Using this code in `index.html`
causes all kind of weird side effects due to race conditions, I assume.
As soon as I removed the async attribute (after one week of trying other things) everything worked, including the splashscreen. I think the splashscreen uses the message queue (event loop) to postpone some of its actions (when fading out) and this collides with the `async` attribute and creates a race-condition. It seems like a bug in wkwebview to not handle `async` script loading properly.