How to display a Leaflet map in modal window?
I found similar issues either with Leaflet or Google maps, like this one here. So I have to reload or re-render the map after the modal window become visible. For this purpose I inserted this jQuery code which does the trick:

drupal_add_js('
jQuery(function($) {
$(document).ready(function(){
$(\'#myMap\').on(\'shown.bs.modal\', function(){
var map = Drupal.settings.leaflet[0].lMap;
map.invalidateSize();
});
});
});
', 'inline');