CmdPal: Prevent unsynchornized access to More commands (#46020)

## Summary of the Pull Request

This PR fixes a crash caused by unsynchronized access to a context menu:

- Fixes `System.ArgumentException: Destination array was not long
enough` crash in `CommandItemViewModel.AllCommands` caused by
`List<T>.AddRange()` racing with background `BuildAndInitMoreCommands`
mutations
- Replaces mutable `List<T>` public surfaces with immutable array
snapshots protected by a `Lock`; writers hold the lock, mutate the
backing list, then atomically publish new snapshots via `volatile`
fields that readers access lock-free
- Applies the same snapshot pattern to `ContentPageViewModel`, using a
bundled `CommandSnapshot` object for atomic publication (since
`PrimaryCommand` drives command invocation there, not just UI hints)
- Narrows `IContextMenuContext.MoreCommands` and `AllCommands` from
`List<T>`/`IEnumerable<T>` to `IReadOnlyList<T>` to prevent consumers
from casting back and mutating
- Moves `SafeCleanup()` calls outside locks in cleanup paths to avoid
holding the lock during cross-process RPC calls

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] Closes: #45975 
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/CommandBarViewModel.cs

@@ -122,11 +122,9 @@ public sealed partial class CommandBarViewModel : ObservableObject,
         }
 
         SecondaryCommand = SelectedItem.SecondaryCommand;
+        var moreCommands = SelectedItem.MoreCommands;
 
-        var hasMoreThanOneContextItem = SelectedItem.MoreCommands.Count() > 1;
+        ShouldShowContextMenu = moreCommands.Count > 1 && SelectedItem.HasMoreCommands;
-        var hasMoreThanOneCommand = SelectedItem.MoreCommands.OfType<CommandContextItemViewModel>().Any();
-
-        ShouldShowContextMenu = hasMoreThanOneContextItem && hasMoreThanOneCommand;
 
         OnPropertyChanged(nameof(HasSecondaryCommand));
         OnPropertyChanged(nameof(SecondaryCommand));

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/CommandItemViewModel.cs

@@ -21,6 +21,12 @@ public partial class CommandItemViewModel : ExtensionObjectViewModel, ICommandBa
 
     private readonly IContextMenuFactory? _contextMenuFactory;
 
+    private readonly Lock _moreCommandsLock = new();
+    private readonly List<IContextItemViewModel> _moreCommands = [];
+    private volatile CommandContextItemViewModel? _secondaryMoreCommand;
+    private volatile IContextItemViewModel[] _moreCommandsSnapshot = [];
+    private volatile IContextItemViewModel[] _allCommandsSnapshot = [];
+
     private ExtensionObject<IExtendedAttributesProvider>? ExtendedAttributesProvider { get; set; }
 
     private readonly ExtensionObject<ICommandItem> _commandItemModel = new(null);
@@ -63,33 +69,22 @@ public partial class CommandItemViewModel : ExtensionObjectViewModel, ICommandBa
 
     public CommandViewModel Command { get; private set; }
 
-    public List<IContextItemViewModel> MoreCommands { get; private set; } = [];
+    // Reuse a cached read-only snapshot so repeated reads don't allocate.
+    public IReadOnlyList<IContextItemViewModel> MoreCommands => _moreCommandsSnapshot;
 
-    IEnumerable<IContextItemViewModel> IContextMenuContext.MoreCommands => MoreCommands;
+    IReadOnlyList<IContextItemViewModel> IContextMenuContext.MoreCommands => _moreCommandsSnapshot;
 
-    private List<CommandContextItemViewModel> ActualCommands => MoreCommands.OfType<CommandContextItemViewModel>().ToList();
+    protected Lock MoreCommandsLock => _moreCommandsLock;
 
-    public bool HasMoreCommands => ActualCommands.Count > 0;
+    protected List<IContextItemViewModel> UnsafeMoreCommands => _moreCommands;
 
-    public string SecondaryCommandName => SecondaryCommand?.Name ?? string.Empty;
+    public bool HasMoreCommands => _secondaryMoreCommand is not null;
+
+    public string SecondaryCommandName => _secondaryMoreCommand?.Name ?? string.Empty;
 
     public CommandItemViewModel? PrimaryCommand => this;
 
