Q:

Google maps - how to add marker without layout refresh

Have problem with marker adding. 
        var marker = new google.maps. Marker({
        position: myLatlng,
        title:"Hello World!" 
    });

    // To add the marker to the map, call setMap();
    marker.setMap(map);

https://jsfiddle.net/73ogq84a/
Simple example, each seconds I put marker, page not reloading, but reloading some layout and we see effect of map reload. 
It is possible to do that put marker smoothly. 
But on this page http://www.flightradar24.com/simple_index.php all working fine, planes are flying and no effect of map reload. 

A:

Make the marker and map global.   Don't recreate the map every time, just move the marker. 
var map, marker, myLatlng;

setInterval(function () {
    if (! marker || ! marker.setPosition) {
        marker = new google.maps. Marker({
            position: myLatlng,
            title: "Hello World!" 
        });

        // To add the marker to the map, call setMap();
        marker.setMap(map);
    } else {
        marker.setPosition(myLatlng);
    }

}, 5000);

updated fiddle

