diff --git a/compat/src/portals.js b/compat/src/portals.js
index c480a3d3a3..b48f2356f5 100644
--- a/compat/src/portals.js
+++ b/compat/src/portals.js
@@ -39,7 +39,7 @@ function Portal(props) {
 			nodeType: 1,
 			parentNode: container,
 			childNodes: [],
-			contains: () => true,
+			isPortal: true,
 			// Technically this isn't needed
 			appendChild(child) {
 				this.childNodes.push(child);
diff --git a/src/diff/children.js b/src/diff/children.js
index 5053a51174..784476fdca 100644
--- a/src/diff/children.js
+++ b/src/diff/children.js
@@ -7,7 +7,7 @@ import {
 	MATCHED,
 	UNDEFINED
 } from '../constants';
-import { isArray } from '../util';
+import { isArray, contains } from '../util';
 import { getDomSibling } from '../component';
 
 /**
@@ -339,7 +339,7 @@ function insert(parentVNode, oldDom, parentDom) {
 
 		return oldDom;
 	} else if (parentVNode._dom != oldDom) {
-		if (oldDom && parentVNode.type && !parentDom.contains(oldDom)) {
+		if (oldDom && parentVNode.type && !contains(parentDom, oldDom)) {
 			oldDom = getDomSibling(parentVNode);
 		}
 		parentDom.insertBefore(parentVNode._dom, oldDom || null);
diff --git a/src/util.js b/src/util.js
index 647519175f..1907e525b9 100644
--- a/src/util.js
+++ b/src/util.js
@@ -25,4 +25,23 @@ export function removeNode(node) {
 	if (node && node.parentNode) node.parentNode.removeChild(node);
 }
 
+/**
+ * Checks whether a given node is contained within a specified parent node's subtree.
+ * This function is a workaround for Internet Explorer 11's limited support for
+ * `Element.prototype.contains()`, which works only for DOM elements and does not
+ * support text nodes.
+ *
+ * @param {import('./internal').PreactElement} parent - The parent node within which to search for the `node`.
+ * @param {import('./index').ContainerNode} node - The node to check for containment within the `parent` subtree.
+ */
+export function contains(parent, node) {
+	if ('isPortal' in parent) return true;
+
+	while (node) {
+		if (node === parent) return true;
+		node = node.parentNode;
+	}
+	return false;
+}
+
 export const slice = EMPTY_ARR.slice;