readonly cells should not support join and split

src/vs/workbench/contrib/notebook/browser/contrib/coreActions.ts

@@ -1333,7 +1333,7 @@ registerAction2(class extends NotebookAction {
 				category: NOTEBOOK_ACTIONS_CATEGORY,
 				menu: {
 					id: MenuId.NotebookCellTitle,
-					when: ContextKeyExpr.and(NOTEBOOK_EDITOR_FOCUSED, NOTEBOOK_CELL_TYPE.isEqualTo('code'), NOTEBOOK_EDITOR_EDITABLE, InputFocusedContext),
+					when: ContextKeyExpr.and(NOTEBOOK_EDITOR_FOCUSED, NOTEBOOK_CELL_TYPE.isEqualTo('code'), NOTEBOOK_EDITOR_EDITABLE, NOTEBOOK_CELL_EDITABLE, InputFocusedContext),
 					order: CellToolbarOrder.SplitCell
 				},
 				icon: { id: 'codicon/split-vertical' },

src/vs/workbench/contrib/notebook/browser/notebookEditorWidget.ts

@@ -896,6 +896,10 @@ export class NotebookEditorWidget extends Disposable implements INotebookEditor
 			return null;
 		}
 
+		if (!cell.metadata?.editable) {
+			return null;
+		}
+
 		let splitPoints = cell.getSelectionsStartPosition();
 		if (splitPoints && splitPoints.length > 0) {
 			await cell.resolveTextModel();
@@ -932,6 +936,10 @@ export class NotebookEditorWidget extends Disposable implements INotebookEditor
 			return null;
 		}
 
+		if (!cell.metadata?.editable) {
+			return null;
+		}
+
 		if (constraint && cell.cellKind !== constraint) {
 			return null;
 		}
@@ -951,6 +959,10 @@ export class NotebookEditorWidget extends Disposable implements INotebookEditor
 				return null;
 			}
 
+			if (!above.metadata?.editable) {
+				return null;
+			}
+
 			await above.resolveTextModel();
 			if (!above.hasModel()) {
 				return null;
@@ -971,6 +983,10 @@ export class NotebookEditorWidget extends Disposable implements INotebookEditor
 				return null;
 			}
 
+			if (!below.metadata?.editable) {
+				return null;
+			}
+
 			await cell.resolveTextModel();
 			if (!cell.hasModel()) {
 				return null;