Topic on Extension talk:JSBreadCrumbs

for default vertical option it is not formatted correctly with the wiki css and this to fix it.

sed -i 's/body/vector-menu-content/' extensions/JSBreadCrumbs/resources/JSBreadCrumbs.js

sed -i 's/portal/mw-portlet mw-portlet-navigation vector-menu vector-menu-portal portal/' extensions/JSBreadCrumbs/resources/JSBreadCrumbs.js

Thank you for reporting this and providing a fix. I created https://phabricator.wikimedia.org/T300256 to track it.