fix(save-attachments): marshal folder-tree mutations to the Dispatcher

v1.2.15's tree picker built the whole tree inside an async method and
mutated the TreeView-bound ObservableCollections after an await. In this
VSTO host the WPF SynchronizationContext is not installed, so
ConfigureAwait(true) resumed on a thread-pool thread and FolderTree.Add
threw "changes to its SourceCollection from a thread different from the
Dispatcher thread" — the exception was swallowed and the tree showed
empty (no folders selectable at all).

Capture the UI Dispatcher in the view-model and marshal every bound
collection mutation (FolderTree + each node's Children) through it, so
the tree populates regardless of which thread the network await resumes
on.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/OutlookAddin.UI/ViewModels/FolderNodeViewModel.cs

@@ -2,6 +2,7 @@ using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Threading.Tasks;
+using System.Windows.Threading;
 using CommunityToolkit.Mvvm.ComponentModel;
 using MarcusLaw.OutlookAddin.Core.Services;
 using Serilog;
@@ -18,6 +19,7 @@ namespace MarcusLaw.OutlookAddin.UI.ViewModels
     {
         private readonly IEspoCrmClient? _client;
         private readonly ILogger? _logger;
+        private readonly Dispatcher? _dispatcher;
         private bool _childrenLoaded;
         private bool _loadingStarted;
 
@@ -58,12 +60,14 @@ namespace MarcusLaw.OutlookAddin.UI.ViewModels
         public FolderNodeViewModel(
             IEspoCrmClient client,
             ILogger logger,
+            Dispatcher dispatcher,
             string name,
             string fullPath,
             string? relativePath)
         {
             _client = client ?? throw new ArgumentNullException(nameof(client));
             _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+            _dispatcher = dispatcher ?? throw new ArgumentNullException(nameof(dispatcher));
             Name = name;
             FullPath = fullPath;
             RelativePath = relativePath;
@@ -132,9 +136,16 @@ namespace MarcusLaw.OutlookAddin.UI.ViewModels
                 _logger?.Warning(ex, "Listing subfolders under '{Path}' failed", FullPath);
             }
             finally
+            {
+                // Marshal onto the Dispatcher: once this node's TreeViewItem
+                // exists, Children is bound and must only be mutated on the UI
+                // thread (the network await above can resume on a thread-pool
+                // thread in this VSTO host).
+                OnUi(() =>
                 {
                     Children.Clear();
                     foreach (var child in loaded) Children.Add(child);
+                });
                 _childrenLoaded = true;
                 IsLoading = false;
             }
@@ -144,7 +155,14 @@ namespace MarcusLaw.OutlookAddin.UI.ViewModels
         {
             var fullPath = string.IsNullOrEmpty(FullPath) ? name : FullPath + "/" + name;
             var relativePath = IsRoot ? name : RelativePath + "/" + name;
-            return new FolderNodeViewModel(_client!, _logger!, name, fullPath, relativePath);
+            return new FolderNodeViewModel(_client!, _logger!, _dispatcher!, name, fullPath, relativePath);
+        }
+
+        private void OnUi(Action action)
+        {
+            var d = _dispatcher;
+            if (d == null || d.CheckAccess()) action();
+            else d.Invoke(action);
         }
     }
 }

src/OutlookAddin.UI/ViewModels/SaveAttachmentsViewModel.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Threading;
 using System.Threading.Tasks;
+using System.Windows.Threading;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 using MarcusLaw.OutlookAddin.Core.Models;
@@ -31,6 +32,7 @@ namespace MarcusLaw.OutlookAddin.UI.ViewModels
 
         private readonly IEspoCrmClient _client;
         private readonly ILogger _logger;
+        private readonly Dispatcher _dispatcher;
         private CancellationTokenSource? _searchCts;
 
         [ObservableProperty]
@@ -109,6 +111,13 @@ namespace MarcusLaw.OutlookAddin.UI.ViewModels
         {
             _client = client ?? throw new ArgumentNullException(nameof(client));
             _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+            // Captured on the UI thread (the VM is constructed on the thread
+            // that shows the dialog). Used to marshal ObservableCollection
+            // mutations back onto the Dispatcher — in this VSTO host the WPF
+            // SynchronizationContext is not installed, so ConfigureAwait(true)
+            // resumes on a thread-pool thread and a bound-collection Add throws
+            // "changes ... from a thread different from the Dispatcher thread".
+            _dispatcher = Dispatcher.CurrentDispatcher;
 
             ResetFolderTree();
 
@@ -150,10 +159,20 @@ namespace MarcusLaw.OutlookAddin.UI.ViewModels
             }
         }
 
+        /// <summary>Runs <paramref name="action"/> on the UI/Dispatcher thread.</summary>
+        private void OnUi(Action action)
+        {
+            if (_dispatcher.CheckAccess()) action();
+            else _dispatcher.Invoke(action);
+        }
+
         private void ResetFolderTree()
+        {
+            OnUi(() =>
             {
                 FolderTree.Clear();
                 SelectedFolder = null;
+            });
         }
 
         partial void OnSearchTextChanged(string value)
@@ -234,10 +253,13 @@ namespace MarcusLaw.OutlookAddin.UI.ViewModels
             // default), expanded, and seeded with the default subfolders so
             // they appear even before the share listing returns. Deeper levels
             // load lazily as the user expands them.
-            var root = new FolderNodeViewModel(_client, _logger, RootSentinel, folderPath, relativePath: null);
+            var root = new FolderNodeViewModel(_client, _logger, _dispatcher, RootSentinel, folderPath, relativePath: null);
+            OnUi(() =>
+            {
                 FolderTree.Add(root);
                 SelectedFolder = root;
                 root.IsSelected = true;
+            });
 
             try
             {
@@ -247,7 +269,7 @@ namespace MarcusLaw.OutlookAddin.UI.ViewModels
             {
                 _logger.Warning(ex, "Listing subfolders for case {CaseId} failed", c.Id);
             }
-            root.IsExpanded = true;
+            OnUi(() => root.IsExpanded = true);
         }
 
         /// <summary>

src/OutlookAddin/Properties/AssemblyInfo.cs

@@ -33,6 +33,6 @@ using System.Security;
 // You can specify all the values or you can default the Build and Revision Numbers 
 // by using the '*' as shown below:
 // [assembly: AssemblyVersion("1.0.*")]
-[assembly: AssemblyVersion("1.2.15.0")]
-[assembly: AssemblyFileVersion("1.2.15.0")]
+[assembly: AssemblyVersion("1.2.16.0")]
+[assembly: AssemblyFileVersion("1.2.16.0")]