diff -r 4b06a20b13b5 -r 814a1ab42d70 src/Pure/System/isabelle_process.ML
--- a/src/Pure/System/isabelle_process.ML	Mon Oct 01 19:30:36 2018 +0200
+++ b/src/Pure/System/isabelle_process.ML	Tue Oct 02 19:02:47 2018 +0200
@@ -71,7 +71,8 @@
           Synchronized.change_result tracing_messages (fn tab =>
             let
               val n = the_default 0 (Inttab.lookup tab exec_id) + 1;
-              val ok = n <= Options.default_int "editor_tracing_messages";
+              val limit = Options.default_int "editor_tracing_messages";
+              val ok = limit <= 0 orelse n <= limit;
             in (ok, Inttab.update (exec_id, n) tab) end);
       in
         if ok then ()