Preserve scroll position when the article list changes underneath you

A background fetch (the 5am alarm or the refresh button) inserting new
articles above the current viewport left LazyColumn pointing at the same
numeric index, which after the insert resolves to a different (newer)
article -- the visible list silently jumped forward instead of staying
where you were reading. Re-anchors to the same article by id at whatever
index it now sits at, restoring the exact scroll offset too.

1 files changed, 25 insertions, 1 deletions

diff --git a/app/src/main/java/org/terst/feedreader/ui/ArticleListScreen.kt b/app/src/main/java/org/terst/feedreader/ui/ArticleListScreen.kt
index 7a99050..d87620d 100644
--- a/app/src/main/java/org/terst/feedreader/ui/ArticleListScreen.kt
+++ b/app/src/main/java/org/terst/feedreader/ui/ArticleListScreen.kt
@@ -12,6 +12,7 @@ import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.items
+import androidx.compose.foundation.lazy.rememberLazyListState
 import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.filled.ArrowBack
@@ -98,6 +99,29 @@ fun ArticleListScreen(
         }
     }.collectAsState(initial = emptyList())
 
+    // Hoisted (not LazyColumn's own default state) so the effect below can read/adjust it.
+    val listState = rememberLazyListState()
+    var previousArticleIds by remember(feedId) { mutableStateOf<List<String>>(emptyList()) }
+
+    // A background fetch (the 5am alarm, or the explicit refresh button) can insert new
+    // articles above whatever's currently on screen. LazyColumn doesn't move
+    // firstVisibleItemIndex just because the backing list changed -- that index is only
+    // ever moved by an actual scroll or an explicit scrollToItem -- so after an insert, the
+    // SAME index now points at a DIFFERENT (newer) article, and the visible content jumps
+    // out from under you instead of staying where you were reading. Re-anchor to the same
+    // article, by id, at whatever index it now sits at, preserving the on-screen offset too
+    // so this doesn't even show as a snap -- just an uninterrupted read.
+    LaunchedEffect(visibleArticles) {
+        val anchorId = previousArticleIds.getOrNull(listState.firstVisibleItemIndex)
+        if (anchorId != null) {
+            val newIndex = visibleArticles.indexOfFirst { it.first.id == anchorId }
+            if (newIndex >= 0 && newIndex != listState.firstVisibleItemIndex) {
+                listState.scrollToItem(newIndex, listState.firstVisibleItemScrollOffset)
+            }
+        }
+        previousArticleIds = visibleArticles.map { it.first.id }
+    }
+
     // Unread/Older/Saved mix articles from every feed, so each row names its source; a
     // single feed's own list already says so in the top bar, so rows there don't repeat it.
     val showFeedName = feedId == UNREAD_FEED_ID || feedId == OLDER_FEED_ID || feedId == SAVED_FEED_ID
@@ -243,7 +267,7 @@ fun ArticleListScreen(
         },
         snackbarHost = { SnackbarHost(snackbarHostState) }
     ) { padding ->
-        LazyColumn(modifier = Modifier.fillMaxSize().padding(padding)) {
+        LazyColumn(modifier = Modifier.fillMaxSize().padding(padding), state = listState) {
             items(visibleArticles, key = { it.first.id }) { (article, result) ->
                 ArticleRow(
                     article = article,