Reland: mus: Enable system modal windows

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 220 matching lines @@
       access_policy_->CanAddTransientWindow(window, transient_window)) {
     Operation op(this, window_server_, OperationType::ADD_TRANSIENT_WINDOW);
     window->AddTransientWindow(transient_window);
     return true;
   }
   return false;
 }
 
 bool WindowTree::SetModal(const ClientWindowId& window_id) {
   ServerWindow* window = GetWindowByClientId(window_id);
+  bool success = false;
   if (window && access_policy_->CanSetModal(window)) {
-    window->SetModal();
     WindowManagerState* wms = GetWindowManagerState(window);
-    if (wms)
+    if (window->transient_parent()) {
+      window->SetModal();
+      success = true;
+    } else {
+      success = user_id_ != InvalidUserId() && wms &&
+                wms->SetSystemModalWindow(window);

[sky, 2016/04/20 20:14:12]

This is problematic because you're assuming there is only ever one system modal
window. Lets say a client creates two system modal windows, A and B, and then
shows A. I believe that won't work well with how you have the code.

[mohsen, 2016/04/21 17:58:43]

What would exactly not work in your example? The client creates A as system
modal, then creates B and tries to set it as system modal, but it fails and its
modality gets reverted. A remains system modal, B remains a regular window.

+    }
+    if (success)
       wms->ReleaseCaptureBlockedByModalWindow(window);
-    return true;
   }
-  return false;
+  return success;
 }
 
 std::vector<const ServerWindow*> WindowTree::GetWindowTree(
     const ClientWindowId& window_id) const {
   const ServerWindow* window = GetWindowByClientId(window_id);
   std::vector<const ServerWindow*> windows;
   if (window)
     GetWindowTreeImpl(window, &windows);
   return windows;
 }
@@ skipping 1150 matching lines @@
 }
 
 bool WindowTree::IsWindowRootOfAnotherTreeForAccessPolicy(
     const ServerWindow* window) const {
   WindowTree* tree = window_server_->GetTreeWithRoot(window);
   return tree && tree != this;
 }
 
 }  // namespace ws
 }  // namespace mus