I don't use DropBox, but what's being described sounds a lot like what I've taken to calling screen veil popups, where these things are not opaque like a screen curtain, but translucent, allowing a sighted person to see what lies beneath on the screen other than where the actual dialog box is showing, but prevents any interaction with the underlying screen until dismissed. And while they don't pose nearly the challenge if you can see, they're still mightily annoying.
Often, but not always, hitting Escape will dismiss these blasted things. If anyone wants to try that (or has) and report back that would be a useful data point.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back