Fix caret jumping in certain cases with cursor blot

[luin]

Closes #1395

Besides the steps described in #1395, there is another bug related to cursor:

1. Type "1234" in the editor.
2. Move the caret between "2" and "3".
3. Press CMD+b.
4. Switch to Chinese IME.
5. Type "w".
6. Cancel the composition session with Escape key.

The expected behavior is the caret to stay between "2" and "3" but the actual behavior is the caret jumps to the end.