Keep a just-read article visible (and Previous-reachable) until the next fetch

The Unread/Older views are live Room Flow queries filtered on
isRead = 0, so marking an article read (which only happens on
Next/Previous, not on open) makes it vanish from the list the instant
Room re-emits -- mid-scroll, or from underneath the reader's own
Previous button, which re-queries this same flow to find its
neighbors. Concretely: Next from C to D marks C read and prunes it
from the live list; Previous from D then can't find C anymore, and
jumps past it to whatever was unread before C. A misclick or the
in-flight prune could cost you the article you meant to keep reading.

Adds ArticleEntity.readAt (set whenever isRead flips true, cleared on
explicit "mark unread"); observeUnread/observeOlderUnread now match
"isRead = 0 OR readAt >= sinceFetch", where sinceFetch is
Settings.lastFetchAt -- the same durable signal the new-articles
snackbar already uses. A read article now lingers (shown dimmed via
the existing showReadState styling) until the next real fetch
actually completes, not the next DB write, and Previous/Next read from
the same still-inclusive query so stepping back to what you just read
finds it right where it was.

3 files changed, 39 insertions, 15 deletions

diff --git a/app/src/main/java/org/terst/feedreader/data/AppDatabase.kt b/app/src/main/java/org/terst/feedreader/data/AppDatabase.kt
index 7f33439..2fd5816 100644
--- a/app/src/main/java/org/terst/feedreader/data/AppDatabase.kt
+++ b/app/src/main/java/org/terst/feedreader/data/AppDatabase.kt
@@ -12,7 +12,7 @@ private const val TAG = "FeedMigration"
 
 @Database(
     entities = [FeedEntity::class, ArticleEntity::class, FilterRuleEntity::class],
-    version = 9,
+    version = 10,
     exportSchema = false
 )
 abstract class AppDatabase : RoomDatabase() {
@@ -258,6 +258,16 @@ abstract class AppDatabase : RoomDatabase() {
             }
         }
 
+        // v10: added ArticleEntity.readAt (epoch millis, null while unread). Plain nullable
+        // ADD COLUMN is safe here the way v2/v3's were -- no column is being removed or
+        // renamed, so there's nothing for Room's exact-match post-migration validation to
+        // trip on (see v4's comment above for what DOES require a full rebuild).
+        private val MIGRATION_9_10 = object : Migration(9, 10) {
+            override fun migrate(db: SupportSQLiteDatabase) {
+                db.execSQL("ALTER TABLE articles ADD COLUMN readAt INTEGER")
+            }
+        }
+
         fun get(context: Context): AppDatabase = instance ?: synchronized(this) {
             instance ?: Room.databaseBuilder(
                 context.applicationContext,
@@ -265,7 +275,7 @@ abstract class AppDatabase : RoomDatabase() {
                 "feedreader.db"
             ).addMigrations(
                 MIGRATION_1_2, MIGRATION_2_3, MIGRATION_3_4, MIGRATION_4_5,
-                MIGRATION_5_6, MIGRATION_6_7, MIGRATION_7_8, MIGRATION_8_9
+                MIGRATION_5_6, MIGRATION_6_7, MIGRATION_7_8, MIGRATION_8_9, MIGRATION_9_10
             ).build().also { instance = it }
         }
     }
diff --git a/app/src/main/java/org/terst/feedreader/data/Daos.kt b/app/src/main/java/org/terst/feedreader/data/Daos.kt
index ecaac43..20632f8 100644
--- a/app/src/main/java/org/terst/feedreader/data/Daos.kt
+++ b/app/src/main/java/org/terst/feedreader/data/Daos.kt
@@ -107,21 +107,30 @@ interface ArticleDao {
     // MainActivity) so a backlog of hundreds of older unread items doesn't bury
     // what's actually new. Undated articles (publishedAt failed to parse) are kept
     // here rather than silently dropped from both views.
+    //
+    // "isRead = 0 OR readAt >= sinceFetch" (not just "isRead = 0"): a just-read article
+    // stays in this view until the NEXT fetch completes, not the instant it's marked
+    // read. Without this, Room's live Flow re-emits the moment isRead flips, so (a) a
+    // read row vanishes out from under you mid-scroll, and (b) the reader's Previous
+    // button -- which re-queries this same flow to find its neighbors -- can no longer
+    // find the article you just came from, since Next already marked it read a moment
+    // earlier. sinceFetch is Settings.lastFetchAt, the same durable "when did a fetch
+    // last finish" signal the new-articles snackbar already uses.
     @Query(
         "SELECT $ARTICLE_LIST_COLUMNS FROM $ARTICLE_LIST_FROM " +
-            "WHERE articles.isRead = 0 AND (articles.publishedAt IS NULL OR articles.publishedAt >= :cutoff) " +
+            "WHERE (articles.isRead = 0 OR articles.readAt >= :sinceFetch) AND (articles.publishedAt IS NULL OR articles.publishedAt >= :cutoff) " +
             "ORDER BY articles.publishedAt DESC"
     )
-    fun observeUnread(cutoff: Long): Flow<List<ArticleListItem>>
+    fun observeUnread(cutoff: Long, sinceFetch: Long): Flow<List<ArticleListItem>>
 
