more hacking around our hacky onGridBlur event

fixes CNVS-10516

Test plan:
  * Turn on the teacher notes column in the gradebook
  * You should be able to click around in the textarea when editing
    custom columns

Change-Id: I290ab6cad6010856eab9cd11c19cbf3f93c66ac6
Reviewed-on: https://gerrit.instructure.com/28644
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Matthew Irish <mirish@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>