Consider IME is associated with the top level widget rather than child widget

ui/views/controls/textfield/native_textfield_views.cc

 // Copyright (c) 2013 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/textfield/native_textfield_views.h"
 
 #include <algorithm>
 #include <set>
 
 #include "base/bind.h"
@@ skipping 949 matching lines @@
         textfield_->obscured_reveal_duration();
     if (reveal_duration != base::TimeDelta()) {
       const size_t change_offset = model_->GetCursorPosition();
       DCHECK_GT(change_offset, 0u);
       RevealObscuredChar(change_offset - 1, reveal_duration);
     }
   }
 }
 
 gfx::NativeWindow NativeTextfieldViews::GetAttachedWindow() const {
-  return GetWidget()->GetNativeWindow();
+  // Imagine the following hierarchy.
+  //   [NativeWidget A] - FocusManager
+  //     [View]
+  //     [NativeWidget B]
+  //       [View]
+  //         [View X]
+  // An important thing is that [NativeWidget A] owns Win32 input focus even
+  // when [View X] is logically focused by FocusManager. As a result, an Win32
+  // IME may want to interact with the native view of [NativeWidget A] rather
+  // than that of [NativeWidget B]. This is why we need to call
+  // GetTopLevelWidget() here.
+  return GetWidget()->GetTopLevelWidget()->GetNativeView();
 }
 
 ui::TextInputType NativeTextfieldViews::GetTextInputType() const {
   return textfield_->GetTextInputType();
 }
 
 bool NativeTextfieldViews::CanComposeInline() const {
   return true;
 }
 
@@ skipping 527 matching lines @@
   if (index != -1) {
     obscured_reveal_timer_.Start(
         FROM_HERE,
         duration,
         base::Bind(&NativeTextfieldViews::RevealObscuredChar,
                    base::Unretained(this), -1, base::TimeDelta()));
   }
 }
 
 }  // namespace views