Openlayers map tiles are not initially loaded in single page app

I have a single page application built with

Express (4.16.3),
Openlayers (5.3) and
Pug (2.0.3 – formerly known as Jade) templating engine.

The map container is loaded and has child elements with ol- classes as well as the zoom controls in the upper left corner. So the Openlayers script is successfully executed. But it doesn't show the map tiles on load.

When I resize the browser the map tiles show up all of a sudden. So I'm wondering:

What is the event that triggers the sudden rendering of the tiles on browser resize, and how can I trigger it myself so the map is getting displayed correctly on load?

My index.pug looks like this:

doctype html
html(lang='de')
  head
    meta(charset='UTF-8')
    meta(http-equiv='X-UA-Compatible', content='ie=edge')
    meta(
      name='viewport'
      content='width=device-width, initial-scale=1')
    title=`myTitle`

    // Stylesheets
    link(
      rel='stylesheet',
      href='https://cdn.rawgit.com/openlayers/openlayers.github.io/master/en/v5.3.0/css/ol.css')
    link(
      rel='stylesheet',
      href='/assets/style.css')

    // Scripts
    script(src='https://cdn.rawgit.com/openlayers/openlayers.github.io/master/en/v5.3.0/build/ol.js')
    script(src='https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList')
    script(src='https://code.jquery.com/jquery-3.3.1.min.js')
    script(data-main='/js/main', src='/js/require.js')

  body
    include header
    include tabs

    main
      #loader Loading...
      include map

    include footer

And in the main part you see the map pug template included that looks like this:

section.component#component-map
  #map.map
  script.
    /**
    * Leaflet Map
    */
    // Create markers and geodata
    const mapCenter = [13.429, 52.494];
    const siteData = !{JSON.stringify(sites)};
    const features = siteData.map(site => {
      return {
        "type": "Feature",
        "geometry": {
          "type": "Point",
          "coordinates": [ Number(site.longitude), Number(site.latitude) ]
        }
      }
    });

    const image = new ol.style.Circle({
      radius: 5,
      fill: null,
      stroke: new ol.style.Stroke({color: "red", width: 1})
    });

    const styles = {
      "Point": new ol.style.Style({
        "image": image
      })
    }
    const styleFunction = function(feature) {
      return styles[feature.getGeometry().getType()];
    };

    const geojsonObject = {
      "type": "FeatureCollection",
      "features": features
    };

    const vectorSource = new ol.source.Vector({
      features: (new ol.format.GeoJSON()).readFeatures(geojsonObject)
    });

    const vectorLayer = new ol.layer.Vector({
      source: vectorSource,
      style: styleFunction
    });

    const map = new ol.Map({
      target: "map",
      layers: [
        new ol.layer.Tile({
          source: new ol.source.OSM()
        }),
        vectorLayer
      ],
      view: new ol.View({
        center: ol.proj.fromLonLat(mapCenter),
        zoom: 17,
      })
    });

Solution

Found the answer myself. My "solution" is more of a workaround than an actual solution. I wait for the map script to be executed (which is the case when the map container has a child element with class name ol-viewport) and then I trigger the browser resize event manually. Other than I expected, the map.render() or map.renderSync() methods do not load the tiles. So the workaround looks like this:

const waitForMap = setInterval(function() {
  if ($('.ol-viewport').length) {
      console.log("Exists!");
      window.dispatchEvent(new Event('resize'));
      clearInterval(waitForMap);
  }
}, 100);