diff --git a/tests/compute_prebuild_queue_check.py b/tests/compute_prebuild_queue_check.py
index 688fb704..d644aa9e 100644
--- a/tests/compute_prebuild_queue_check.py
+++ b/tests/compute_prebuild_queue_check.py
@@ -20,8 +20,10 @@ static struct {
     volatile int64_t next;
     volatile int32_t stop;
     size_t n, n_keep;
-    int idle, tail_one;
+    int idle, tail_one, bg_threads;
 } LB;
+static int bf_wait_now, bg_threads = 8;   // bf_waiting(): the start waits for the whole warm
+static int lb_idle(void) { return LB.idle && !bf_wait_now; }
 static volatile LONG seen[2000];
 static HANDLE start_event;
 '''
@@ -34,13 +36,14 @@ static DWORD WINAPI worker(void *arg) {
         size_t i=lb_next(id);
         if (i>=LB.n) break;
         assert(InterlockedIncrement(&seen[i])==1);
-        if (LB.idle && LB.tail_one && i>=LB.n_keep) assert(id==0);
+        if (LB.idle && !bf_wait_now && LB.tail_one && i>=LB.n_keep) assert(id==0);
+        if (LB.idle && !bf_wait_now) assert(id<LB.bg_threads);
         if (i%7==0) SwitchToThread();
     }
     return 0;
 }
 static void run(size_t prefix, int automatic, int idle) {
-    LB.next=0;LB.n=2000;LB.n_keep=prefix;LB.stop=0;LB.tail_one=automatic;LB.idle=idle;
+    LB.next=0;LB.n=2000;LB.n_keep=prefix;LB.stop=0;LB.tail_one=automatic;LB.idle=idle;LB.bg_threads=bg_threads;
     for (int i=0;i<2000;i++) seen[i]=0;
     ResetEvent(start_event);
     HANDLE threads[8];
@@ -57,13 +60,21 @@ int main(void) {
         run(289,1,1);run(0,1,1);run(2000,1,1);run(1,1,1);
         run(289,0,1);run(289,1,0);
     }
-    LB.next=289;LB.n_keep=289;LB.tail_one=1;LB.idle=1;
+    // The boot's builders (idle) beside the game: only the first bg_threads workers go on; while the start waits
+    // for the whole warm (bf_waiting) every worker does.
+    bg_threads=2;run(289,1,1);run(0,0,1);bf_wait_now=1;run(289,1,1);bf_wait_now=0;bg_threads=8;
+    LB.next=289;LB.n_keep=289;LB.tail_one=1;LB.idle=1;LB.bg_threads=8;
     assert(lb_next(1)==LB.n && LB.next==289);
     assert(lb_next(0)==289);
     LB.stop=1;
     assert(lb_next(0)==LB.n && LB.next==290);
     LB.stop=0;LB.tail_one=0;
     assert(lb_next(1)==290);
+    LB.bg_threads=2;LB.idle=1;
+    assert(lb_next(2)==LB.n && LB.next==291);   // worker 2 of 2 stops beside the game
+    assert(lb_next(1)==291);
+    LB.idle=0;
+    assert(lb_next(2)==292);                    // and goes on in a preparation (idle 0)
     CloseHandle(start_event);
     puts("Prebuild queue: concurrent exactly-once coverage, recent/tail boundary, explicit override and stop checks passed");
     return 0;