-    public CommandItemViewModel? SecondaryCommand
+    public CommandItemViewModel? SecondaryCommand => _secondaryMoreCommand;
-    {
-        get
-        {
-            if (HasMoreCommands)
-            {
-                if (MoreCommands[0] is CommandContextItemViewModel command)
-                {
-                    return command;
-                }
-            }
-
-            return null;
-        }
-    }
 
     public bool ShouldBeVisible => !string.IsNullOrEmpty(Name);
 
@@ -101,18 +96,7 @@ public partial class CommandItemViewModel : ExtensionObjectViewModel, ICommandBa
 
     public DataPackageView? DataPackage { get; private set; }
 
-    public List<IContextItemViewModel> AllCommands
+    public IReadOnlyList<IContextItemViewModel> AllCommands => _allCommandsSnapshot;
-    {
-        get
-        {
-            List<IContextItemViewModel> l = _defaultCommandContextItemViewModel is null ?
-                new() :
-                [_defaultCommandContextItemViewModel];
-
-            l.AddRange(MoreCommands);
-            return l;
-        }
-    }
 
     private static readonly IconInfoViewModel _errorIcon;
 
@@ -246,6 +230,11 @@ public partial class CommandItemViewModel : ExtensionObjectViewModel, ICommandBa
             UpdateDefaultContextItemIcon();
         }
 
+        lock (_moreCommandsLock)
+        {
+            RefreshMoreCommandStateUnsafe();
+        }
+
         Initialized |= InitializedState.SelectionInitialized;
         UpdateProperty(nameof(MoreCommands));
         UpdateProperty(nameof(AllCommands));
@@ -265,7 +254,7 @@ public partial class CommandItemViewModel : ExtensionObjectViewModel, ICommandBa
             Command = new(null, PageContext);
             _itemTitle = "Error";
             Subtitle = "Item failed to load";
-            MoreCommands = [];
+            ClearMoreCommands();
             _icon = _errorIcon;
             _titleCache.Invalidate();
             _subtitleCache.Invalidate();
@@ -304,7 +293,7 @@ public partial class CommandItemViewModel : ExtensionObjectViewModel, ICommandBa
             Command = new(null, PageContext);
             _itemTitle = "Error";
             Subtitle = "Item failed to load";
-            MoreCommands = [];
+            ClearMoreCommands();
             _icon = _errorIcon;
             _titleCache.Invalidate();
             _subtitleCache.Invalidate();
@@ -385,9 +374,7 @@ public partial class CommandItemViewModel : ExtensionObjectViewModel, ICommandBa
 
             case nameof(model.MoreCommands):
                 BuildAndInitMoreCommands();
-                UpdateProperty(nameof(SecondaryCommand));
+                UpdateProperty(nameof(SecondaryCommand), nameof(SecondaryCommandName), nameof(HasMoreCommands), nameof(AllCommands));
-                UpdateProperty(nameof(SecondaryCommandName));
-                UpdateProperty(nameof(HasMoreCommands));
 
                 break;
             case nameof(DataPackage):
@@ -478,9 +465,10 @@ public partial class CommandItemViewModel : ExtensionObjectViewModel, ICommandBa
         var results = factory.UnsafeBuildAndInitMoreCommands(more, this);
 
         List<IContextItemViewModel>? freedItems;
-        lock (MoreCommands)
+        lock (_moreCommandsLock)
         {
-            ListHelpers.InPlaceUpdateList(MoreCommands, results, out freedItems);
+            ListHelpers.InPlaceUpdateList(_moreCommands, results, out freedItems);
+            RefreshMoreCommandStateUnsafe();
         }
 
         freedItems.OfType<CommandContextItemViewModel>()
@@ -516,20 +504,30 @@ public partial class CommandItemViewModel : ExtensionObjectViewModel, ICommandBa
     {
         base.UnsafeCleanup();
 
