindow* window) const {
   WindowTree* tree = window_server_->GetTreeWithRoot(window);
   return tree && tree != this;
 }
 
 }  // namespace ws
 }  // namespace mus