     // The rest of the unread backlog, older than the cutoff -- its own view rather
-    // than mixed into the main one.
+    // than mixed into the main one. See observeUnread for why sinceFetch is here.
     @Query(
         "SELECT $ARTICLE_LIST_COLUMNS FROM $ARTICLE_LIST_FROM " +
-            "WHERE articles.isRead = 0 AND articles.publishedAt IS NOT NULL AND articles.publishedAt < :cutoff " +
+            "WHERE (articles.isRead = 0 OR articles.readAt >= :sinceFetch) AND articles.publishedAt IS NOT NULL AND articles.publishedAt < :cutoff " +
             "ORDER BY articles.publishedAt DESC"
     )
-    fun observeOlderUnread(cutoff: Long): Flow<List<ArticleListItem>>
+    fun observeOlderUnread(cutoff: Long, sinceFetch: Long): Flow<List<ArticleListItem>>
 
     @Query("SELECT COUNT(*) FROM articles WHERE isRead = 0 AND (publishedAt IS NULL OR publishedAt >= :cutoff)")
     fun observeUnreadCount(cutoff: Long): Flow<Int>
@@ -157,16 +166,20 @@ interface ArticleDao {
     @Insert(onConflict = OnConflictStrategy.REPLACE)
     suspend fun upsert(article: ArticleEntity)
 
-    @Query("UPDATE articles SET isRead = :read WHERE id = :id")
-    suspend fun setRead(id: String, read: Boolean)
+    // readAt is caller-supplied (not CURRENT_TIMESTAMP in SQL) so it's directly comparable
+    // to Settings.lastFetchAt, which is also a Kotlin System.currentTimeMillis() epoch
+    // value, not a SQLite datetime string. Pass null when read=false (explicit "mark
+    // unread") so a stale readAt can't linger on a row that's unread again.
+    @Query("UPDATE articles SET isRead = :read, readAt = :readAt WHERE id = :id")
+    suspend fun setRead(id: String, read: Boolean, readAt: Long?)
 
     // Explicit bulk action ("mark read from here") -- batched per confirmed tap,
     // never called implicitly (e.g. by scrolling).
-    @Query("UPDATE articles SET isRead = 1 WHERE id IN (:ids)")
-    suspend fun markReadBatch(ids: List<String>)
+    @Query("UPDATE articles SET isRead = 1, readAt = :readAt WHERE id IN (:ids)")
+    suspend fun markReadBatch(ids: List<String>, readAt: Long)
 
-    @Query("UPDATE articles SET isRead = 1 WHERE feedId = :feedId")
-    suspend fun markAllReadForFeed(feedId: String)
+    @Query("UPDATE articles SET isRead = 1, readAt = :readAt WHERE feedId = :feedId")
+    suspend fun markAllReadForFeed(feedId: String, readAt: Long)
 
     // Run when unsubscribing from a feed. Saved articles are deliberately kept
     // regardless (isSaved is a separate, durable "keep forever" concept,
@@ -196,8 +209,8 @@ interface ArticleDao {
 // and SQLite has a hard per-statement limit on bound variables (historically 999 on many
 // Android SQLite builds). Chunking well under any realistic limit means this can't throw
 // "too many SQL variables" and crash on an otherwise routine bulk mark-read tap.
-suspend fun ArticleDao.markReadBatchChunked(ids: List<String>) {
-    ids.chunked(500).forEach { chunk -> markReadBatch(chunk) }
+suspend fun ArticleDao.markReadBatchChunked(ids: List<String>, readAt: Long = System.currentTimeMillis()) {
+    ids.chunked(500).forEach { chunk -> markReadBatch(chunk, readAt) }
 }
 
 @Dao
diff --git a/app/src/main/java/org/terst/feedreader/data/Entities.kt b/app/src/main/java/org/terst/feedreader/data/Entities.kt
index 5f6a28c..58c9f79 100644
--- a/app/src/main/java/org/terst/feedreader/data/Entities.kt
+++ b/app/src/main/java/org/terst/feedreader/data/Entities.kt
@@ -31,6 +31,7 @@ data class ArticleEntity(
     val extractedContent: String? = null,    // Readability-extracted full text, filled in on first read if feedContentHtml was thin
     val imageUrl: String? = null,            // thumbnail, from media:thumbnail/enclosure or the first <img> in the feed content -- see FeedParser
     val isRead: Boolean = false,
+    val readAt: Long? = null,                 // when isRead flipped true; null while unread or after an explicit "mark unread". Lets a just-read article linger in the Unread view until the next fetch instead of vanishing the instant it's read -- see observeUnread/observeOlderUnread.
     val isSaved: Boolean = false,             // explicitly kept (independent of read state) -- triggers a Stash save
     val stashArtifactId: String? = null,      // set once successfully archived to Stash; null means not yet saved or save failed
     val createdAt: Long