How do I scroll a slide within a vertical slide and have it snap to the next slide and fade?

[raiden-kaihatsu]

As the title says, how do I scroll through a slide that has 500vh and then snap to the next slide and fade?

https://codepen.io/Echo1017/pen/NPWrBGq?editors=1010

I want to keep the behaviour as it is and allow the first slide to be scrollable.
I have tried free mode, but after a little scrolling it goes back to the top of the slide.