The company's Jen Gentleman notes that there's already a fix in place for Office Insiders who are on the Slow test ring (that is, the safest). That suggests that a patch is coming sooner rather than later, and you can get rid of the window bug right now if you're willing to use a less-than-stable version of the productivity suite. Still, this is bound to be irritating -- especially if you're on a work computer and can't try preview software.