diff -r f627ab8c276c -r 2024a2298d7a src/Pure/Concurrent/consumer_thread.scala
--- a/src/Pure/Concurrent/consumer_thread.scala	Tue Oct 31 14:35:51 2023 +0100
+++ b/src/Pure/Concurrent/consumer_thread.scala	Tue Oct 31 15:40:46 2023 +0100
@@ -45,7 +45,7 @@
   /* thread */
 
   private var active = true
-  private val mailbox = Mailbox[Option[Request]]
+  private val mailbox = Mailbox[Option[Request]]()
 
   private val thread = Isabelle_Thread.fork(name = name, daemon = daemon) { main_loop(Nil) }
   def is_active(): Boolean = active && thread.isAlive