How to keep popup remains open in mapbox-gl?

Question

var geojson = { "type": "FeatureCollection", "features": [{ "type": "Feature", "geometry": { "type": "Point", "coordinates": [-77.032, 38.913] }, "properties": { "title": "Mapbox", "description": "Washington, D.C." } }, { "type": "Feature", "geometry": { "type": "Point", "coordinates": [-122.414, 37.776] }, "properties": { "title": "Mapbox", "description": "San Francisco, California" } }] };

var map = new mapboxgl.Map({ container: 'map', style: 'mapbox://styles/mapbox/light-v10', center: [-96, 37.8], zoom: 3 });

// add markers to map

geojson.features.forEach(function(marker) {

// create a HTML element for each feature
var el = document.createElement('div');
el.className = 'marker';

// make a marker for each feature and add it to the map
new mapboxgl.Marker(el)
    .setLngLat(marker.geometry.coordinates)
    .setPopup(new mapboxgl.Popup({offset: 25}) // add popups
        .setHTML('<h3>' + marker.properties.title + '</h3><p>' + marker.properties.description + '</p>'))
    .addTo(map);

});