diff --git a/crates/grill-perf/src/process.rs b/crates/grill-perf/src/process.rs
index 6e23326..0409ca2 100644
--- a/crates/grill-perf/src/process.rs
+++ b/crates/grill-perf/src/process.rs
@@ -55,7 +55,7 @@ pub mod program_signals {
     static ACTIVE: Mutex<()> = Mutex::new(());
     static CANCELLED: AtomicBool = AtomicBool::new(false);
 
-    extern "C" fn cancel(_: libc::c_int) {
+    pub(super) extern "C" fn cancel(_: libc::c_int) {
         CANCELLED.store(true, Ordering::SeqCst);
     }
 
@@ -433,8 +433,9 @@ mod tests {
             if cancel {
                 // The leader is already reaped, so cancellation must interrupt
                 // normal-exit drainage rather than the running-program path.
-                // SAFETY: the collector guard has installed the SIGINT handler.
-                assert_eq!(unsafe { libc::raise(libc::SIGINT) }, 0);
+                // A raised SIGINT would reach whichever handler a concurrent
+                // test installed last, so invoke the guard's handler directly.
+                program_signals::cancel(libc::SIGINT);
                 let completed = completion.recv_timeout(Duration::from_secs(5)).is_ok();
                 drop((stdout, stderr));
                 assert!(completed, "cancellation waited for pipe EOF");
diff --git a/crates/grill-perf/tests/support/systemone.rs b/crates/grill-perf/tests/support/systemone.rs
index e2323c4..11cbabf 100644
--- a/crates/grill-perf/tests/support/systemone.rs
+++ b/crates/grill-perf/tests/support/systemone.rs
@@ -404,21 +404,44 @@ fn inspect_detects_changed_retained_response_bytes() {
 #[test]
 fn compare_reports_a_declared_change_under_the_observed_envelope() {
     let temp = Temp::new();
-    let slow = DecisionServer::new(|request| {
-        thread::sleep(Duration::from_millis(40));
-        immediate(request)
-    });
     let fast = DecisionServer::new(immediate);
-    // Hosted macOS runners oversleep the 40 ms fixture delay by up to ~150 ms, so a
+    // Hosted macOS runners oversleep the fixture delay by up to ~150 ms, so a
     // tight reference-spread bound tests the host timer, not the comparison.
     let mut wide = plan_json();
     for gate in wide["gates"].as_array_mut().unwrap() {
         gate["max_reference_spread_bps"] = json!(1000000);
     }
     let plan = fixtures(&temp, "compare", &wide);
-    successful(&capture_server(&temp, &slow, &plan, 0, "a"));
+    // The candidate is captured first so every reference request can outlast
+    // its slowest burst, however long load stretched that burst.
     successful(&capture(&temp, &fast.endpoint, &plan, 1, "b"));
+    let waves = |arm: &str| {
+        (0..)
+            .map(|index| temp.path(&format!("{arm}/wave-{index:06}.json")))
+            .take_while(|path| path.exists())
+            .map(value)
+            .collect::<Vec<_>>()
+    };
+    let slowest_burst = |waves: &[Value]| {
+        waves
+            .iter()
+            .map(|wave| wave["makespan_us"].as_u64().unwrap())
+            .max()
+            .unwrap()
+    };
+    let b = waves("b");
+    let delay = Arc::new(std::sync::atomic::AtomicU64::new(slowest_burst(&b)));
+    let server_delay = delay.clone();
+    let slow = DecisionServer::new(move |request| {
+        thread::sleep(Duration::from_micros(server_delay.load(Ordering::SeqCst)));
+        immediate(request)
+    });
+    successful(&capture_server(&temp, &slow, &plan, 0, "a"));
+    let a = waves("a");
+    // A2 must extend A's envelope even when scheduling stretches A's capture.
+    delay.store(slowest_burst(&a), Ordering::SeqCst);
     successful(&capture_server(&temp, &slow, &plan, 2, "a2"));
+    let a2 = waves("a2");
     let output = cli()
         .args(["systemone", "compare", "--plan"])
         .arg(&plan)
@@ -438,11 +461,41 @@ fn compare_reports_a_declared_change_under_the_observed_envelope() {
     assert_eq!(report["acquisitions"].as_array().unwrap().len(), 3);
     let gates = report["gates"].as_array().unwrap();
     assert_eq!(gates.len(), 2);
-    for gate in gates {
+    for (gate, declared) in gates.iter().zip(wide["gates"].as_array().unwrap()) {
         assert_eq!(gate["counts"], json!([3, 3, 3]));
         assert_eq!(gate["outcome"], json!("PASS"));
-        assert!(gate["reference"].is_array());
-        assert!(gate["candidate"].is_array());
+        let extrema = |arms: &[&[Value]]| {
+            let mut samples = Vec::new();
+            for wave in arms
+                .iter()
+                .flat_map(|waves| waves.iter())
+                .filter(|wave| wave["phase"] == "measured" && wave["cell"] == declared["cell"])
+            {
+                assert_eq!(wave["complete"], json!(true));
+                match declared["metric"].as_str().unwrap() {
+                    "latency_us" => {
+                        for attempt in wave["attempts"].as_array().unwrap() {
+                            samples.push((attempt["latency_us"].as_u64().unwrap(), 1));
+                        }
+                    }
+                    "decisions_per_second" => samples.push((
+                        wave["decisions"].as_u64().unwrap() * 1_000_000,
+                        wave["makespan_us"].as_u64().unwrap(),
+                    )),
+                    metric => panic!("unexpected fixture metric {metric}"),
+                }
+            }
+            samples.sort_unstable_by(|&(an, ad), &(bn, bd)| {
+                (u128::from(an) * u128::from(bd)).cmp(&(u128::from(bn) * u128::from(ad)))
+            });
+            [samples[0], *samples.last().unwrap()].map(|(numerator, denominator)| {
+                json!({"numerator": numerator, "denominator": denominator})
+            })
+        };
+        let reference = extrema(&[&a, &a2]);
+        assert_ne!(reference, extrema(&[&a]), "A2 must extend the envelope");
+        assert_eq!(gate["reference"], json!(reference));
+        assert_eq!(gate["candidate"], json!(extrema(&[&b])));
     }
     let latency = &gates[0]["candidate"][1];
     let reference = &gates[0]["reference"][0];