diff -r 8ec5c82b67dc -r b9a5eb0f3b43 src/Pure/Tools/debugger.scala
--- a/src/Pure/Tools/debugger.scala	Mon Apr 06 12:36:00 2020 +0200
+++ b/src/Pure/Tools/debugger.scala	Mon Apr 06 12:53:45 2020 +0200
@@ -155,7 +155,7 @@
   private val state = Synchronized(Debugger.State())
 
   private val delay_update =
-    Isabelle_Thread.delay_first(session.output_delay) {
+    Delay.first(session.output_delay) {
       session.debugger_updates.post(Debugger.Update)
     }