diff --git a/JitHub.WinUI.Tests/Services/ProductPerformanceObservationTests.cs b/JitHub.WinUI.Tests/Services/ProductPerformanceObservationTests.cs
index 88e36397c..48c441d9d 100644
--- a/JitHub.WinUI.Tests/Services/ProductPerformanceObservationTests.cs
+++ b/JitHub.WinUI.Tests/Services/ProductPerformanceObservationTests.cs
@@ -6,8 +6,6 @@ namespace JitHub.WinUI.Tests.Services;
 
 public sealed class ProductPerformanceObservationTests
 {
-    private static readonly object ReadinessGate = new();
-
     [Fact]
     public void ContentTransition_DoesNotPassWhenInvokeReturnsButDestinationIdentityDoesNotChange()
     {
@@ -210,75 +208,6 @@ public void ContentTransition_UsesAppSideCompositionTimestampsInsteadOfObserverL
         Assert.Equal(TimeSpan.FromMilliseconds(38), tracker.SettledDataContent);
     }
 
-    [Fact]
-    public void TraversalCommitEndsGenerationAndRejectsLaterStages()
-    {
-        lock (ReadinessGate)
-        {
-            string? previous = Environment.GetEnvironmentVariable("JITHUB_PERFORMANCE_FIXTURE");
-            List<ProductPerformanceTraversalStage> stages = [];
-            EventHandler<ProductPerformanceTraversalStage> handler = (_, stage) => stages.Add(stage);
-            ProductPerformanceReadiness.TraversalStageRecorded += handler;
-            try
-            {
-                Environment.SetEnvironmentVariable("JITHUB_PERFORMANCE_FIXTURE", "1");
-                ProductPerformanceReadiness.BeginTraversal("stars", "repo:42", "repo_code");
-                ProductPerformanceReadiness.RecordTraversalStage("before.commit");
-                ProductPerformanceReadiness.CommitTraversal("repo_code", "repo:42");
-                ProductPerformanceReadiness.RecordTraversalStage("after.commit");
-
-                ProductPerformanceTraversalStage stage = Assert.Single(
-                    stages,
-                    candidate => candidate.Stage is "before.commit" or "after.commit");
-                Assert.Equal("before.commit", stage.Stage);
-                Assert.True(stage.Generation > 0);
-            }
-            finally
-            {
-                ProductPerformanceReadiness.CancelTraversal();
-                ProductPerformanceReadiness.TraversalStageRecorded -= handler;
-                Environment.SetEnvironmentVariable("JITHUB_PERFORMANCE_FIXTURE", previous);
-            }
-        }
-    }
-
-    [Fact]
-    public void TraversalMeasurement_ResumesProbeAfterCommitOrCancellation()
-    {
-        lock (ReadinessGate)
-        {
-            string? previous = Environment.GetEnvironmentVariable("JITHUB_PERFORMANCE_FIXTURE");
-            int armed = 0;
-            int completed = 0;
-            EventHandler armedHandler = (_, _) => armed++;
-            EventHandler completedHandler = (_, _) => completed++;
-            ProductPerformanceReadiness.TraversalMeasurementArmed += armedHandler;
-            ProductPerformanceReadiness.TraversalMeasurementCompleted += completedHandler;
-            try
-            {
-                Environment.SetEnvironmentVariable("JITHUB_PERFORMANCE_FIXTURE", "1");
-
-                ProductPerformanceReadiness.ArmTraversalMeasurement();
-                ProductPerformanceReadiness.BeginTraversal("repo_code", "file:1", "repo_code");
-                ProductPerformanceReadiness.CommitTraversal("repo_code", "file:1");
-
-                ProductPerformanceReadiness.ArmTraversalMeasurement();
-                ProductPerformanceReadiness.BeginTraversal("repo_code", "file:2", "repo_code");
-                ProductPerformanceReadiness.CancelTraversal();
-
-                Assert.Equal(2, armed);
-                Assert.Equal(2, completed);
-            }
-            finally
-            {
-                ProductPerformanceReadiness.CancelTraversal();
-                ProductPerformanceReadiness.TraversalMeasurementArmed -= armedHandler;
-                ProductPerformanceReadiness.TraversalMeasurementCompleted -= completedHandler;
-                Environment.SetEnvironmentVariable("JITHUB_PERFORMANCE_FIXTURE", previous);
-            }
-        }
-    }
-
     private static ProductPerformanceContentObservation Observation(
         string identity,
         long frame,
diff --git a/JitHub.WinUI.Tests/Services/ProductPerformanceReadinessTraversalTests.cs b/JitHub.WinUI.Tests/Services/ProductPerformanceReadinessTraversalTests.cs
new file mode 100644
index 000000000..1ed14e538
--- /dev/null
+++ b/JitHub.WinUI.Tests/Services/ProductPerformanceReadinessTraversalTests.cs
@@ -0,0 +1,91 @@
+using System.Collections.Concurrent;
+using JitHub.Services;
+using Xunit;
+
+namespace JitHub.WinUI.Tests.Services;
+
+/// <summary>
+/// Tests that turn on <c>JITHUB_PERFORMANCE_FIXTURE</c> and drive the static
+/// <see cref="ProductPerformanceReadiness"/> traversal state. Both are process-wide:
+/// while the variable is set, product code under test in other classes (for example
+/// <c>RepoCodePageViewModel.SelectFileAsync</c>) records traversal stages into the same
+/// static traversal, and the variable changes <see cref="ProductPerformanceReadiness.IsEnabled"/>
+/// for every test in the process. xUnit runs this collection on its own, after the
+/// parallel collections finish.
