diff --git a/docs/pregate.md b/docs/pregate.md
index 9a962cf8e..b6610fe5e 100644
--- a/docs/pregate.md
+++ b/docs/pregate.md
@@ -448,7 +448,7 @@ test-tier command:
   runner. Other runners (node --test, vitest, jest, pytest, playwright test, flutter test, dart test, cargo test) get heartbeats
   only. A heartbeat follows each 60 s with no progress line;
 - is killed as **stalled** when its log does not grow and its process group
-  uses no CPU for the stall timeout (`--stall-timeout <s>`, else
+  stays under a tenth of one CPU for the stall timeout (`--stall-timeout <s>`, else
   `stall_timeout_s`, else 300; `0` disables): the check is `incomplete` with
   `PREGATE_STALLED` and names the last file started; the next serial file
   still runs. The group counts as busy when its CPU time, sampled each second,
diff --git a/lib/pregate/supervise.ts b/lib/pregate/supervise.ts
index 3c4c3412e..37cc3210e 100644
--- a/lib/pregate/supervise.ts
+++ b/lib/pregate/supervise.ts
@@ -220,10 +220,8 @@ export function supervise(o: SuperviseOptions): Promise<Supervised> {
       if (now >= o.deadline) { stop('timeout'); return; }
       if (o.stallMs > 0 && process.platform !== 'win32' && now - cpu.at >= CPU_SAMPLE_MS) {
         const used = groupCpuMs(child.pid!);
-        if (used !== null) {
-          if (used - cpu.ms >= CPU_BUSY_SHARE * (now - cpu.at)) lastBusy = now;
-          cpu = { at: now, ms: used };
-        }
+        if (used !== null && used - cpu.ms >= CPU_BUSY_SHARE * (now - cpu.at)) lastBusy = now;
+        cpu = { at: now, ms: used ?? cpu.ms };
       }
       if (o.stallMs > 0 && now - Math.max(lastGrowth, lastBusy) >= o.stallMs) { stop('stalled'); return; }
       if (now - lastProgress < heartbeatMs) return;