The notification drawer can only be closed by clicking the bell icon in the masthead a second time (as far as I could discover). I'd expect there to be a close `x` icon in the drawer itself. I was also surprised that clicking outside the drawer didn't close it.
Moving bug to upcoming sprint, will add an issue to patternfly core / react to get an X added to the drawer to close it.
Added issue here: https://github.com/patternfly/patternfly-react/issues/4201
will fix with contribution from upstream when ready moving to upcoming sprint
Update on this, the pf notification drawer has a pr to allow for action when clicking outside the drawer. PR: https://github.com/patternfly/patternfly-react/pull/4587
Created attachment 1712788 [details] Verification screenshot
Attached screenshot, this could be verified on 4.6.0-0.nightly-2020-08-27-030439
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory (OpenShift Container Platform 4.6 GA Images), and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHBA-2020:4196