The link (http://tolmasky.com/letmeshowyou/webkit/focusAndSelect.mov) shows a movie of this effect. It happens when drag-selecting from the edge of form elements.
<rdar://problem/7321601>
It looks like this bug might have been fixed since it was originally filed. I cannot repro it. Suggest to move it to resolved.
WebKit QA cannot reproduce this on current releases.