Handle IMR_QUERYCHARPOSITION even when there is no composition

content/browser/renderer_host/render_widget_host_view_win.cc

 // Copyright (c) 2012 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 "content/browser/renderer_host/render_widget_host_view_win.h"
 
 #include <algorithm>
 #include <map>
 #include <peninputpanel_i.c>
 #include <stack>
@@ skipping 3006 matching lines @@
   // According to Microsft API document, IMR_RECONVERTSTRING and
   // IMR_DOCUMENTFEED should return reconv, but some applications return
   // need_size.
   return reinterpret_cast<LRESULT>(reconv);
 }
 
 LRESULT RenderWidgetHostViewWin::OnQueryCharPosition(
     IMECHARPOSITION* position) {
   DCHECK(position);
 
-  if (!ime_input_.is_composing() || composition_range_.is_empty() ||
-      position->dwSize < sizeof(IMECHARPOSITION) ||
-      position->dwCharPos >= composition_character_bounds_.size()) {
+  if (position->dwSize < sizeof(IMECHARPOSITION))
+    return 0;
+
+  RECT target_rect = {};
+  if (ime_input_.is_composing() && !composition_range_.is_empty() &&
+      position->dwCharPos < composition_character_bounds_.size()) {
+    target_rect =
+        composition_character_bounds_[position->dwCharPos].ToRECT();
+  } else if (position->dwCharPos == 0) {
+    // When there is no on-going composition but |position->dwCharPos| is 0,
+    // use the caret rect. This behavior is the same to RichEdit. In fact,
+    // CUAS (Cicero Unaware Application Support) relies on this behavior to
+    // implement ITfContextView::GetTextExt on top of IMM32-based applications.
+    target_rect = caret_rect_.ToRECT();
+  } else {
     return 0;
   }
-
-  RECT target_rect =
-      composition_character_bounds_[position->dwCharPos].ToRECT();
   ClientToScreen(&target_rect);
 
   RECT document_rect = GetViewBounds().ToRECT();
   ClientToScreen(&document_rect);
 
   position->pt.x = target_rect.left;
   position->pt.y = target_rect.top;
   position->cLineHeight = target_rect.bottom - target_rect.top;
   position->rcDocument = document_rect;
   return 1;
@@ skipping 37 matching lines @@
   // receive a focus change in the context of a pointer down message, it means
   // that the pointer down message occurred on the edit field and we should
   // display the on screen keyboard
   if (!received_focus_change_after_pointer_down_ && virtual_keyboard_)
     DisplayOnScreenKeyboardIfNeeded();
   received_focus_change_after_pointer_down_ = false;
   pointer_down_context_ = false;
 }
 
 }  // namespace content