-        lock (MoreCommands)
+        List<IContextItemViewModel> freedItems;
+        CommandContextItemViewModel? freedDefault;
+        lock (_moreCommandsLock)
         {
-            MoreCommands.OfType<CommandContextItemViewModel>()
+            freedItems = [.. _moreCommands];
-                        .ToList()
+            _moreCommands.Clear();
-                        .ForEach(c => c.SafeCleanup());
+
-            MoreCommands.Clear();
+            // Null out here so the single RefreshMoreCommandStateUnsafe call
+            // produces an _allCommandsSnapshot that excludes the default command.
+            freedDefault = _defaultCommandContextItemViewModel;
+            _defaultCommandContextItemViewModel = null;
+
+            RefreshMoreCommandStateUnsafe();
         }
 
+        // Cleanup outside lock to avoid holding it during RPC calls
+        freedItems.OfType<CommandContextItemViewModel>()
+                  .ToList()
+                  .ForEach(c => c.SafeCleanup());
+        freedDefault?.SafeCleanup();
+
         // _listItemIcon.SafeCleanup();
         _icon = new(null); // necessary?
 
-        _defaultCommandContextItemViewModel?.SafeCleanup();
-        _defaultCommandContextItemViewModel = null;
-
         Command.PropertyChanged -= Command_PropertyChanged;
         Command.SafeCleanup();
 
@@ -545,6 +543,40 @@ public partial class CommandItemViewModel : ExtensionObjectViewModel, ICommandBa
         base.SafeCleanup();
         Initialized |= InitializedState.CleanedUp;
     }
+
+    protected void RefreshMoreCommandStateUnsafe()
+    {
+        _moreCommandsSnapshot = [.. _moreCommands];
+
+        _secondaryMoreCommand = null;
+        foreach (var item in _moreCommands)
+        {
+            if (item is CommandContextItemViewModel command)
+            {
+                _secondaryMoreCommand = command;
+                break;
+            }
+        }
+
+        _allCommandsSnapshot = _defaultCommandContextItemViewModel is null ?
+            _moreCommandsSnapshot :
+            [_defaultCommandContextItemViewModel, .. _moreCommandsSnapshot];
+    }
+
+    private void ClearMoreCommands()
+    {
+        List<IContextItemViewModel> freedItems;
+        lock (_moreCommandsLock)
+        {
+            freedItems = [.. _moreCommands];
+            _moreCommands.Clear();
+            RefreshMoreCommandStateUnsafe();
+        }
+
+        freedItems.OfType<CommandContextItemViewModel>()
+                  .ToList()
+                  .ForEach(c => c.SafeCleanup());
+    }
 }
 
 [Flags]

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ContentPageViewModel.cs

@@ -17,13 +17,15 @@ namespace Microsoft.CmdPal.UI.ViewModels;
 public partial class ContentPageViewModel : PageViewModel, ICommandBarContext
 {
     private readonly ExtensionObject<IContentPage> _model;
+    private readonly Lock _commandsLock = new();
+    private volatile CommandSnapshot _snapshot = CommandSnapshot.Empty;
 
     [ObservableProperty]
     public partial ObservableCollection<ContentViewModel> Content { get; set; } = [];
 
-    public List<IContextItemViewModel> Commands { get; private set; } = [];
+    private List<IContextItemViewModel> Commands { get; } = [];
 
-    public bool HasCommands => ActualCommands.Count > 0;
+    public bool HasCommands => _snapshot.PrimaryCommand is not null;
 
     public DetailsViewModel? Details { get; private set; }
 
@@ -31,19 +33,17 @@ public partial class ContentPageViewModel : PageViewModel, ICommandBarContext
     public bool HasDetails => Details is not null;
 
     /////// ICommandBarContext ///////
-    public IEnumerable<IContextItemViewModel> MoreCommands => Commands.Skip(1);
+    public IReadOnlyList<IContextItemViewModel> MoreCommands => _snapshot.MoreCommands;
 
-    private List<CommandContextItemViewModel> ActualCommands => Commands.OfType<CommandContextItemViewModel>().ToList();
+    public bool HasMoreCommands => _snapshot.SecondaryCommand is not null;
 
