src/Tools/VSCode/src/vscode_resources.scala

     val theory = Thy_Header.thy_name(uri).getOrElse("")
     val master_dir =
       if (!VSCode_Resources.is_wellformed(uri) || theory == "") ""
       else VSCode_Resources.canonical_file(uri).getParent
     Document.Node.Name(uri, master_dir, theory)
+  }
+
+  override def import_name(qualifier: String, master: Document.Node.Name, s: String)
+    : Document.Node.Name =
+  {
+    val name = super.import_name(qualifier, master, s)
+    if (name.node.startsWith("file://") || name.node.forall(c => c != '/' && c != '\\')) name
+    else name.copy(node = "file://" + name.node)
   }
 
 
   /* document models */