Created attachment 257642 [details]
Screenshot showing the problem
It happens when one of the first elements are selected, because we try to center the current selection on the combo element. When the menu is large enough to make the element centered we need to leave empty space at the beginning. See the attached screenshot.
Created attachment 257643 [details]
This fixes the issue.
Created attachment 257644 [details]
Screenshot with the patch applied
Committed r187542: <http://trac.webkit.org/changeset/187542>