Create SidebarViewController, a UICollectionViewController subclass, to replace the UITableViewController subclass previously in use. This will get us the modern sidebar appearance.

iOS/MainWindow/Sidebar/SidebarViewController.swift

@@ -0,0 +1,77 @@
+//
+//  SidebarViewController.swift
+//  NetNewsWire-iOS
+//
+//  Created by Brent Simmons on 2/2/25.
+//  Copyright © 2025 Ranchero Software. All rights reserved.
+//
+
+import Foundation
+import UIKit
+
+final class SidebarViewController: UICollectionViewController {
+
+	enum Section {
+		case smartFeeds
+	}
+
+	struct SidebarItem: Hashable, Identifiable {
+		let id: UUID = UUID()
+		let title: String
+		let icon: UIImage?
+	}
+
+	typealias DataSource = UICollectionViewDiffableDataSource<Section, SidebarViewController.SidebarItem>
+	private lazy var dataSource = createDataSource()
+
+	init() {
+		super.init(collectionViewLayout: Self.createSidebarLayout())
+	}
+
+	required init?(coder: NSCoder) {
+		fatalError("init(coder:) has not been implemented")
+	}
+
+	override func viewDidLoad() {
+
+		super.viewDidLoad()
+
+		applySnapshot()
+	}
+}
+
+private extension SidebarViewController {
+
+	static func createSidebarLayout() -> UICollectionViewLayout {
+		let configuration = UICollectionLayoutListConfiguration(appearance: .sidebar)
+		return UICollectionViewCompositionalLayout.list(using: configuration)
+	}
+
+	private func createDataSource() -> DataSource {
+		let cellRegistration = UICollectionView.CellRegistration<UICollectionViewListCell, SidebarItem> { (cell, indexPath, item) in
+			var content = UIListContentConfiguration.cell()
+			content.text = item.title
+			content.image = item.icon
+			cell.contentConfiguration = content
+		}
+
+		dataSource = UICollectionViewDiffableDataSource<Section, SidebarItem>(collectionView: collectionView) { (collectionView, indexPath, item) in
+			return collectionView.dequeueConfiguredReusableCell(using: cellRegistration, for: indexPath, item: item)
+		}
+
+		return dataSource
+	}
+
+	private func applySnapshot() {
+		var snapshot = NSDiffableDataSourceSnapshot<Section, SidebarItem>()
+
+		snapshot.appendSections([.smartFeeds])
+		snapshot.appendItems([
+			SidebarItem(title: "Today", icon: AppImage.today),
+			SidebarItem(title: "All Unread", icon: AppImage.allUnread),
+			SidebarItem(title: "Starred", icon: AppImage.starred)
+		])
+
+		dataSource.apply(snapshot, animatingDifferences: true)
+	}
+}