-    public bool HasMoreCommands => ActualCommands.Count > 1;
+    public string SecondaryCommandName => _snapshot.SecondaryCommand?.Name ?? string.Empty;
 
-    public string SecondaryCommandName => SecondaryCommand?.Name ?? string.Empty;
+    public CommandItemViewModel? PrimaryCommand => _snapshot.PrimaryCommand;
 
-    public CommandItemViewModel? PrimaryCommand => HasCommands ? ActualCommands[0] : null;
+    public CommandItemViewModel? SecondaryCommand => _snapshot.SecondaryCommand;
 
-    public CommandItemViewModel? SecondaryCommand => HasMoreCommands ? ActualCommands[1] : null;
+    public IReadOnlyList<IContextItemViewModel> AllCommands => _snapshot.AllCommands;
-
-    public List<IContextItemViewModel> AllCommands => Commands;
 
     // Remember - "observable" properties from the model (via PropChanged)
     // cannot be marked [ObservableProperty]
@@ -109,28 +109,14 @@ public partial class ContentPageViewModel : PageViewModel, ICommandBarContext
             return; // throw?
         }
 
-        Commands = model.Commands
+        var commands = BuildCommandViewModels(model.Commands);
-                .ToList()
+        InitializeCommandViewModels(commands, static contextItem => contextItem.InitializeProperties());
-                .Select<IContextItem, IContextItemViewModel>(item =>
-                {
-                    if (item is ICommandContextItem contextItem)
-                    {
-                        return new CommandContextItemViewModel(contextItem, PageContext);
-                    }
-                    else
-                    {
-                        return new SeparatorViewModel();
-                    }
-                })
-                .ToList();
 
-        Commands
+        lock (_commandsLock)
-            .OfType<CommandContextItemViewModel>()
+        {
-            .ToList()
+            ListHelpers.InPlaceUpdateList(Commands, commands);
-            .ForEach(contextItem =>
+            RefreshCommandSnapshotsUnsafe();
-            {
+        }
-                contextItem.InitializeProperties();
-            });
 
         var extensionDetails = model.Details;
         if (extensionDetails is not null)
@@ -168,37 +154,29 @@ public partial class ContentPageViewModel : PageViewModel, ICommandBarContext
                 var more = model.Commands;
                 if (more is not null)
                 {
-                    var newContextMenu = more
+                    var newContextMenu = BuildCommandViewModels(more);
-                            .ToList()
+                    InitializeCommandViewModels(newContextMenu, static contextItem => contextItem.SlowInitializeProperties());
-                            .Select(item =>
-                            {
-                                if (item is ICommandContextItem contextItem)
-                                {
-                                    return new CommandContextItemViewModel(contextItem, PageContext) as IContextItemViewModel;
-                                }
-                                else
-                                {
-                                    return new SeparatorViewModel();
-                                }
-                            })
-                            .ToList();
 
-                    lock (Commands)
+                    List<IContextItemViewModel> removedItems;
+                    lock (_commandsLock)
                     {
-                        ListHelpers.InPlaceUpdateList(Commands, newContextMenu);
+                        ListHelpers.InPlaceUpdateList(Commands, newContextMenu, out removedItems);
+                        RefreshCommandSnapshotsUnsafe();
                     }
 
-                    Commands
+                    CleanupCommandViewModels(removedItems);
-                        .OfType<CommandContextItemViewModel>()
-                        .ToList()
-                        .ForEach(contextItem =>
-                        {
-                            contextItem.SlowInitializeProperties();
-                        });
                 }
                 else
                 {
-                    Commands.Clear();
+                    List<IContextItemViewModel> removedItems;
+                    lock (_commandsLock)
+                    {
+                        removedItems = [.. Commands];
+                        Commands.Clear();
+                        RefreshCommandSnapshotsUnsafe();
+                    }
+
+                    CleanupCommandViewModels(removedItems);
                 }
 
