more robust (see https://code.visualstudio.com/docs/extensionAPI/document-selectors);

--- a/src/Tools/VSCode/extension/src/extension.ts	Wed Nov 21 14:45:24 2018 +0100
+++ b/src/Tools/VSCode/extension/src/extension.ts	Wed Nov 21 15:19:11 2018 +0100
@@ -41,7 +41,11 @@
         { command: isabelle_tool,
           args: ["vscode_server"].concat(standard_args, isabelle_args) };
     const language_client_options: LanguageClientOptions = {
-      documentSelector: ["isabelle", "isabelle-ml", "bibtex"]
+      documentSelector: [
+        { language: "isabelle", scheme: "file" },
+        { language: "isabelle-ml", scheme: "file" },
+        { language: "bibtex", scheme: "file" }
+      ]
     };
 
     const language_client =