fix(otel): only set Scope.Transaction in SentrySpanProcessor when null

src/Sentry.OpenTelemetry/SentrySpanProcessor.cs

@@ -172,7 +172,15 @@ private void CreateRootSpan(Activity data)
             tracer.Contexts.Trace.Origin = OpenTelemetryOrigin;
             tracer.StartTimestamp = data.StartTimeUtc;
         }
-        _hub.ConfigureScope(static (scope, transaction) => scope.Transaction = transaction, transaction);
+        // Only set Scope.Transaction when null to avoid clobbering an existing transaction
+        // when two root OTel activities overlap. See #5314.
+        _hub.ConfigureScope(static (scope, transaction) =>
+        {
+            if (scope.Transaction is null)
+            {
+                scope.Transaction = transaction;
+            }
+        }, transaction);
         transaction.SetFused(data);
         _map[data.SpanId] = transaction;
     }

test/Sentry.OpenTelemetry.Tests/SentrySpanProcessorTests.cs

@@ -1012,4 +1012,29 @@ public void ParseOtelSpanDescription_HttpClient()
         description.Should().Be("POST https://example.com/foo");
         source.Should().Be(TransactionNameSource.Custom);
     }
+
+    [Fact]
+    public void OnStart_WithExistingTransactionOnScope_DoesNotOverwriteExistingTransaction()
+    {
+        // Arrange
+        _fixture.Options.Instrumenter = Instrumenter.OpenTelemetry;
+        _fixture.Options.TracesSampleRate = 1.0;
+        _fixture.ScopeManager = Substitute.For<IInternalScopeManager>();
+        var scope = new Scope();
+        var clientScope = new KeyValuePair<Scope, ISentryClient>(scope, _fixture.Client);
+        _fixture.ScopeManager.GetCurrent().Returns(clientScope);
+        var sut = _fixture.GetSut();
+
+        using var firstActivity = Tracer.StartActivity("Existing");
+        sut.OnStart(firstActivity!);
+        var existingTransaction = scope.Transaction;
+
+        // Act: second root activity starts while Existing is still on the scope
+        using var secondActivity = Tracer.StartActivity("NewRootActivity");
+        sut.OnStart(secondActivity!);
+
+        // Assert
+        scope.Transaction.Should().BeSameAs(existingTransaction,
+            "CreateRootSpan should not overwrite an already-set Scope.Transaction");
+    }
 }