@swissspidy Can you share a bit more context? What leads to it being "too late"? I have never encountered the problem, so would like to understand better.

When I run this in WP core‘s Docker environment, the callback in the shutdown hook is never fired, so it‘s not sending the header. I assume it‘s because the shutdown handler is called long after the page has been served.
Weirdly it‘s not an issue when using this with LocalWP, just with the core env.

@swissspidy That's odd. I have been using this in the WP core Docker env several times without problems 🤔 Maybe it's something more specific than that?

IDK 🤷 Maybe I can debug at contributor day or so

Will the removal of the ob_get_clean() call in this PR lead to issues? Not very familiar with how output buffers work under the hood, but I always assumed you should also "end" an output buffer that you start.

Any open buffers are automatically flushed when the page response is finished, per docs:

If output buffering is still active when the script ends, PHP outputs the contents automatically.