added TOP_WINDOW constant

lib/single-file/frame-tree.js

@@ -24,9 +24,14 @@ this.frameTree = this.frameTree || (() => {
 
 	const MESSAGE_PREFIX = "__frameTree__";
 	const TIMEOUT_INIT_REQUEST_MESSAGE = 500;
+	const TOP_WINDOW = window == top;
 
-	let sessionId = 0, sessions = new Map(), windowId;
+	let sessionId, sessions, windowId;
 
+	if (TOP_WINDOW) {
+		sessions = new Map();
+		sessionId = 0;
+	}
 	addEventListener("message", event => {
 		if (typeof event.data == "string" && event.data.startsWith(MESSAGE_PREFIX + "::")) {
 			const message = JSON.parse(event.data.substring(MESSAGE_PREFIX.length + 2));
@@ -64,7 +69,7 @@ this.frameTree = this.frameTree || (() => {
 	}
 
 	function initResponse(message) {
-		if (window == top) {
+		if (TOP_WINDOW) {
 			const windowData = sessions.get(message.sessionId);
 			if (windowData) {
 				message.framesData.forEach(messageFrameData => {