                 UpdateProperty(nameof(PrimaryCommand));
@@ -206,6 +184,7 @@ public partial class ContentPageViewModel : PageViewModel, ICommandBarContext
                 UpdateProperty(nameof(SecondaryCommandName));
                 UpdateProperty(nameof(HasCommands));
                 UpdateProperty(nameof(HasMoreCommands));
+                UpdateProperty(nameof(MoreCommands));
                 UpdateProperty(nameof(AllCommands));
                 DoOnUiThread(
                 () =>
@@ -243,6 +222,72 @@ public partial class ContentPageViewModel : PageViewModel, ICommandBarContext
             });
     }
 
+    private List<IContextItemViewModel> BuildCommandViewModels(IContextItem[]? items)
+    {
+        if (items is null)
+        {
+            return [];
+        }
+
+        return items
+            .Select<IContextItem, IContextItemViewModel>(item =>
+            {
+                if (item is ICommandContextItem contextItem)
+                {
+                    return new CommandContextItemViewModel(contextItem, PageContext);
+                }
+
+                return new SeparatorViewModel();
+            })
+            .ToList();
+    }
+
+    private static void InitializeCommandViewModels(IEnumerable<IContextItemViewModel> commands, Action<CommandContextItemViewModel> initialize)
+    {
+        foreach (var contextItem in commands.OfType<CommandContextItemViewModel>())
+        {
+            initialize(contextItem);
+        }
+    }
+
+    private static void CleanupCommandViewModels(IEnumerable<IContextItemViewModel> commands)
+    {
+        foreach (var contextItem in commands.OfType<CommandContextItemViewModel>())
+        {
+            contextItem.SafeCleanup();
+        }
+    }
+
+    private void RefreshCommandSnapshotsUnsafe()
+    {
+        var allCommands = (IContextItemViewModel[])[.. Commands];
+        var moreCommands = allCommands.Length > 1
+            ? allCommands[1..]
+            : [];
+
+        CommandContextItemViewModel? primary = null;
+        CommandContextItemViewModel? secondary = null;
+        foreach (var item in allCommands)
+        {
+            if (item is not CommandContextItemViewModel command)
+            {
+                continue;
+            }
+
+            if (primary is null)
+            {
+                primary = command;
+            }
+            else if (secondary is null)
+            {
+                secondary = command;
+                break;
+            }
+        }
+
+        _snapshot = new(allCommands, moreCommands, primary, secondary);
+    }
+
     // InvokeItemCommand is what this will be in Xaml due to source generator
     // this comes in on Enter keypresses in the SearchBox
     [RelayCommand]
@@ -270,12 +315,15 @@ public partial class ContentPageViewModel : PageViewModel, ICommandBarContext
 
         Details?.SafeCleanup();
 
-        Commands
+        List<IContextItemViewModel> removedItems;
-            .OfType<CommandContextItemViewModel>()
+        lock (_commandsLock)
-            .ToList()
+        {
-            .ForEach(item => item.SafeCleanup());
+            removedItems = [.. Commands];
+            Commands.Clear();
+            RefreshCommandSnapshotsUnsafe();
+        }
 
-        Commands.Clear();
+        CleanupCommandViewModels(removedItems);
 
         foreach (var item in Content)
         {
@@ -290,4 +338,25 @@ public partial class ContentPageViewModel : PageViewModel, ICommandBarContext
             model.ItemsChanged -= Model_ItemsChanged;
         }
     }
+
+    /// <summary>
+    /// Immutable bundle of derived command state, published atomically via a
+    /// single volatile write so readers never see a torn snapshot.
+    /// </summary>
+    private sealed class CommandSnapshot(
+        IContextItemViewModel[] allCommands,
+        IContextItemViewModel[] moreCommands,
+        CommandContextItemViewModel? primaryCommand,
+        CommandContextItemViewModel? secondaryCommand)
+    {
+        public static CommandSnapshot Empty { get; } = new([], [], null, null);
+
+        public IContextItemViewModel[] AllCommands { get; } = allCommands;
+
+        public IContextItemViewModel[] MoreCommands { get; } = moreCommands;
+
+        public CommandContextItemViewModel? PrimaryCommand { get; } = primaryCommand;
+
+        public CommandContextItemViewModel? SecondaryCommand { get; } = secondaryCommand;
+    }
 }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ListItemViewModel.cs

