diff --git a/extensions/vscode-notebook-tests/src/notebook.test.ts b/extensions/vscode-notebook-tests/src/notebook.test.ts
index 062c55cb1cb..d512d5e0ff0 100644
--- a/extensions/vscode-notebook-tests/src/notebook.test.ts
+++ b/extensions/vscode-notebook-tests/src/notebook.test.ts
@@ -328,7 +328,7 @@ suite('metadata', () => {
 
 		await waitFor(500);
 		assert.equal(vscode.notebook.activeNotebookEditor !== undefined, true, 'notebook first');
-		assert.equal(vscode.notebook.activeNotebookEditor!.document.metadata.custom['testMetadata'] as boolean, false);
+		assert.equal(vscode.notebook.activeNotebookEditor!.document.metadata.custom!['testMetadata'] as boolean, false);
 		assert.equal(vscode.notebook.activeNotebookEditor!.selection?.metadata.custom!['testCellMetadata'] as number, 123);
 		assert.equal(vscode.notebook.activeNotebookEditor!.selection?.language, 'typescript');
 	});
diff --git a/src/vs/vscode.proposed.d.ts b/src/vs/vscode.proposed.d.ts
index 48556963886..ddc152deb05 100644
--- a/src/vs/vscode.proposed.d.ts
+++ b/src/vs/vscode.proposed.d.ts
@@ -1642,9 +1642,9 @@ declare module 'vscode' {
 		displayOrder?: GlobPattern[];
 
 		/**
-		 * Additional attributes of a cell metadata.
+		 * Additional attributes of the document metadata.
 		 */
-		[key: string]: any;
+		custom?: { [key: string]: any };
 	}
 
 	export interface NotebookDocument {
@@ -1685,7 +1685,9 @@ declare module 'vscode' {
 		 * The primary selected cell on this notebook editor.
 		 */
 		readonly selection?: NotebookCell;
+
 		viewColumn?: ViewColumn;
+
 		/**
 		 * Fired when the output hosting webview posts a message.
 		 */
diff --git a/src/vs/workbench/contrib/notebook/browser/notebookEditor.ts b/src/vs/workbench/contrib/notebook/browser/notebookEditor.ts
index 93173d625e2..c6e063464b0 100644
--- a/src/vs/workbench/contrib/notebook/browser/notebookEditor.ts
+++ b/src/vs/workbench/contrib/notebook/browser/notebookEditor.ts
@@ -124,6 +124,7 @@ export class NotebookEditor extends BaseEditor {
 	}
 
 	clearInput(): void {
+		// de-ref widget
 		super.clearInput();
 	}
 
diff --git a/src/vs/workbench/contrib/notebook/browser/notebookEditorInput.ts b/src/vs/workbench/contrib/notebook/browser/notebookEditorInput.ts
index af58a89697b..3fc6eb2d78d 100644
--- a/src/vs/workbench/contrib/notebook/browser/notebookEditorInput.ts
+++ b/src/vs/workbench/contrib/notebook/browser/notebookEditorInput.ts
@@ -119,6 +119,7 @@ export class NotebookEditorInput extends EditorInput {
 		return this._move(group, target)?.editor;
 	}
 
+	// called when users rename a notebook document
 	move(group: GroupIdentifier, target: URI): IMoveResult | undefined {
 		if (this.textModel) {
 			const contributedNotebookProviders = this.notebookService.getContributedNotebookProviders(target);