Fix ReactiveUI scheduler hang in unit batches

ReactiveUI 24's coalescing Avalonia scheduler can retain queued dispatcher work across headless view-model instances. Scope the scheduler to each MainWindowViewModel and run the settings-panel flow test in Avalonia's async test context so CI unit batch 7 completes reliably.

Author: StefanKoell

src/RoyalTerminal.Avalonia.App/ViewModels/MainWindowViewModel.cs

@@ -10,6 +10,7 @@
 using System.Reactive;
 using System.Reactive.Linq;
 using System.Runtime.InteropServices;
+using Avalonia.Threading;
 using RoyalTerminal.Avalonia.App;
 using RoyalTerminal.Avalonia.Controls;
 using RoyalTerminal.Avalonia.Rendering;
@@ -58,6 +59,7 @@ public sealed class MainWindowViewModel : ReactiveObject
     private TerminalRenderMode _activeRenderMode = TerminalRenderMode.RenderedAuto;
     private readonly ITerminalModeResolver _modeResolver;
     private readonly ITerminalThemeCatalog _themeCatalog;
+    private readonly AvaloniaScheduler _uiScheduler;
     private readonly IReadOnlyList<TerminalThemePreset> _themePresets;
     private readonly bool _showMacOsTitleBarLogos;
     private readonly Dictionary<TerminalRenderMode, ModeThemeState> _modeThemes = [];
@@ -213,6 +215,7 @@ internal MainWindowViewModel(
         _modeResolver = modeResolver ?? throw new ArgumentNullException(nameof(modeResolver));
         _themeCatalog = themeCatalog ?? throw new ArgumentNullException(nameof(themeCatalog));
         ArgumentNullException.ThrowIfNull(shellOptions);
+        _uiScheduler = new AvaloniaScheduler(Dispatcher.UIThread);
 
         _showMacOsTitleBarLogos = shellOptions.ShowMacOsTitleBarLogos;
         _themePresets = _themeCatalog.Presets;
@@ -2334,7 +2337,7 @@ private IObservable<Unit> PrepareSettingsPanel()
     {
         return PrepareSettingsPanelInteraction
             .Handle(Unit.Default)
-            .ObserveOn(AvaloniaScheduler.Instance)
+            .ObserveOn(_uiScheduler)
             .Do(_ => IsSettingsPanelOpen = true);
     }
 

tests/RoyalTerminal.Tests/MainWindowViewModelFlowTests.cs

@@ -2187,20 +2187,20 @@ public void PaneCommands_RouteSplitFocusAndResizeRequests()
         Assert.Equal(1, closeRequests);
     }
 
-    [Fact]
-    public void SettingsPanel_OpenClose_TogglesOverlayAfterPreparation()
+    [AvaloniaFact]
+    public async Task SettingsPanel_OpenClose_TogglesOverlayAfterPreparation()
     {
         MainWindowViewModel viewModel = new();
         using IDisposable preparationRegistration = viewModel.PrepareSettingsPanelInteraction.RegisterHandler(context =>
         {
             context.SetOutput(Unit.Default);
         });
 
-        viewModel.PrepareSettingsPanelCommand.Execute().Wait();
+        await viewModel.PrepareSettingsPanelCommand.Execute().ToTask();
 
         Assert.True(viewModel.IsSettingsPanelOpen);
 
-        viewModel.CloseSettingsPanelCommand.Execute().Wait();
+        await viewModel.CloseSettingsPanelCommand.Execute().ToTask();
 
         Assert.False(viewModel.IsSettingsPanelOpen);
     }