fix #6792 -- persist code folding state

Author: williamstein

src/packages/frontend/codemirror/util.ts

@@ -2,7 +2,7 @@ import { set, get, del } from "@cocalc/frontend/misc/local-storage-typed";
 import { isEqual } from "lodash";
 
 export function getFoldedLines(cm): number[] {
-  if (cm.foldCode == null) {
+  if (cm?.foldCode == null) {
     // not enabled
     return [];
   }
@@ -13,7 +13,7 @@ export function getFoldedLines(cm): number[] {
 }
 
 export function setFoldedLines(cm, lines: number[]) {
-  if (cm.foldCode == null) {
+  if (cm?.foldCode == null) {
     // not enabled
     return;
   }

src/packages/frontend/frame-editors/code-editor/codemirror-editor.tsx

@@ -36,6 +36,7 @@ import { GutterMarkers } from "./codemirror-gutter-markers";
 import { Actions } from "./actions";
 import { EditorState } from "../frame-tree/types";
 import { Path } from "../frame-tree/path";
+import { initFold, saveFold } from "@cocalc/frontend/codemirror/util";
 
 const STYLE = {
   width: "100%",
@@ -290,6 +291,16 @@ export const CodemirrorEditor: React.FC<Props> = React.memo((props) => {
     }
     cmRef.current.setOption("readOnly", props.read_only);
     cm_refresh();
+
+    const foldKey = `${props.path}\\${props.id}`;
+    const saveFoldState = () => {
+      if (cmRef.current != null) {
+        saveFold(cmRef.current,foldKey);
+      }
+    };
+    cmRef.current.on("fold" as any, saveFoldState);
+    cmRef.current.on("unfold" as any, saveFoldState);
+    initFold(cmRef.current, foldKey);
   }
 
   function init_new_codemirror(): void {