work around chrome position: absolute scrolling bug
work around chrome position: absolute scrolling bug

test plan:
 - in chrome 87, visit a course or account page where the
   page content is shorter than the course/account nav
 - you should be able to scroll down far enough to see the
   entire course nav
 - the course nav should still appear correct

flag = none
kinda fixes LS-1630

this is a hack and we should probably rework the layout
not to do the weird stuff it's doing (using position: absolute
to put the course nav into the main content margin)

or just wait for chromium to fix issue 1150774 and then
revert this

Change-Id: Ia08235e7beab71b2013e32cc9e0413bb55107e94
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/253673
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Nate Armstrong <619b324efbf5f7848de055bdc8bef4c0dd7c8ef5@instructure.com>
QA-Review: Nate Armstrong <619b324efbf5f7848de055bdc8bef4c0dd7c8ef5@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms