From 3cd5b49f987e158a082a221dee84689dbabe59ca Mon Sep 17 00:00:00 2001
From: atsunatsu <atsunatsu@users.noreply.github.com>
Date: Fri, 18 Sep 2026 02:04:04 +0800
Subject: [PATCH] feat(mutual): keep pass list scroll position across page
 switches

The mutual pass list LazyColumn reset to the top on every return to the page.
Scroll position is now stored in the Activity-scoped MutualViewModel and
restored on re-entry; a new query (queryGeneration bump) rebuilds the list at
the top so stale positions never leak into fresh results.
---
 .../look4sat/feature/mutual/MutualScreen.kt   | 26 +++++++++++++++++++
 .../feature/mutual/MutualViewModel.kt         | 14 ++++++++++
 2 files changed, 40 insertions(+)

diff --git a/feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/MutualScreen.kt b/feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/MutualScreen.kt
index f2e1ccc0..79e5ae45 100644
--- a/feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/MutualScreen.kt
+++ b/feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/MutualScreen.kt
@@ -32,6 +32,7 @@ import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.layout.width
 import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.LazyListState
 import androidx.compose.foundation.lazy.itemsIndexed
 import androidx.compose.foundation.text.KeyboardOptions
 import androidx.compose.material3.Button
@@ -49,11 +50,13 @@ import androidx.compose.material3.Slider
 import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableFloatStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
+import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.res.painterResource
@@ -74,6 +77,7 @@ import java.text.SimpleDateFormat
 import java.util.Date
 import java.util.Locale
 import java.util.TimeZone
+import kotlinx.coroutines.flow.distinctUntilChanged
 
 @Composable
 fun MutualScreen(
@@ -150,6 +154,7 @@ fun MutualScreen(
         }
     ) { isVertical ->
         MutualContent(
+            viewModel = viewModel,
             state = state,
             isVertical = isVertical,
             onQuery = viewModel::queryMutualPasses,
@@ -172,6 +177,7 @@ fun MutualScreen(
 
 @Composable
 private fun MutualContent(
+    viewModel: MutualViewModel,
     state: MutualUiState,
     isVertical: Boolean,
     onQuery: () -> Unit,
@@ -195,8 +201,28 @@ private fun MutualContent(
         }
     }
 
+    // List state survives page switches via the Activity-scoped ViewModel: the
+    // initial position is restored from the VM, and scrolling writes back to it,
+    // so returning to this page keeps exactly where the user left off. A new
+    // query (queryGeneration bump) rebuilds the state at the top.
+    val listState = remember(viewModel.queryGeneration) {
+        LazyListState(
+            firstVisibleItemIndex = viewModel.listScrollIndex,
+            firstVisibleItemScrollOffset = viewModel.listScrollOffset
+        )
+    }
+    LaunchedEffect(listState) {
+        snapshotFlow { listState.firstVisibleItemIndex to listState.firstVisibleItemScrollOffset }
+            .distinctUntilChanged()
+            .collect { (index, offset) ->
+                viewModel.listScrollIndex = index
+                viewModel.listScrollOffset = offset
+            }
+    }
+
     LazyColumn(
         modifier = Modifier.fillMaxSize(),
+        state = listState,
         verticalArrangement = Arrangement.spacedBy(6.dp)
     ) {
         // Error message
diff --git a/feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/MutualViewModel.kt b/feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/MutualViewModel.kt
index 7520f5a9..66abd1ec 100644
--- a/feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/MutualViewModel.kt
+++ b/feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/MutualViewModel.kt
@@ -69,6 +69,15 @@ class MutualViewModel(
     private val _uiState = MutableStateFlow(MutualUiState())
     val uiState: StateFlow<MutualUiState> = _uiState.asStateFlow()
 
+    // Session-scoped scroll position of the pass list (Activity-scoped VM, so it
+    // survives switching to another page and back). Reset to the top whenever a
+    // new query replaces the results.
+    var listScrollIndex: Int = 0
+    var listScrollOffset: Int = 0
+
+    /** Bumped on every new query so the list scrolls back to the top. */
+    var queryGeneration: Int = 0
+
     init {
         // Pre-fill station A with the user's current station position (as grid),
         // and default min elevation to the same value used by the main radar passes
@@ -201,6 +210,11 @@ class MutualViewModel(
                 selectedPassIndex = -1
             )
         }
+        // The results list is about to be replaced, so the scroll position must
+        // not leak from the previous query's list.
+        listScrollIndex = 0
+        listScrollOffset = 0
+        queryGeneration += 1
 
         viewModelScope.launch {
             val time = System.currentTimeMillis()