Due to the fix for https://webkit.org/b/115968 (which landed in http://trac.webkit.org/changeset/149942), the captions container is no longer positioned correctly when full-screen since the CSS positioning is no longer relative to the controller (but rather the media element).
Created attachment 201708 [details]
Comment on attachment 201708 [details]
Clearing flags on attachment: 201708
Committed r150075: <http://trac.webkit.org/changeset/150075>
All reviewed patches have been landed. Closing bug.