On my web app the user can generate a CVS file that can get pretty large 10+ Mb sometimes. The report obviously can take some time to generate. I want to display a throbber for the user while the report is being generated, once they are prompted to save/run I want the throbber to hide. Is this possible?
No, it's not really possible to detect when the file arrives and the user saves it. You're pretty much stuck just updating your throbber and offering a continue link when they're ready to continue.
If this was possible then the download sites would use it to auto-forward you to the download list when you're finished at the file's landing page.