I have made a simpler test case: https://codesandbox.io/s/swiper-svelte-fork-forked-rgxie?file=/App.svelte.
Inside the swiper.virtual.prependSlide method, the things that are happening are basically the following:

1. The new slide is unshifted to swiper.virtual.slides.
2. swiper.virtual.update(true) is called.
3. swiper.slideTo() is called to get to the new index.

I have implemented this behaviour in the prependDate function on line 46, but skipped the slideTo call.
You can see that when you click on the prepend button when you are on the first slide, nothing gets rerendered. If then the slideTo method is called inside the prependSlide method, the swiper goes 'back' to the next slide, because the new slide is not rendered yet.
But if you skip the slideTo call and swipe forward a few times to force a rerender and then swipe back, you can see that the new slide gets rendered correctly.

So the problem is that the new slides do not get rendered when prepending a slide, and not an issue in the slideTo method as I initially thought.

Here in Svelte, you shouldn't use swiper.virtual.prependSlide/appendSlide methods, you should just modify your initial virtualSlide array with push/unshift but I see it also have some issue

Fixes in Swiper 6.6.2. Here is the full working example with logic for example https://github.com/nolimits4web/swiper/blob/master/playground/svelte/Virtual.svelte

Hello @nolimits4web the svelte example is missing. Any advice how to use it with svelte?