GWT - easiest way to do a simple loading screen until file is loaded

Question

When clicking a button, my GWT application returns a PDF file embedded in an HTML page which looks something like:

Answer 1

Did you see this stackoverflow question Detect when browser receives file download? Basically the answer given is that you set a cookie in the return response and wait on the client side for this cookie to be set. This can be done easily with GWT as it has a Scheduler (for the repeated timer check) and easy access to Cookies. You still need to make some server changes, but you don't have to create a background process.