@@ -159,7 +159,6 @@ public partial class ListItemViewModel : CommandItemViewModel
                 UpdateShowDetailsCommand();
                 break;
             case nameof(model.MoreCommands):
-                UpdateProperty(nameof(MoreCommands));
                 AddShowDetailsCommands();
                 break;
             case nameof(model.Title):
@@ -195,19 +194,27 @@ public partial class ListItemViewModel : CommandItemViewModel
             pageContext is ListViewModel listViewModel &&
             !listViewModel.ShowDetails)
         {
-            // Check if "Show Details" action already exists to prevent duplicates
+            var addedCommand = false;
-            if (!MoreCommands.Any(cmd => cmd is CommandContextItemViewModel contextItemViewModel &&
+            lock (MoreCommandsLock)
-                                        contextItemViewModel.Command.Id == ShowDetailsCommand.ShowDetailsCommandId))
             {
-                // Create the view model for the show details command
+                // Check if "Show Details" action already exists to prevent duplicates
-                var showDetailsCommand = new ShowDetailsCommand(Details);
+                if (!UnsafeMoreCommands.Any(cmd => cmd is CommandContextItemViewModel contextItemViewModel &&
-                var showDetailsContextItem = new CommandContextItem(showDetailsCommand);
+                                                  contextItemViewModel.Command.Id == ShowDetailsCommand.ShowDetailsCommandId))
-                var showDetailsContextItemViewModel = new CommandContextItemViewModel(showDetailsContextItem, PageContext);
+                {
-                showDetailsContextItemViewModel.SlowInitializeProperties();
+                    var showDetailsCommand = new ShowDetailsCommand(Details);
-                MoreCommands.Add(showDetailsContextItemViewModel);
+                    var showDetailsContextItem = new CommandContextItem(showDetailsCommand);
+                    var showDetailsContextItemViewModel = new CommandContextItemViewModel(showDetailsContextItem, PageContext);
+                    showDetailsContextItemViewModel.SlowInitializeProperties();
+                    UnsafeMoreCommands.Add(showDetailsContextItemViewModel);
+                    RefreshMoreCommandStateUnsafe();
+                    addedCommand = true;
+                }
             }
 
-            UpdateProperty(nameof(MoreCommands), nameof(AllCommands));
+            if (addedCommand)
+            {
+                UpdateProperty(nameof(MoreCommands), nameof(AllCommands));
+            }
         }
     }
 
@@ -222,22 +229,27 @@ public partial class ListItemViewModel : CommandItemViewModel
             pageContext is ListViewModel listViewModel &&
             !listViewModel.ShowDetails)
         {
-            var existingCommand = MoreCommands.FirstOrDefault(cmd =>
+            CommandContextItemViewModel? oldCommand = null;
-                                        cmd is CommandContextItemViewModel contextItemViewModel &&
+            lock (MoreCommandsLock)
-                                        contextItemViewModel.Command.Id == ShowDetailsCommand.ShowDetailsCommandId);
-
-            // If the command already exists, remove it to update with the new details
-            if (existingCommand is not null)
             {
-                MoreCommands.Remove(existingCommand);
+                oldCommand = UnsafeMoreCommands
+                    .OfType<CommandContextItemViewModel>()
+                    .FirstOrDefault(contextItemViewModel => contextItemViewModel.Command.Id == ShowDetailsCommand.ShowDetailsCommandId);
+
+                if (oldCommand is not null)
+                {
+                    UnsafeMoreCommands.Remove(oldCommand);
+                }
+
+                var showDetailsCommand = new ShowDetailsCommand(Details);
+                var showDetailsContextItem = new CommandContextItem(showDetailsCommand);
+                var showDetailsContextItemViewModel = new CommandContextItemViewModel(showDetailsContextItem, PageContext);
+                showDetailsContextItemViewModel.SlowInitializeProperties();
+                UnsafeMoreCommands.Add(showDetailsContextItemViewModel);
+                RefreshMoreCommandStateUnsafe();
             }
 
