Fix DCHECK/crash in WindowManagerState

components/mus/ws/window_manager_state.cc

 // Copyright 2016 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_manager_state.h"
 
 #include "components/mus/ws/connection_manager.h"
 #include "components/mus/ws/display_manager.h"
 #include "components/mus/ws/platform_display.h"
 #include "components/mus/ws/server_window.h"
@@ skipping 257 matching lines @@
   }
 
   // If the event is in the non-client area the event goes to the owner of
   // the window. Otherwise if the window is an embed root, forward to the
   // embedded window.
   WindowTree* tree =
       in_nonclient_area
           ? connection_manager()->GetTreeWithId(target->id().connection_id)
           : connection_manager()->GetTreeWithRoot(target);
   if (!tree) {
-    DCHECK(!in_nonclient_area);
-    tree = connection_manager()->GetTreeWithId(target->id().connection_id);
+    if (in_nonclient_area) {
+      // Being the root of the tree means we may get events outside the bounds
+      // of the platform window. Because the root has a connection id of 0,
+      // no WindowTree is found for it and we have to special case it here.
+      DCHECK_EQ(target, root_.get());
+      tree = tree_;
+    } else {
+      tree = connection_manager()->GetTreeWithId(target->id().connection_id);
+    }
   }
 
   // TOOD(sad): Adjust this delay, possibly make this dynamic.
   const base::TimeDelta max_delay = base::debug::BeingDebugged()
                                         ? base::TimeDelta::FromDays(1)
                                         : GetDefaultAckTimerDelay();
   event_ack_timer_.Start(FROM_HERE, max_delay, this,
                          &WindowManagerState::OnEventAckTimeout);
 
   tree_awaiting_input_ack_ = tree;
@@ skipping 41 matching lines @@
         new ProcessedEventTarget(target, in_nonclient_area));
     QueueEvent(event, std::move(processed_event_target));
     return;
   }
 
   DispatchInputEventToWindowImpl(target, in_nonclient_area, event);
 }
 
 }  // namespace ws
 }  // namespace mus