fix: prevent setting bar too left and getting off the left edge

When the most left side of the text is selected, the bar is positioned too left and is getting off the left edge of the browser, see screenshot.

![screen shot 2013-07-15 at 22 20 52](https://f.cloud.github.com/assets/442278/798148/fd279050-ed5a-11e2-823a-e333e3d1d8d2.png)