singleFetch set headers (like Cache-Control) depending initial doc or data requests

[nickluger]

Before using the singleFetch-flag one was able to set different headers on the initial doc/HTML request vs. subsequent data-only/JSON requests.

This for example was helpful to avoid caching docs/HTML to assert that references and JS are always fresh, but do cache subsequent data requests. The data requests were than varied in CDN via app-version-cookie and could be cached without risking version skew.

This seems to be more difficult because single fetch seems not to distinguish between data requests and the initial doc fetch in loaders. I could parse e.g. a request header like Accept in the loader or similar, but it seems a convoluted workaround as Remix probably already knows what kind of request this is.

Is there some flag or any other indicator that could be used?