CmdPal: add locking TopLevelCommandManager.DockBands (#45898)

## Summary of the Pull Request

- Put enumeration of DockBands in
TopLevelCommandManager.ExtensionService_OnExtensionRemoved under correct
lock
- Return a snapshot of the dock bands list to prevent reading DockBands
without lock outside of TopLevelCommandManager

## PR Checklist

- [x] Closes: #45893

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Dock/DockViewModel.cs

@@ -29,7 +29,7 @@ public sealed partial class DockViewModel
 
     public ObservableCollection<DockBandViewModel> EndItems { get; } = new();
 
-    public ObservableCollection<TopLevelViewModel> AllItems => _topLevelCommandManager.DockBands;
+    public IReadOnlyList<TopLevelViewModel> AllItems => _topLevelCommandManager.GetDockBandsSnapshot();
 
     public DockViewModel(
         TopLevelCommandManager tlcManager,

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

@@ -585,9 +585,9 @@ public sealed partial class TopLevelCommandManager : ObservableObject,
                 // Then find all the top-level commands that belonged to that extension
                 List<TopLevelViewModel> commandsToRemove = [];
                 List<TopLevelViewModel> bandsToRemove = [];
-                lock (TopLevelCommands)
+                foreach (var extension in extensions)
                 {
-                    foreach (var extension in extensions)
+                    lock (TopLevelCommands)
                     {
                         foreach (var command in TopLevelCommands)
                         {
@@ -597,7 +597,10 @@ public sealed partial class TopLevelCommandManager : ObservableObject,
                                 commandsToRemove.Add(command);
                             }
                         }
+                    }
 
+                    lock (_dockBandsLock)
+                    {
                         foreach (var band in DockBands)
                         {
                             var host = band.ExtensionHost;
@@ -675,6 +678,14 @@ public sealed partial class TopLevelCommandManager : ObservableObject,
         return null;
     }
 
+    public List<TopLevelViewModel> GetDockBandsSnapshot()
+    {
+        lock (_dockBandsLock)
+        {
+            return [.. DockBands];
+        }
+    }
+
     public void Receive(ReloadCommandsMessage message) =>
         _ = ReloadAllCommandsAsync();
 

src/modules/cmdpal/Microsoft.CmdPal.UI/Settings/DockSettingsPage.xaml.cs

@@ -178,7 +178,7 @@ public sealed partial class DockSettingsPage : Page
 
         var tlcManager = App.Current.Services.GetService<TopLevelCommandManager>()!;
 
-        foreach (var item in tlcManager.DockBands)
+        foreach (var item in tlcManager.GetDockBandsSnapshot())
         {
             if (item.IsDockBand)
             {
@@ -197,7 +197,7 @@ public sealed partial class DockSettingsPage : Page
         var tlcManager = App.Current.Services.GetService<TopLevelCommandManager>()!;
         var settingsModel = App.Current.Services.GetService<SettingsModel>()!;
         var dockViewModel = App.Current.Services.GetService<DockViewModel>()!;
-        var allBands = tlcManager.DockBands;
+        var allBands = tlcManager.GetDockBandsSnapshot();
         foreach (var band in allBands)
         {
             var setting = band.DockBandSettings;