fdo#59329 fix docked panels borders

Change-Id: Ia918d644fb57d6fbaeef90c8c1a6806a8d7e589c
Reviewed-on: https://gerrit.libreoffice.org/8410
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>