BE HEARD - SUBMIT A SUGGESTION TO MICROSOFT at this link:
http://office.microsoft.com/en-us/suggestions.aspx
If everyone who reads this post trying to find a solution, let's MS know how frustrating this new "feature" is, maybe it will get some priority and get fixed. Or, MS will at least provide a setting so the user has the option to not have their scroll bar disappear.
BTW, MS must have included a way for a page's HTML to stop the disappearing scroll bar - notice it doesn't disappear on this site. Makes me wonder if they added this "feature" deliberately so people would accidentally click on an ad, and MS could charge the advertiser for the click-through. If I was an advertiser, I'd be upset that I was paying for unintentional clicks that are unlikely to result in a sale and very likely to result in the "visitor" having a negative experience associated with my site.