Hi, there has been a regression in the past few days of Discourse commits where the browser / OS scrollbar appears always visible with a few pixels of vertical slop when the Sidebar Theme Toggle is enabled. It’s happening here on Discourse Meta and also on my forum, currently on 3.1.0.beta3 (6b5743ba3c). It happens with Firefox and Chromium. I imagine that nobody has noticed it because most OSes these days are defaulting to skinny / disappearing scrollbars, but I’m rather old-school. 
Edit: Looks like I can get rid of it with this quick dirty workaround for now:
.sidebar-footer-container {
padding-bottom: .5em !important;
}