I have found the solution.
The reason why it didn’t work, is because the onload event is set in the following way:

window.onload = function() {
  // code here
};

Instead, it should be set this way:

window.addEventListener('load', function() {
  // code here
});

Would you be willing to change this in file /js/scripts/global/helpers.min.js, so that we don’t lose our fixes after updating the theme?