support folding

Author: williamstein

src/packages/frontend/codemirror/util.ts

@@ -2,13 +2,21 @@ import { set, get, del } from "@cocalc/frontend/misc/local-storage-typed";
 import { isEqual } from "lodash";
 
 export function getFoldedLines(cm): number[] {
+  if (cm.foldCode == null) {
+    // not enabled
+    return [];
+  }
   return cm
     .getAllMarks()
     .filter((mark) => mark.__isFold)
     .map((mark) => mark.find().from.line);
 }
 
 export function setFoldedLines(cm, lines: number[]) {
+  if (cm.foldCode == null) {
+    // not enabled
+    return;
+  }
   lines.reverse();
   for (const n of lines) {
     cm.foldCode(n);

src/packages/frontend/jupyter/codemirror-editor.tsx

@@ -644,13 +644,6 @@ export const CodeMirrorEditor: React.FC<CodeMirrorEditorProps> = ({
       options0.extraKeys["Ctrl-Enter"] = () => {};
       options0.extraKeys["Alt-Enter"] = () => {};
       options0.extraKeys["Cmd-Enter"] = () => {};
-      /*
-      Disabled for now since fold state isn't preserved.
-      if (options0.foldGutter) {
-        options0.extraKeys["Ctrl-Q"] = cm => cm.foldCodeSelectionAware();
-        options0.gutters = ["CodeMirror-linenumbers", "CodeMirror-foldgutter"];
-      }
-      */
     } else {
       options0.readOnly = true;
     }