more explicit jEdit file operations;
less redundant JEdit_Resources.node_name();

--- a/src/Tools/jEdit/src/document_model.scala	Wed Apr 12 17:48:19 2017 +0200
+++ b/src/Tools/jEdit/src/document_model.scala	Wed Apr 12 19:56:47 2017 +0200
@@ -310,7 +310,7 @@
     last_perspective: Document.Node.Perspective_Text = Document.Node.no_perspective_text,
     pending_edits: List[Text.Edit] = Nil): File_Model =
   {
-    val file = PIDE.resources.node_name_file(node_name)
+    val file = JEdit_Lib.check_file(node_name.node)
     file.foreach(PIDE.plugin.file_watcher.register_parent(_))
 
     val content = Document_Model.File_Content(text)

--- a/src/Tools/jEdit/src/jedit_lib.scala	Wed Apr 12 17:48:19 2017 +0200
+++ b/src/Tools/jEdit/src/jedit_lib.scala	Wed Apr 12 19:56:47 2017 +0200
@@ -9,6 +9,7 @@
 
 import isabelle._
 
+import java.io.{File => JFile}
 import java.awt.{Component, Container, GraphicsEnvironment, Point, Rectangle, Dimension}
 import java.awt.event.{InputEvent, KeyEvent, KeyListener}
 import javax.swing.{Icon, ImageIcon, JWindow, SwingUtilities}
@@ -16,6 +17,7 @@
 import scala.util.parsing.input.CharSequenceReader
 
 import org.gjt.sp.jedit.{jEdit, Buffer, View, GUIUtilities, Debug, EditPane}
+import org.gjt.sp.jedit.io.{VFS, FileVFS, VFSManager}
 import org.gjt.sp.jedit.gui.KeyEventWorkaround
 import org.gjt.sp.jedit.buffer.{JEditBuffer, LineManager}
 import org.gjt.sp.jedit.textarea.{JEditTextArea, TextArea, TextAreaPainter}
@@ -93,6 +95,15 @@
   }
 
 
+  /* files */
+
+  def is_file(name: String): Boolean =
+    VFSManager.getVFSForPath(name).isInstanceOf[FileVFS]
+
+  def check_file(name: String): Option[JFile] =
+    if (is_file(name)) Some(new JFile(name)) else None
+
+
   /* buffers */
 
   def buffer_text(buffer: JEditBuffer): String =
@@ -111,10 +122,12 @@
     }
   }
 
+  def buffer_line_manager(buffer: JEditBuffer): LineManager =
+    Untyped.get[LineManager](buffer, "lineMgr")
+
   def buffer_name(buffer: Buffer): String = buffer.getSymlinkPath
 
-  def buffer_line_manager(buffer: JEditBuffer): LineManager =
-    Untyped.get[LineManager](buffer, "lineMgr")
+  def buffer_file(buffer: Buffer): Option[JFile] = check_file(buffer_name(buffer))
 
 
   /* main jEdit components */

--- a/src/Tools/jEdit/src/jedit_resources.scala	Wed Apr 12 17:48:19 2017 +0200
+++ b/src/Tools/jEdit/src/jedit_resources.scala	Wed Apr 12 19:56:47 2017 +0200
@@ -25,14 +25,6 @@
 {
   /* document node name */
 
-  def node_name(buffer: Buffer): Document.Node.Name =
-  {
-    val node = JEdit_Lib.buffer_name(buffer)
-    val theory = Thy_Header.theory_name(node)
-    val master_dir = if (theory == "") "" else buffer.getDirectory
-    Document.Node.Name(node, master_dir, theory)
-  }
-
   def node_name(path: String): Document.Node.Name =
   {
     val vfs = VFSManager.getVFSForPath(path)
@@ -42,11 +34,8 @@
     Document.Node.Name(node, master_dir, theory)
   }
 
-  def node_name_file(name: Document.Node.Name): Option[JFile] =
-  {
-    val vfs = VFSManager.getVFSForPath(name.node)
-    if (vfs.isInstanceOf[FileVFS]) Some(new JFile(name.node)) else None
-  }
+  def node_name(buffer: Buffer): Document.Node.Name =
+    node_name(JEdit_Lib.buffer_name(buffer))
 
   def theory_node_name(buffer: Buffer): Option[Document.Node.Name] =
   {