I'm using a premium plugin (Superfly Menu). The issue doesn't seem to be caused by this plugin or any other, though is associated with this plugin. It looks like the cause is somewhere else.
On a test site, I ran identical tests. First I installed the 2017 theme with the Superfly Menu plugin installed. There was no extra scrollbar. Then I switched to the same theme use on the live site (via Upfront). Still no extra scrollbar issue. So then I started to mimic each individual setting for the plugin. Yet I still couldn't replicate the issue.
Then I implemented a plugin conflict test and still found no issue with any other plugins. I even enabled them all on the test site and couldn't replicate the issue.
I added the same Global CSS as well with no go.
I'm not sure how else to proceed to solve this issue and was hoping you may shed some clarity on this matter.