avoid top toolbar overflow

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

@@ -26,6 +26,10 @@
 	z-index: 100;
 }
 
+.monaco-workbench .notebookOverlay .cell-list-container {
+	overflow: hidden;
+}
+
 .monaco-workbench .notebookOverlay .cell-list-container .overflowingContentWidgets > div {
 	z-index: 600 !important;
 	/* @rebornix: larger than the editor title bar */

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

@@ -1678,6 +1678,7 @@ export class NotebookEditorWidget extends Disposable implements INotebookEditor
 
 		this._localStore.clear();
 		this._list?.dispose();
+		this._listTopCellToolbar?.dispose();
 
 		this._overlayContainer.remove();
 		this.viewModel?.dispose();

src/vs/workbench/contrib/notebook/test/testNotebookEditor.ts

@@ -33,6 +33,7 @@ import { IConfigurationService } from 'vs/platform/configuration/common/configur
 import { TestConfigurationService } from 'vs/platform/configuration/test/common/testConfigurationService';
 import { IThemeService } from 'vs/platform/theme/common/themeService';
 import { TestThemeService } from 'vs/platform/theme/test/common/testThemeService';
+import { ScrollEvent } from 'vs/base/common/scrollable';
 
 export class TestCell extends NotebookCellTextModel {
 	constructor(
@@ -69,6 +70,7 @@ export class TestNotebookEditor implements INotebookEditor {
 	multipleKernelsAvailable: boolean = false;
 	onDidChangeAvailableKernels: Event<void> = new Emitter<void>().event;
 	onDidChangeActiveCell: Event<void> = new Emitter<void>().event;
+	onDidScroll = new Emitter<ScrollEvent>().event;
 
 	uri?: URI | undefined;
 	textModel?: NotebookTextModel | undefined;