Wierd white line between slides

[samsnori]

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:

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

Or just setting the outline to hidden.

Any ideas what might cause this?