src/Pure/Tools/debugger.scala

--- a/src/Pure/Tools/debugger.scala	Sat Apr 04 21:38:20 2020 +0200
+++ b/src/Pure/Tools/debugger.scala	Sun Apr 05 13:05:40 2020 +0200
@@ -155,7 +155,7 @@
   private val state = Synchronized(Debugger.State())
 
   private val delay_update =
-    Standard_Thread.delay_first(session.output_delay) {
+    Isabelle_Thread.delay_first(session.output_delay) {
       session.debugger_updates.post(Debugger.Update)
     }