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

ui/views/controls/textfield/native_textfield_views.cc

Index: ui/views/controls/textfield/native_textfield_views.cc
diff --git a/ui/views/controls/textfield/native_textfield_views.cc b/ui/views/controls/textfield/native_textfield_views.cc
index 0e807556ef17f3f3ed9fc71fa049f1fdfb8492cf..80acc84e7ce0d6ada214246d6e4826c2777261e0 100644
--- a/ui/views/controls/textfield/native_textfield_views.cc
+++ b/ui/views/controls/textfield/native_textfield_views.cc
@@ -967,7 +967,18 @@ void NativeTextfieldViews::InsertChar(char16 ch, int flags) {
 }
 
 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 {