The Settings sliders called persistFuelSavingsSettings() on every value change,
which rewrote all four keys — four UserDefaults writes and four keychain round
trips per step, on the main thread, up to 90 times per drag. That is what made
them stutter.
saveFuelSavingsField(_:from:) writes just the field that moved, so a slider
gesture touches one key.
writeKeychain now uses SecItemUpdate and only falls back to SecItemAdd on
errSecItemNotFound. Delete-then-add cost two round trips to securityd and
briefly left the key holding no value at all — a window where the keychain, the
authoritative mirror on read, could disagree with the app-group defaults.
Tests pin the store half: a single-field save leaves the other three keys alone,
and the whole-settings save still writes all four.
FuelBoard Contributor
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