TypeError: undefined is not a constructor (evaluating 'new window.IntersectionObserver(handler, opts)')

[sonn-gamm]

I get this error only with Safari 12.0.1, the page in Chrome and Firefox works fine. It's line 25 of index.js.

This is how I build the image

${state.cache(MonoImage, post.files[0].filename).render({
      sizes: { 600: post.files[0].url },
      dimensions: { ratio: 100 }
    }, {
      inline: true,
    })
}

More context here. I could possibly be doing something wrong?