Implement Pango RenderText for Linux.

ui/gfx/render_text.h

Index: ui/gfx/render_text.h
===================================================================
--- ui/gfx/render_text.h	(revision 97029)
+++ ui/gfx/render_text.h	(working copy)
@@ -121,9 +121,12 @@
  private:
   void Init(size_t start, size_t end, size_t pos, CaretPlacement status);
 
-  // Logical selection start. If there is non-empty selection, the selection
-  // always starts visually at the leading edge of the selection_start. So, we
-  // do not need extra information for visual selection bounding.
+  // Logical selection start. If there is non-empty selection, if
+  // selection_start_ is less than selection_end_, the selection starts visually
+  // at the leading edge of the selection_start_. If selection_start_ is greater
+  // than selection_end_, the selection starts visually at the trailing edge of
+  // selection_start_'s previous grapheme. So, we do not need extra information

[msw, 2011/08/19 23:16:59]

Is that true? Isn't it always leading the selection start?
What does the selection range of "abc[DE]F" look like? I don't think there's any
component of that selection that trails 'c', it's just "abcF[ED]".

Can you show an example of when MoveCursorLeft/Right should move the
cursor&caret to trail the selection start's previous grapheme?

[xji, 2011/08/22 23:57:28]

The above example wont show any difference.
But the difference shows up when you select "abc[FE]D".

logically, the selection range is the same no matter you select from left to
right or right to left. For the above example, the selection range is (6, 4) or
(4, 6).

But the visual starting point is different between select from right to left and
from left to right.

If you click right half of 'E' and move mouse left, the selection visually start
from (4, leading).
If you click left half of 'F' and move mouse right, the selection visually start
from (5, trailing), but (6, leading).

The difference probably wont affect the selection's substring bounds.
But it affects when moving cursor left or right when there is selection, in
which case, the selection should be collapsed to visual left or right.
For the above example, when selection range (6, 4), selection_start is at
visually left when using (5, trailing) as visual start point. But selection_end
is at visually left when using (6, 0) as visual start point, which is wrong.

[msw, 2011/08/23 08:01:01]

Ok, I think I understand and it seems that you are correct.

+  // for visual bounding.
   size_t selection_start_;
 
   // The logical cursor position that next character will be inserted into.
@@ -298,6 +301,10 @@
   // cursor is within the visible display area.
   void UpdateCachedBoundsAndOffset();
 
+  // Returns the selection model of selection_start_.
+  // The returned value represents a cursor/caret position without a selection.
+  SelectionModel GetSelectionModelForSelectionStart();
+
   // Logical UTF-16 string data to be drawn.
   string16 text_;