Enable cell folding all the time

Fix #132800

src/vs/workbench/contrib/notebook/browser/media/notebook.css

@@ -908,3 +908,7 @@
 .hc-black .notebookOverlay .monaco-list.selection-multiple:focus-within .monaco-list-row.selected:not(.focused) .cell-inner-container:not(.cell-editor-focus) .cell-focus-indicator-left:before { border-left-style: dotted; }
 .hc-black .notebookOverlay .monaco-list.selection-multiple:focus-within .monaco-list-row.selected:not(.focused) .cell-inner-container:not(.cell-editor-focus) .cell-focus-indicator-right:before  { border-right-style: dotted; }
 
+.monaco-workbench .notebookOverlay .cell-editor-container .monaco-editor .margin-view-overlays .codicon-folding-expanded,
+.monaco-workbench .notebookOverlay .cell-editor-container .monaco-editor .margin-view-overlays .codicon-folding-collapsed {
+	margin-left: 0;
+}

src/vs/workbench/contrib/notebook/browser/view/renderers/cellEditorOptions.ts

@@ -38,7 +38,7 @@ export class CellEditorOptions extends Disposable {
 		selectOnLineNumbers: false,
 		lineNumbers: 'off',
 		lineDecorationsWidth: 0,
-		folding: false,
+		folding: true,
 		fixedOverflowWidgets: true,
 		minimap: { enabled: false },
 		renderValidationDecorations: 'on',
@@ -106,7 +106,7 @@ export class CellEditorOptions extends Disposable {
 		const computed = {
 			...editorOptions,
 			...CellEditorOptions.fixedEditorOptions,
-			... { lineNumbers, folding: lineNumbers === 'on' },
+			... { lineNumbers },
 			...editorOptionsOverride,
 			...{ padding: { top: 12, bottom: 12 } },
 			readOnly: this.notebookEditor.isReadOnly
@@ -139,10 +139,8 @@ export class CellEditorOptions extends Disposable {
 			const renderLiNumbers = this.configurationService.getValue<'on' | 'off'>('notebook.lineNumbers') === 'on';
 			const lineNumbers: LineNumbersType = renderLiNumbers ? 'on' : 'off';
 			this._value.lineNumbers = lineNumbers;
-			this._value.folding = lineNumbers === 'on';
 		} else {
 			this._value.lineNumbers = lineNumbers as LineNumbersType;
-			this._value.folding = lineNumbers === 'on';
 		}
 		this._onDidChange.fire();
 	}