-            // Create the view model for the show details command
+            oldCommand?.SafeCleanup();
-            var showDetailsCommand = new ShowDetailsCommand(Details);
-            var showDetailsContextItem = new CommandContextItem(showDetailsCommand);
-            var showDetailsContextItemViewModel = new CommandContextItemViewModel(showDetailsContextItem, PageContext);
-            showDetailsContextItemViewModel.SlowInitializeProperties();
-            MoreCommands.Add(showDetailsContextItemViewModel);
 
             UpdateProperty(nameof(MoreCommands), nameof(AllCommands));
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Messages/UpdateCommandBarMessage.cs

@@ -18,11 +18,11 @@ public record UpdateCommandBarMessage(ICommandBarContext? ViewModel)
 
 public interface IContextMenuContext : INotifyPropertyChanged
 {
-    public IEnumerable<IContextItemViewModel> MoreCommands { get; }
+    public IReadOnlyList<IContextItemViewModel> MoreCommands { get; }
 
     public bool HasMoreCommands { get; }
 
-    public List<IContextItemViewModel> AllCommands { get; }
+    public IReadOnlyList<IContextItemViewModel> AllCommands { get; }
 
     /// <summary>
     /// Generates a mapping of key -> command item for this particular item's

src/modules/cmdpal/Tests/Microsoft.CmdPal.UI.ViewModels.UnitTests/CommandItemViewModelTests.cs

@@ -0,0 +1,79 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Threading.Tasks;
+using Microsoft.CmdPal.UI.ViewModels.Models;
+using Microsoft.CommandPalette.Extensions.Toolkit;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace Microsoft.CmdPal.UI.ViewModels.UnitTests;
+
+[TestClass]
+public class CommandItemViewModelTests
+{
+    private sealed class TestPageContext : IPageContext
+    {
+        public TaskScheduler Scheduler => TaskScheduler.Default;
+
+        public ICommandProviderContext ProviderContext => CommandProviderContext.Empty;
+
+        public void ShowException(Exception ex, string? extensionHint = null)
+        {
+            throw new AssertFailedException($"Unexpected exception from view model: {ex}");
+        }
+    }
+
+    [TestMethod]
+    public void MoreCommandsAndAllCommands_ReturnSnapshots()
+    {
+        // The public getters should return cached read-only snapshots, so
+        // repeated reads don't allocate a new list when the backing data hasn't
+        // changed.
+        var pageContext = new TestPageContext();
+        var item = new CommandItem(new NoOpCommand { Name = "Primary" })
+        {
+            Title = "Primary",
+            MoreCommands =
+            [
+                new CommandContextItem(new NoOpCommand { Name = "Secondary" }),
+            ],
+        };
+
+        var viewModel = new CommandItemViewModel(new(item), new(pageContext), DefaultContextMenuFactory.Instance);
+        viewModel.SlowInitializeProperties();
+
+        var moreCommands = viewModel.MoreCommands;
+        var allCommands = viewModel.AllCommands;
+
+        Assert.AreSame(moreCommands, viewModel.MoreCommands);
+        Assert.AreSame(allCommands, viewModel.AllCommands);
+        Assert.AreEqual(1, moreCommands.Count);
+        Assert.AreEqual(2, allCommands.Count);
+    }
+
+    [TestMethod]
+    public void SecondaryCommand_IgnoresLeadingSeparators()
+    {
+        // SecondaryCommand/HasMoreCommands should be derived from the first actual command item,
+        // not from the raw first entry in MoreCommands.
+        var pageContext = new TestPageContext();
+        var item = new CommandItem(new NoOpCommand { Name = "Primary" })
+        {
+            Title = "Primary",
+            MoreCommands =
+            [
+                new Separator("Group"),
+                new CommandContextItem(new NoOpCommand { Name = "Secondary" }),
+            ],
+        };
+
+        var viewModel = new CommandItemViewModel(new(item), new(pageContext), DefaultContextMenuFactory.Instance);
+        viewModel.SlowInitializeProperties();
+
+        Assert.IsTrue(viewModel.HasMoreCommands);
+        Assert.IsNotNull(viewModel.SecondaryCommand);
+        Assert.AreEqual("Secondary", viewModel.SecondaryCommand.Name);
+    }
+}

src/modules/cmdpal/Tests/Microsoft.CmdPal.UI.ViewModels.UnitTests/ContentPageViewModelTests.cs

