diff -r dde5ecbd5e10 -r e4699ef5cf90 src/Pure/GUI/gui_thread.scala
--- a/src/Pure/GUI/gui_thread.scala	Sat Sep 19 19:40:09 2015 +0200
+++ b/src/Pure/GUI/gui_thread.scala	Sat Sep 19 20:31:57 2015 +0200
@@ -49,9 +49,9 @@
 
   /* delayed events */
 
-  def delay_first(delay: => Time)(event: => Unit): Simple_Thread.Delay =
-    Simple_Thread.delay_first(delay) { later { event } }
+  def delay_first(delay: => Time, cancel: () => Unit = () => ())(event: => Unit)
+    : Simple_Thread.Delay = Simple_Thread.delay_first(delay, cancel) { later { event } }
 
-  def delay_last(delay: => Time)(event: => Unit): Simple_Thread.Delay =
-    Simple_Thread.delay_last(delay) { later { event } }
+  def delay_last(delay: => Time, cancel: () => Unit = () => ())(event: => Unit)
+    : Simple_Thread.Delay = Simple_Thread.delay_last(delay, cancel) { later { event } }
 }