I am working on an Outlook mail app, which will be available to user on web only (not of outlook desktop). A file need to be uploaded from app to azure via custom control in compose form of App, meanwhile the file is uploaded to Azure user should not be able to close the mail. If they try to do so, a warning should be given to them.
Unfortunately Office.js API does not have the feature you are inquiring. You will not be able to disallow user interaction, as such closing the compose window or closing your add-in. As the API doesn't have "OnSend" or "OnClose" events you will not be able to display any warning either. If this is the new feature you would like to add you may try to send request via Office Developers User Voice. As the work around you should clearly indicate for the users that they need to wait and do not interup operation. You should display activity indicator, indeed. And finally you should be prepare user still interupt the operation in the middle and work properly with the error occur. Hope this helps.