fix know issues in RenderText

ui/gfx/render_text.h

Index: ui/gfx/render_text.h
===================================================================
--- ui/gfx/render_text.h	(revision 100008)
+++ ui/gfx/render_text.h	(working copy)
@@ -193,7 +193,9 @@
   // Set the selection_model_ to the value of |selection|.
   // The selection model components are modified if invalid.
   // Returns true if the cursor position or selection range changed.
-  // TODO(xji): need to check the cursor is set at grapheme boundary.
+  // If |selectin_start_| or |selection_end_| or |caret_pos_| in
+  // |selection_model| is not a cursorable position (not on grapheme boundary),
+  // it is a NO-OP and returns false.
   bool MoveCursorTo(const SelectionModel& selection_model);

[msw, 2011/09/09 23:03:24]

Is it possible that the text could end with a non-cursorable position? For
example, if the logical text ends with the first half of a surrogate pair? What
happens then?

[xji, 2011/09/12 21:31:48]

We do not allow that.
When I choose whether to use u_strToUTF8 or u_strToUTF8WithSub for utf8 and
utf16 conversion, I talked with Junghsik. And we think since we are dealing with
text field, user either input surrogate pair directly, in which, the IME should
not allow half surrogate pair, or if user paste text, the pasted text would be
garbage character if the source contains half surrogate.
So, I used u_strToUTF8 and assert that there wont be INVALID  character in the
source string.

 
   // Move the cursor to the position associated with the clicked point.
@@ -254,6 +256,9 @@
   // Subsequent text, cursor, or bounds changes may invalidate returned values.
   const Rect& GetUpdatedCursorBounds();
 
+  // Get the logical index of the grapheme following the argument |position|.
+  virtual size_t GetIndexOfNextGrapheme(size_t position);
+
  protected:
   RenderText();
 
@@ -288,6 +293,10 @@
   // TODO(msw) Re-evaluate this function's necessity and signature.
   virtual std::vector<Rect> GetSubstringBounds(size_t from, size_t to);
 
+  // Return true if cursor can appear in front of the character at |position|,
+  // which means it is a grapheme boundary or the first character in the text.
+  virtual bool IsCursorablePosition(size_t position);
+
   // Apply composition style (underline) to composition range and selection
   // style (foreground) to selection range.
   void ApplyCompositionAndSelectionStyles(StyleRanges* style_ranges) const;
@@ -311,10 +320,10 @@
   // Set the cursor to |position|, with the caret trailing the previous
   // grapheme, or if there is no previous grapheme, leading the cursor position.
   // If |select| is false, the selection start is moved to the same position.
+  // If the |position| is not a cursorable position (not on grapheme boundary),
+  // it is a NO-OP.
   void MoveCursorTo(size_t position, bool select);
 
-  bool IsPositionAtWordSelectionBoundary(size_t pos);
-
   // Update the cached bounds and display offset to ensure that the current
   // cursor is within the visible display area.
   void UpdateCachedBoundsAndOffset();