[ryanbr]

Found while reviewing #1590, which is the second consumer of this flow. Independent of that PR — no conflict, either order.

The bug

combine(recentDays, flowOf(NoopPrefs.spo2CandidateDisplay(appContext))) { days, toggleOn -> … }

flowOf emits once and completes, so combine holds that first value for the ViewModel's whole life. The toggle is read at construction and never again.

The symptom is a setting that does nothing:

you do	what happens
turn "show SpO₂ candidate" off	Key Metrics tile keeps showing strap estimates until the process restarts
turn it on	nothing appears until the process restarts

Both directions, with nothing in the app to indicate the value in use is stale.

iOS reads PuffinExperiment.spo2CandidateDisplayEnabled per render and has never had the lag, so this was a live parity divergence too.

The fix

NoopPrefs.spo2CandidateDisplayFlow wraps the same key in a callbackFlow over OnSharedPreferenceChangeListener — the idiom already used by SettingsScreen (experiment keys) and NoopMotion — seeded with the current value and distinctUntilChanged so an unrelated write to the prefs file can't re-run the database union behind this flow.

Two details carried over from those existing listener sites rather than rediscovered:

• A strong local reference. Android holds these listeners weakly; one kept alive only by the register call is collected and silently stops firing. SettingsScreen has a comment saying exactly this.
• The null-key case. key is @Nullable on modern SDKs — it arrives null when the whole file is cleared, which reads as "everything changed". Required to compile, not merely defensive.

Named for the one key rather than made generic, so it sits beside the getter it mirrors and the two can't drift on the default. flowOf had no other caller in AppViewModel, so its import went with it.

Scope

The flowOf(NoopPrefs…) pattern appears exactly once in the codebase — this line. Not systemic, and now gone.

Verification, and one honest gap

• compileFullDebugKotlin clean, full unit suite 4294 tests / 0 failures, assembleFullDebug clean, doc lint clean.
• Not unit-tested. The listener needs a real SharedPreferences, and this module is deliberately Robolectric-free (AndroidDiagnosticsTest and friends note the same constraint), so there's no way to pin the re-emit without adding that dependency for one test. I'd rather say that than claim coverage that isn't there.
• The behaviour needs a device: toggle it in Settings and the Blood Oxygen tile should change without restarting the app.

Android-only — iOS already reads the flag per render.