X_TEST_ISLAND_STATE_UNKNOWN

an island state manifest declares no state with that id

mountIslandState(manifest, id) or describeIslandState(manifest, id, …) named a state the manifest does not declare. A renamed state and a typo are one symptom, so cause lists every declared id

name one of the ids the cause lists — or declare the state in the <name>.island.states.ts beside the island: { id: 'idle', title: 'the first paint', props: {} }