@@ -0,0 +1,104 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Threading.Tasks;
+using Microsoft.CommandPalette.Extensions;
+using Microsoft.CommandPalette.Extensions.Toolkit;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace Microsoft.CmdPal.UI.ViewModels.UnitTests;
+
+[TestClass]
+public partial class ContentPageViewModelTests
+{
+    private sealed partial class TestAppExtensionHost : AppExtensionHost
+    {
+        public override string? GetExtensionDisplayName() => "Test Host";
+    }
+
+    private sealed partial class TestContentPage : ContentPage
+    {
+        public override IContent[] GetContent() => [];
+    }
+
+    private static CommandContextItem Command(string name) => new(new NoOpCommand { Name = name });
+
+    private static ContentPageViewModel CreateViewModel(TestContentPage page) =>
+        new(page, TaskScheduler.Default, new TestAppExtensionHost(), CommandProviderContext.Empty);
+
+    [TestMethod]
+    public void AllCommandsAndMoreCommands_ReturnCachedSnapshots()
+    {
+        // Content pages should expose stable snapshots, not the live Commands
+        // list, so repeated reads don't allocate and callers can't observe
+        // in-place list mutations.
+        var page = new TestContentPage
+        {
+            Id = "content.page",
+            Name = "Content Page",
+            Title = "Content Page",
+            Commands =
+            [
+                Command("Primary"),
+                Command("Secondary"),
+            ],
+        };
+
+        var viewModel = CreateViewModel(page);
+        viewModel.InitializeProperties();
+
+        var allCommands = viewModel.AllCommands;
+        var moreCommands = viewModel.MoreCommands;
+
+        Assert.AreSame(allCommands, viewModel.AllCommands);
+        Assert.AreSame(moreCommands, viewModel.MoreCommands);
+        Assert.AreEqual(2, allCommands.Count);
+        Assert.AreEqual(1, moreCommands.Count);
+        Assert.AreEqual("Primary", viewModel.PrimaryCommand?.Name);
+        Assert.AreEqual("Secondary", viewModel.SecondaryCommand?.Name);
+    }
+
+    [TestMethod]
+    public void CommandsUpdate_RefreshesSnapshotsConsistently()
+    {
+        // Updating the model commands should swap in a new coherent snapshot.
+        // The old snapshots stay intact, and the new cached values agree on
+        // counts, primary/secondary commands, and "has more" state.
+        var page = new TestContentPage
+        {
+            Id = "content.page",
+            Name = "Content Page",
+            Title = "Content Page",
+            Commands =
+            [
+                Command("Primary"),
+                Command("Secondary"),
+            ],
+        };
+
+        var viewModel = CreateViewModel(page);
+        viewModel.InitializeProperties();
+
+        var oldAllCommands = viewModel.AllCommands;
+        var oldMoreCommands = viewModel.MoreCommands;
+
+        page.Commands =
+        [
+            Command("Updated Primary"),
+            new Separator("Group"),
+            Command("Updated Secondary"),
+        ];
+
+        Assert.AreEqual(2, oldAllCommands.Count);
+        Assert.AreEqual(1, oldMoreCommands.Count);
+
+        Assert.AreEqual(3, viewModel.AllCommands.Count);
+        Assert.AreEqual(2, viewModel.MoreCommands.Count);
+        Assert.IsTrue(viewModel.HasCommands);
+        Assert.IsTrue(viewModel.HasMoreCommands);
+        Assert.AreEqual("Updated Primary", viewModel.PrimaryCommand?.Name);
+        Assert.AreEqual("Updated Secondary", viewModel.SecondaryCommand?.Name);
+        Assert.AreEqual("Updated Secondary", viewModel.SecondaryCommandName);
+    }
+}