If you accidentally close the window or tab during an upload, it would be nice to give a warning saying that the upload will be canceled if you proceed, so that you don't lose the entire upload and have to restart. If you're uploading a file on the Dropbox website and you do this, it gives a popup warning that says "Are you sure you want to leave this page?" and shows the option to click one of two buttons: "stay on page" or "leave page".