When inside an event handler for the "selectstart" event, the selection should be preventable, either by returning false, or by calling the Event.preventDefault method. This was working in earlier nightlies. I have narrowed it down to some change between r16968 (working) and r16977 (broken).
Created attachment 13260 [details] Test case reduction Open the file, follow the instructions.
Maybe <http://trac.webkit.org/projects/webkit/changeset/16975>
Created attachment 13345 [details] Go through shouldSelect() in the selection-via-autoscroll case too Includes change log and layout test.
Comment on attachment 13345 [details] Go through shouldSelect() in the selection-via-autoscroll case too Woops, what are the pixel results doing there?!
Created attachment 13346 [details] Go through shouldSelect() in the selection-via-autoscroll case too
Comment on attachment 13346 [details] Go through shouldSelect() in the selection-via-autoscroll case too r=me
Landed in r19835.