Fix older notifications not loading when initially visible set fits on one screen

Closes #346

Tusker/Screens/Notifications/NotificationsCollectionViewController.swift

@@ -539,6 +539,10 @@ extension NotificationsCollectionViewController: UICollectionViewDelegate {
         let itemsInSection = collectionView.numberOfItems(inSection: indexPath.section)
         if indexPath.row == itemsInSection - 1 {
             Task {
+                // Because of grouping, all cells from the first load may fit on screen,
+                // in which case, we try to load older while still in the loadingInitial state.
+                // So, wait for that to finish before trying to load more.
+                await controller.finishPendingOperation()
                 await controller.loadOlder()
             }
         }

Tusker/TimelineLikeController.swift

@@ -8,6 +8,7 @@
 
 import Foundation
 import OSLog
+import Combine
 
 protocol TimelineLikeControllerDelegate<TimelineItem>: AnyObject {
     associatedtype TimelineItem: Sendable
@@ -42,7 +43,7 @@ class TimelineLikeController<Item: Sendable> {
     private unowned var delegate: any TimelineLikeControllerDelegate<Item>
     private let ownerType: String
     
-    private(set) var state = State.notLoadedInitial {
+    @AsyncObservable private(set) var state = State.notLoadedInitial {
         willSet {
             guard state.canTransition(to: newValue) else {
                 logger.error("\(self.ownerType, privacy: .public) State \(self.state.debugDescription, privacy: .public) cannot transition to \(newValue.debugDescription, privacy: .public)")
@@ -57,6 +58,19 @@ class TimelineLikeController<Item: Sendable> {
         self.ownerType = ownerType
     }
     
+    /// Waits for the controller to finish the current operation and arrive at the idle state.
+    ///
+    /// If the current state is `notLoadedInitial`, this will wait until the controller
+    /// settles after the initial load.
+    func finishPendingOperation() async {
+        guard state != .idle else {
+            return
+        }
+        for await state in $state where state == .idle {
+            break
+        }
+    }
+    
     func loadInitial() async {
         guard state == .notLoadedInitial || state == .idle else {
             return
@@ -369,3 +383,17 @@ enum TimelineGapDirection {
         }
     }
 }
+
+// I would love to be able to do this with @Observable, but it's not clear how to do so.
+@propertyWrapper
+private class AsyncObservable<Value>: ObservableObject {
+    @Published var wrappedValue: Value
+    
+    var projectedValue: AsyncPublisher<Published<Value>.Publisher> {
+        $wrappedValue.values
+    }
+    
+    init(wrappedValue: Value) {
+        self.wrappedValue = wrappedValue
+    }
+}