`useSWRImmutable` ignores hook-level `refreshInterval` since 2.4.0 (regression of #4208)

[lukasvice]

Bug report

Description / Observed Behavior

Since v2.4.0, useSWRImmutable ignores a refreshInterval passed directly to the hook, not just one inherited from a global <SWRConfig>.

#4208 ("fix: Ensure useSWRImmutable overrides global refreshInterval") added this line to the immutable middleware:

config.refreshInterval = 0

However, middleware runs after the hook-level options have been merged with the context config.

At that point the middleware cannot distinguish a globally configured refreshInterval from one passed as hook options, so it silently disables both. Polling that worked on ≤ 2.3.x stops with no error or warning after upgrading.

Expected Behavior

A refreshInterval passed directly to useSWRImmutable(key, fetcher, { refreshInterval }) should still be honored — only a refreshInterval inherited from a parent <SWRConfig> (the case #4208 was about) should be ignored.

If ignoring the hook-level option is intentional, it would be worth documenting as a breaking change, since it silently changes runtime behavior in a minor release.

Repro Steps / Code Example

import useSWRImmutable from 'swr/immutable'

function Status() {
  // On swr 2.3.6: fetches every second.
  // On swr >= 2.4.0: fetches exactly once, refreshInterval is discarded.
  const { data } = useSWRImmutable('/api/status', fetcher, {
    refreshInterval: 1000,
  })

  return <span>{data?.status}</span>
}

No global <SWRConfig> is involved.

https://codesandbox.io/p/sandbox/thqfwl

Additional Context

SWR version: 2.5.0 (regression introduced in 2.4.0 via #4208; 2.3.6 behaves as expected).

[NektarTheo]

Hello, I'm a new contributor and I've done some work on this issue in #4325.

From my unserstanding, the root cause is that the immutable middleware runs after the context config and hook options have been merged into one object, so it can't tell which source refreshInterval came from and clears both. The PR applies refreshInterval: 0 as a default before that merge instead, so a global interval is still ignored while one passed directly to the hook is applied.

Happy to adjust the approach if the maintainers would prefer something different 👍