diff --git a/Sources/SaplingApp/SaplingApp.swift b/Sources/SaplingApp/SaplingApp.swift
index 06cc216..daa625d 100644
--- a/Sources/SaplingApp/SaplingApp.swift
+++ b/Sources/SaplingApp/SaplingApp.swift
@@ -164,19 +164,25 @@ private final class SaplingAppModel: ObservableObject {
         guard let url = item.containerURL?.standardizedFileURL else { return }
         guard workspaceChildren[url] == nil, !loadingTreeItemURLs.contains(url) else { return }
 
-        loadingTreeItemURLs.insert(url)
+        var loadingURLs = loadingTreeItemURLs
+        loadingURLs.insert(url)
+        loadingTreeItemURLs = loadingURLs
         let workspaceManager = self.workspaceManager
         Task {
             do {
                 let children = try await Task.detached {
                     try await workspaceManager.loadItems(in: url)
                 }.value
-                workspaceChildren[url] = children
+                var loadedChildren = workspaceChildren
+                loadedChildren[url] = children
+                workspaceChildren = loadedChildren
             } catch {
                 editorErrorMessage = "Unable to load \(item.displayName): \(error.localizedDescription)"
                 logger.error("Failed to load workspace tree item: \(error)", category: .workspace)
             }
-            loadingTreeItemURLs.remove(url)
+            var loadingURLs = loadingTreeItemURLs
+            loadingURLs.remove(url)
+            loadingTreeItemURLs = loadingURLs
         }
     }