How to implement EventSource on Codeignigter app?

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So after more experimentation, I found that the form data will POST to the controller via fetch if I remove the header, but the progress messages that echo during the process aren't fired until the process is done. The strange thing is that I even flush() inside the process, and the messages are sent as a response to my fetch request. 

I hardcoded the values I'm trying to pass, and I got the regular progress messages to eventsource throughout the process (instead of until it was done). Something about POSTing this data without a header prevents these messages from sending...