This event is emitted after a new overlay was placed into the DOM.
The event is emitted right before the opening animation starts. Because the overlay
has not been rendered by the browser, this makes it a good occasion to
customize overlay elements:
up.on('up:layer:opened', function(event) {
if (isChristmas()) {
up.element.affix(event.layer.element, '.santa-hat', text: 'Merry Christmas!')
}
})
