如何向响应流写入文件，并显示/隐藏“工作”模式窗口?

I need to to export a file to the user. It takes 1-2 min to generate the file so I'd like to have the page go into a kind-of modal mode with a layover on the page and a 'Working' spinner showing. The problem is I can't make the modal stuff go away after Save File dialog is closed.I need to to export a file to the user. It take