ui: preserve pending older queue loads

app/src/main/kotlin/com/subtracks/ui/playback/QueueScreen.kt

@@ -120,6 +120,7 @@ class QueueViewModel(
         private set
 
     private var reorderPending = false
+    private var olderLoadPending = false
     private var dragRollback: ReorderRollback? = null
 
     private var nextId = 0L
@@ -189,7 +190,7 @@ class QueueViewModel(
         val scanned = loaded.map { rowFor(it, reused, snapshot.resetVersion) }
         val firstRow = rows.minByOrNull { it.position }
         val windowStart =
-            if (firstRow == null) {
+            if (firstRow == null || olderLoadPending) {
                 start
             } else {
                 scanned.firstOrNull { it.id == firstRow.id && it.position < firstPosition }?.position ?: firstPosition
@@ -214,7 +215,9 @@ class QueueViewModel(
     fun loadOlder() {
         val top = rows.minOfOrNull { it.position } ?: return
         if (top <= 0L) return
+        olderLoadPending = true
         viewModelScope.launch {
+            try {
                 mutex.withLock {
                     val first = rows.minOfOrNull { it.position } ?: return@withLock
                     if (first <= 0L) return@withLock
@@ -230,6 +233,9 @@ class QueueViewModel(
                     rows.addAll(0, collected.map { newRow(it, snapshot.resetVersion) })
                     while (rows.size > QUEUE_WINDOW_ROWS) rows.removeAt(rows.size - 1)
                 }
+            } finally {
+                olderLoadPending = false
+            }
         }
     }