diff --git a/tests/AcDream.Launcher.Tests/AcDream.Launcher.Tests.csproj b/tests/AcDream.Launcher.Tests/AcDream.Launcher.Tests.csproj
index 78b2ec1b..23aedde8 100644
--- a/tests/AcDream.Launcher.Tests/AcDream.Launcher.Tests.csproj
+++ b/tests/AcDream.Launcher.Tests/AcDream.Launcher.Tests.csproj
@@ -23,14 +23,4 @@
   <ItemGroup>
     <ProjectReference Include="..\..\src\AcDream.Launcher\AcDream.Launcher.csproj" />
   </ItemGroup>
-  <ItemGroup>
-    <!-- Avalonia's headless session is thread-affine, and its per-test cleanup
-         runs on whichever thread xUnit hands it. Under the default parallel
-         collections MainWindowViewTests failed in Test Case Cleanup with
-         "The calling thread cannot access this object because a different
-         thread owns it" while passing in isolation. Same serialization
-         settings AcDream.Core.Tests already uses. -->
-    <None Update="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
-  </ItemGroup>
-
 </Project>
diff --git a/tests/AcDream.Launcher.Tests/TestAppBuilder.cs b/tests/AcDream.Launcher.Tests/TestAppBuilder.cs
index 4245f0e4..777f559b 100644
--- a/tests/AcDream.Launcher.Tests/TestAppBuilder.cs
+++ b/tests/AcDream.Launcher.Tests/TestAppBuilder.cs
@@ -1,9 +1,18 @@
 using Avalonia;
 using Avalonia.Headless;
+using Xunit;
 
 [assembly: AvaloniaTestApplication(typeof(AcDream.Launcher.Tests.TestAppBuilder))]
 [assembly: AvaloniaTestIsolation(AvaloniaTestIsolationLevel.PerAssembly)]
 
+// Compiled INTO the assembly on purpose. A xunit.runner.json expresses the same
+// intent but only works if the file is copied beside the test DLL; under CI's
+// `dotnet build` + `dotnet test --no-build` split it did not arrive, so the
+// suite ran with parallel collections and MainWindowViewTests failed in Test
+// Case Cleanup ("The calling thread cannot access this object") while passing
+// locally. An assembly attribute cannot fail to deploy.
+[assembly: CollectionBehavior(DisableTestParallelization = true)]
+
 namespace AcDream.Launcher.Tests;
 
 /// <summary>
diff --git a/tests/AcDream.Launcher.Tests/xunit.runner.json b/tests/AcDream.Launcher.Tests/xunit.runner.json
deleted file mode 100644
index c3155894..00000000
--- a/tests/AcDream.Launcher.Tests/xunit.runner.json
+++ /dev/null
@@ -1,6 +0,0 @@
-{
-  "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
-  "parallelizeAssembly": false,
-  "parallelizeTestCollections": false,
-  "maxParallelThreads": 1
-}