client/index.html currently includes leaflet.css over http, which will be blocked by some browsers when accessing the app over https. (Also, Chrome reports, "getCurrentPosition() and watchPosition() are deprecated on insecure origins, and support will be removed in the future. You should consider switching your application to a secure origin, such as HTTPS. See https://goo.gl/rStTGz for more details." so the app should probably move to require https soon.)
One hitch: cdn.leafletjs.com doesn't have a valid ssl cert. As an alternative, leaflet is available from cdnjs, which does offer https: https://cdnjs.com/libraries/leaflet/
client/index.html currently includes leaflet.css over http, which will be blocked by some browsers when accessing the app over https. (Also, Chrome reports, "getCurrentPosition() and watchPosition() are deprecated on insecure origins, and support will be removed in the future. You should consider switching your application to a secure origin, such as HTTPS. See https://goo.gl/rStTGz for more details." so the app should probably move to require https soon.)
One hitch: cdn.leafletjs.com doesn't have a valid ssl cert. As an alternative, leaflet is available from cdnjs, which does offer https: https://cdnjs.com/libraries/leaflet/