diff -r 3300847d75ae -r 45dc9de1bd33 src/Pure/Tools/simplifier_trace.scala
--- a/src/Pure/Tools/simplifier_trace.scala	Tue Sep 07 15:05:57 2021 +0200
+++ b/src/Pure/Tools/simplifier_trace.scala	Tue Sep 07 15:10:53 2021 +0200
@@ -287,7 +287,7 @@
 
   def the_manager(session: Session): Consumer_Thread[Any] =
     managers.value.get(session) match {
-      case Some(thread) if thread.is_active => thread
+      case Some(thread) if thread.is_active() => thread
       case _ => error("Bad Simplifier_Trace.manager thread")
     }