In the attached test file, when you click on the text in the contenteditable div, you'll see an odd focus ring.
Instead of stopping at the bottom of the div, it outlines the text below that is in the overflow area.
Created attachment 5097 [details]
<rdar://problem/4381360> REGRESSION: Focus ring on contenteditable div outlines text in overflow area
instead of div (6100)
Hyatt proposed a fix for this, and I'm testing it right now.