Sidebar problem

I posted this in the Themes forum, also, but I haven’t gotten any answers, so maybe someone here knows…
I have boxes on my sidebar with different content. Well, one of these boxes turns kind of transparent when the page scrolls.
https://www.sarahsblogging.com
It’s the ‘Top Posts’ box. I don’t know if it’s an issue with the plug-in, or it’s some kind of formatting thing that I’ve done.
I’m still a bit of a beginner, so if anyone out there could help, I would really appreciate it! Thanks so much!