re #99399.

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

@@ -686,7 +686,9 @@
 	top: 0;
 	left: 0;
 	right: 0;
-	height: 100%;
+	width: 26px;
+	height: 26px;
+	cursor: pointer;
 }
 
 .monaco-workbench .notebookOverlay > .cell-list-container .notebook-folding-indicator .codicon {

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

@@ -345,7 +345,7 @@ export class MarkdownCellRenderer extends AbstractCellRenderer implements IListR
 		editorPart.style.display = 'none';
 
 		const innerContent = DOM.append(container, $('.cell.markdown'));
-		const foldingIndicator = DOM.append(focusIndicator, DOM.$('.notebook-folding-indicator'));
+		const foldingIndicator = DOM.append(container, DOM.$('.notebook-folding-indicator'));
 
 		const bottomCellContainer = DOM.append(container, $('.cell-bottom-toolbar-container'));
 		const betweenCellToolbar = disposables.add(this.createBetweenCellToolbar(bottomCellContainer, disposables, contextKeyService));