So the issue in our setup was the fact that we use a really large Iframe - 5000px - set in the viz options. The solution is going to be finding where to modify the js for the positioning of the export dialog. We also had to make this update in the 9.2 js api - vqlweb.js. In this version the changes have to go in vqlweb.bundle.js. Also noteworthy, the vqlweb.bundle.js.gz file is read first if available. So be sure and remove that file from the directory while developing changes
We're having the same issue except in our case it's 2000 px. I'm hoping to find a way to position the "Image generated" popup without having to modify the vqlweb.css file. I'll update this comment if I do find something.