Wierd white line between slides

I've got a basic carousel with ~25 videos. It's working fine, however sometimes when I drag the slides I see this strange vertical line appearing between the slides. The line flickers and sometimes it has the same height as the video/slide and sometimes just as small as in the screenshot. My guess is that it's something with the `outline`, however I've tried to disable it using:

```css
* {
outline: 2px solid black !important;
}
```

Or just setting the outline to `hidden`.

Any ideas what might cause this?