diff -r 091f57432f05 -r af09cc4792dc src/Pure/PIDE/session.scala
--- a/src/Pure/PIDE/session.scala	Sat Jan 12 19:53:57 2019 +0100
+++ b/src/Pure/PIDE/session.scala	Sat Jan 12 20:14:05 2019 +0100
@@ -9,6 +9,7 @@
 
 
 import scala.collection.immutable.Queue
+import scala.annotation.tailrec
 
 
 object Session
@@ -640,6 +641,16 @@
       pending_edits: List[Text.Edit] = Nil): Document.Snapshot =
     global_state.value.snapshot(name, pending_edits)
 
+  @tailrec final def await_stable_snapshot(): Document.Snapshot =
+  {
+    val snapshot = this.snapshot()
+    if (snapshot.is_outdated) {
+      Thread.sleep(output_delay.ms)
+      await_stable_snapshot()
+    }
+    else snapshot
+  }
+
   def start(start_prover: Prover.Receiver => Prover)
   {
     file_formats