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Only scroll to tabbable when it is different from before

modulexcite/nbgrader,alope107/nbgrader,jhamrick/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader,modulexcite/nbgrader,EdwardJKim/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,dementrock/nbgrader,ellisonbg/nbgrader,jhamrick/nbgrader,MatKallada/nbgrader,alope107/nbgrader,MatKallada/nbgrader,jupyter/nbgrader,dementrock/nbgrader