diff --git a/src/vs/workbench/contrib/notebook/browser/view/renderers/webviewPreloads.ts b/src/vs/workbench/contrib/notebook/browser/view/renderers/webviewPreloads.ts
index 0a59c685ad0..7303ddef0dd 100644
--- a/src/vs/workbench/contrib/notebook/browser/view/renderers/webviewPreloads.ts
+++ b/src/vs/workbench/contrib/notebook/browser/view/renderers/webviewPreloads.ts
@@ -401,6 +401,17 @@ async function webviewPreloads(ctx: PreloadContext) {
 		});
 	}
 
+	function isAncestor(testChild: Node | null, testAncestor: Node | null): boolean {
+		while (testChild) {
+			if (testChild === testAncestor) {
+				return true;
+			}
+			testChild = testChild.parentNode;
+		}
+
+		return false;
+	}
+
 	function _internalHighlightRange(range: Range, tagName = 'mark', attributes = {}) {
 		// derived from https://github.com/Treora/dom-highlight-range/blob/master/highlight-range.js