In the attached test case, the descenders of the lowercase Ps are clipped out. I think the regression happened in <http://trac.webkit.org/projects/webkit/changeset/24604>. This is in Radar as <rdar://problem/5464301>.
Created attachment 16220 [details] Reduction
Created attachment 16221 [details] Patch
Landed by Tristan in <http://trac.webkit.org/projects/webkit/changeset/25471>.