+/// </summary>
+[CollectionDefinition(Name, DisableParallelization = true)]
+public sealed class ProductPerformanceReadinessCollection
+{
+    public const string Name = "Product performance readiness (process-wide)";
+}
+
+[Collection(ProductPerformanceReadinessCollection.Name)]
+public sealed class ProductPerformanceReadinessTraversalTests
+{
+    private const string FixtureVariable = "JITHUB_PERFORMANCE_FIXTURE";
+
+    [Fact]
+    public void TraversalCommitEndsGenerationAndRejectsLaterStages()
+    {
+        string? previous = Environment.GetEnvironmentVariable(FixtureVariable);
+        // The static event may be raised from any thread, so collect stages thread-safely
+        // and assert on one snapshot.
+        ConcurrentQueue<ProductPerformanceTraversalStage> stages = new();
+        EventHandler<ProductPerformanceTraversalStage> handler = (_, stage) => stages.Enqueue(stage);
+        ProductPerformanceReadiness.TraversalStageRecorded += handler;
+        try
+        {
+            Environment.SetEnvironmentVariable(FixtureVariable, "1");
+            ProductPerformanceReadiness.BeginTraversal("stars", "repo:42", "repo_code");
+            ProductPerformanceReadiness.RecordTraversalStage("before.commit");
+            ProductPerformanceReadiness.CommitTraversal("repo_code", "repo:42");
+            ProductPerformanceReadiness.RecordTraversalStage("after.commit");
+
+            ProductPerformanceTraversalStage stage = Assert.Single(
+                stages.ToArray(),
+                candidate => candidate.Stage is "before.commit" or "after.commit");
+            Assert.Equal("before.commit", stage.Stage);
+            Assert.True(stage.Generation > 0);
+        }
+        finally
+        {
+            ProductPerformanceReadiness.CancelTraversal();
+            ProductPerformanceReadiness.TraversalStageRecorded -= handler;
+            Environment.SetEnvironmentVariable(FixtureVariable, previous);
+        }
+    }
+
+    [Fact]
+    public void TraversalMeasurement_ResumesProbeAfterCommitOrCancellation()
+    {
+        string? previous = Environment.GetEnvironmentVariable(FixtureVariable);
+        int armed = 0;
+        int completed = 0;
+        EventHandler armedHandler = (_, _) => Interlocked.Increment(ref armed);
+        EventHandler completedHandler = (_, _) => Interlocked.Increment(ref completed);
+        ProductPerformanceReadiness.TraversalMeasurementArmed += armedHandler;
+        ProductPerformanceReadiness.TraversalMeasurementCompleted += completedHandler;
+        try
+        {
+            Environment.SetEnvironmentVariable(FixtureVariable, "1");
+
+            ProductPerformanceReadiness.ArmTraversalMeasurement();
+            ProductPerformanceReadiness.BeginTraversal("repo_code", "file:1", "repo_code");
+            ProductPerformanceReadiness.CommitTraversal("repo_code", "file:1");
+
+            ProductPerformanceReadiness.ArmTraversalMeasurement();
+            ProductPerformanceReadiness.BeginTraversal("repo_code", "file:2", "repo_code");
+            ProductPerformanceReadiness.CancelTraversal();
+
+            Assert.Equal(2, Volatile.Read(ref armed));
+            Assert.Equal(2, Volatile.Read(ref completed));
+        }
+        finally
+        {
+            ProductPerformanceReadiness.CancelTraversal();
+            ProductPerformanceReadiness.TraversalMeasurementArmed -= armedHandler;
+            ProductPerformanceReadiness.TraversalMeasurementCompleted -= completedHandler;
+            Environment.SetEnvironmentVariable(FixtureVariable, previous);
+        }
+    }
+}