Customize: Fix mobile placement of panes for available widgets and nav menu items.

Props Mahvash-Fatima.
Amends [41893].
See #42215.
Fixes #42359.

Built from https://develop.svn.wordpress.org/trunk@42036


git-svn-id: https://core.svn.wordpress.org/trunk@41870 1a063a9b-81f0-0310-95a4-ce76da25c4cd