src/Tools/jEdit/src/document_model.scala

--- a/src/Tools/jEdit/src/document_model.scala	Wed Mar 14 15:23:50 2012 +0100
+++ b/src/Tools/jEdit/src/document_model.scala	Wed Mar 14 15:37:51 2012 +0100
@@ -61,12 +61,15 @@
   /* header */
 
   def node_header(): Document.Node_Header =
-    Isabelle.swing_buffer_lock(buffer) {
+  {
+    Swing_Thread.require()
+    Isabelle.buffer_lock(buffer) {
       Exn.capture {
         Isabelle.thy_load.check_header(name,
           Thy_Header.read(buffer.getSegment(0, buffer.getLength)))
       }
     }
+  }
 
 
   /* perspective */