implement tab drag-drop and copy across windows, plus fixed asset race

App.axaml.cs

@@ -24,6 +24,7 @@ public override void OnFrameworkInitializationCompleted()
     {
         if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
         {
+            EmbeddedAssets.WarmUp();
             desktop.MainWindow = services.GetRequiredService<MainWindowFactory>().Create();
             SystemMonospaceFonts.WarmUp();
         }

Services/DocumentFileIdentity.cs

@@ -0,0 +1,10 @@
+namespace Inlay;
+
+internal static class DocumentFileIdentity
+{
+    public static StringComparer Comparer { get; } =
+        OperatingSystem.IsWindows() ? StringComparer.OrdinalIgnoreCase : StringComparer.Ordinal;
+
+    public static bool Matches(IDocumentFile? file, string identity) =>
+        file is not null && Comparer.Equals(file.Identity, identity);
+}

Services/EmbeddedAssets.cs

@@ -0,0 +1,16 @@
+using Avalonia.Platform;
+
+namespace Inlay;
+
+internal static class EmbeddedAssets
+{
+    public static Uri Uri(string name) => new($"avares://Inlay/Assets/{name}");
+
+    // The toolbar's Svg controls each load on their own thread as the first window
+    // is built. Reading one asset here builds this assembly's resource table once,
+    // on the UI thread, before those loads start racing for it.
+    public static void WarmUp()
+    {
+        using var stream = AssetLoader.Open(Uri("new-document.svg"));
+    }
+}

Services/TabDragDrop.cs

@@ -0,0 +1,13 @@
+using Avalonia.Input;
+using Inlay.ViewModels;
+
+namespace Inlay;
+
+internal sealed class TabDragPayload
+{
+    public static readonly DataFormat<TabDragPayload> Format =
+        DataFormat.CreateInProcessFormat<TabDragPayload>("application/x-inlay-tab");
+
+    public required MainWindow SourceWindow { get; init; }
+    public required DocumentTabViewModel SourceTab { get; init; }
+}

Tests/Behavior/Application/ApplicationBehaviorTests.cs

@@ -118,10 +118,32 @@ public void NewWindowsHaveIndependentDocumentState()
         }
     }
 
+    [AvaloniaFact]
+    public void ClosingAWindowReleasesItsTabsWithoutDisturbingTheBoundSelection()
+    {
+        var (window, viewModel) = MainWindowTestHost.CreateWindow();
+        var tab = viewModel.SelectedDocument!;
+        var editor = MainWindowTestHost.FindEditor(window);
+
+        window.Close();
+
+        // The window is still bound to Documents while it closes, so emptying the
+        // collection here would push a null selection back through SelectedDocument
+        // and every SelectedDocument.* binding would fail.
+        Assert.Same(tab, viewModel.SelectedDocument);
+        Assert.Single(viewModel.Documents);
+
+        // The tab is disposed all the same, so its editor no longer feeds it changes.
+        editor.Text = "Edited after the window closed";
+        Assert.False(tab.IsDirty);
+    }
+
     private static MainWindow CreateWindow(FakeInteractionService interaction) =>
+#pragma warning disable CA2000 // Ownership passes to the caller.
         new(new MainWindowViewModel(
             new JsonTemplateDocumentService(),
             new FakeStorageService(),
             interaction,
             new FakeApplicationService()));
+#pragma warning restore CA2000
 }

Tests/Behavior/MainWindow/LineLengthBehaviorTests.cs

@@ -56,7 +56,7 @@ public void FlyoutControlsUpdateTheSelectedDocumentAndEditor()
     [AvaloniaFact]
     public async Task OpeningADocumentAppliesItsLineSettingsWithoutMarkingItDirty()
     {
-        var context = MainWindowViewModelTestContext.Create();
+        using var context = MainWindowViewModelTestContext.Create();
         var file = new MemoryDocumentFile("configured.itd");
         file.SetRawContents("""
             {

Tests/Behavior/MainWindow/MainWindowCommandTests.cs

@@ -203,7 +203,7 @@ public void ViewCommandsUpdateRenderedState()
     [AvaloniaFact]
     public async Task FontCommandUpdatesTheRenderedEditorFamily()
     {
-        var context = MainWindowViewModelTestContext.Create();
+        using var context = MainWindowViewModelTestContext.Create();
         var initialFont = context.ViewModel.EditorFontFamily;
         context.Interaction.SelectedFont = new FontFamily("DejaVu Sans Mono");
         var window = new MainWindow(context.ViewModel);