Strange! I nailed it down to one thing:
Disqus comments loaded on that page, and it only happens with the Facebook link. Somehow, the scripts loaded by Disqus are interfering or hi-jacking the Facebook share link, preventing default behavior.
I will do some more research tomorrow. For now, your only "workaround" is to set
"target": "_blank" for the Facebook item in
Settings > Toolbar > Items. The Facebook property would then look like this:
{
"id": "facebook",
"target": "_blank",
"icon": "facebook",
"label": "Share on Facebook",
"url": "https://www.facebook.com/sharer/sharer.php?u={{url}}"
},
This will open share in a new window instead of popup, and is just a workaround (for now), if you need to avoid the issue.