fix(stop-detail): retain header across rotation to avoid refetch

mobile/feature/stop-detail/src/main/kotlin/ac/jfx/openptv/feature/stopdetail/StopDetailViewModel.kt

@@ -274,7 +274,16 @@ class StopDetailViewModel
             }
         }
 
+        /**
+         * One-shot header fetch. Idempotent on a successful load: once the header is
+         * [HeaderState.Loaded] we don't re-fetch, so a config change (rotation) that recreates the
+         * ViewModel — or simply re-enters `init` — never re-hits `stops/{id}/route_type/{rt}`
+         * (issue #161). The genuine first load still runs (header is `Loading`), and
+         * [retryHeader] resets the state back to `Loading` before calling, so retry-after-error
+         * still re-fetches.
+         */
         private fun loadHeader() {
+            if (_uiState.value.header is HeaderState.Loaded) return
             viewModelScope.launch {
                 val result: Result<StopDetail> = getStopDetail(stopId, routeType)
                 _uiState.update { current ->
@@ -292,7 +301,15 @@ class StopDetailViewModel
 
         private fun StopDetailUiState.applyDepartureResult(result: Result<List<Departure>>): StopDetailUiState =
             when (result) {
-                is Result.Loading -> copy(departures = DeparturesState.Loading)
+                is Result.Loading ->
+                    // Don't blow away an already-loaded list when the poll restarts. On a config
+                    // change (rotation) `repeatOnLifecycle(RESUMED)` re-subscribes, and the
+                    // repository's first re-emission is `Loading`; flipping back to the loading
+                    // skeleton there is the visible "reload flicker" from issue #161. Keep the
+                    // last-good `Loaded` rows on screen — the fresh data lands on the next
+                    // `Success` tick and replaces them in place. Only the genuine first load (no
+                    // rows yet) shows the skeleton.
+                    if (departures is DeparturesState.Loaded) this else copy(departures = DeparturesState.Loading)
                 is Result.Success -> {
                     lastHeadPoll = result.data
                     val merged = mergeDepartures(headPoll = result.data)

mobile/feature/stop-detail/src/test/kotlin/ac/jfx/openptv/feature/stopdetail/StopDetailViewModelTest.kt

@@ -100,6 +100,40 @@ class StopDetailViewModelTest {
             assertThat(viewModel.uiState.value.header).isEqualTo(HeaderState.Loaded(detail))
         }
 
+    @Test
+    fun `header is fetched exactly once on first load — issue 161`() =
+        runTest(dispatcher) {
+            stopDetailRepository.enqueueSuccess(StopDetailMother.aStopDetail().build())
+            val viewModel = newViewModel()
+            advanceUntilIdle()
+
+            assertThat(viewModel.uiState.value.header).isInstanceOf(HeaderState.Loaded::class.java)
+            assertThat(stopDetailRepository.requestedKeys).hasSize(1)
+        }
+
+    @Test
+    fun `restarting the departures poll does not refetch the header — issue 161`() =
+        runTest(dispatcher) {
+            // A config change (rotation) re-runs the repeatOnLifecycle(RESUMED) block, which calls
+            // startObserving() again. That must not drag the one-shot header fetch along with it —
+            // the header stays Loaded and `stops/{id}/route_type/{rt}` is hit only once.
+            stopDetailRepository.enqueueSuccess(StopDetailMother.aStopDetail().build())
+            val viewModel = newViewModel()
+            advanceUntilIdle()
+            viewModel.startObserving()
+            advanceUntilIdle()
+            departureRepository.emitSuccess(listOf(DepartureMother.aDeparture().build()))
+            advanceUntilIdle()
+
+            // Simulate the rotation Pause -> Resume: the lifecycle owner re-enters the block.
+            viewModel.stopObserving()
+            viewModel.startObserving()
+            advanceUntilIdle()
+
+            assertThat(viewModel.uiState.value.header).isInstanceOf(HeaderState.Loaded::class.java)
+            assertThat(stopDetailRepository.requestedKeys).hasSize(1)
+        }
+
     @Test
     fun `header Error renders the user-facing reason and retry restores Loading`() =
         runTest(dispatcher) {
@@ -246,22 +280,42 @@ class StopDetailViewModelTest {
         }
 
     @Test
-    fun `loading emission flips departures back to Loading`() =
+    fun `first loading emission shows the loading skeleton`() =
         runTest(dispatcher) {
             stopDetailRepository.enqueueSuccess(StopDetailMother.aStopDetail().build())
             val viewModel = newViewModel()
             advanceUntilIdle()
             viewModel.startObserving()
             advanceUntilIdle()
 
+            // No rows have landed yet — a Loading emission should surface the skeleton.
+            departureRepository.emit(Result.Loading)
+            advanceUntilIdle()
+            assertThat(viewModel.uiState.value.departures).isEqualTo(DeparturesState.Loading)
+        }
+
+    @Test
+    fun `loading emission after data keeps the last-good list on screen — issue 161`() =
+        runTest(dispatcher) {
+            // Rotation re-subscribes the poll via repeatOnLifecycle(RESUMED); the repository's
+            // first re-emission is Loading. We must NOT flip back to the skeleton — that's the
+            // reload flicker issue #161 fixes. The previously-loaded rows stay visible until the
+            // next Success tick replaces them.
+            stopDetailRepository.enqueueSuccess(StopDetailMother.aStopDetail().build())
+            val viewModel = newViewModel()
+            advanceUntilIdle()
+            viewModel.startObserving()
+            advanceUntilIdle()
+
             departureRepository.emitSuccess(listOf(DepartureMother.aDeparture().build()))
             advanceUntilIdle()
-            assertThat(viewModel.uiState.value.departures)
-                .isInstanceOf(DeparturesState.Loaded::class.java)
+            val loaded = viewModel.uiState.value.departures
+            assertThat(loaded).isInstanceOf(DeparturesState.Loaded::class.java)
 
             departureRepository.emit(Result.Loading)
             advanceUntilIdle()
-            assertThat(viewModel.uiState.value.departures).isEqualTo(DeparturesState.Loading)
+            // Same Loaded list, not the skeleton.
+            assertThat(viewModel.uiState.value.departures).isEqualTo(loaded)
         }
 
     @Test