Track visited parents and null out cycles

lib/browser/guest-window-manager.js

@@ -7,19 +7,25 @@ const hasProp = {}.hasOwnProperty
 const frameToGuest = {}
 
 // Copy attribute of |parent| to |child| if it is not defined in |child|.
-const mergeOptions = function (child, parent) {
-  let key, value
-  for (key in parent) {
+const mergeOptions = function (child, parent, visited) {
+  // Check for circular reference.
+  if (visited == null) visited = new Set()
+  if (visited.has(parent)) return
+
+  visited.add(parent)
+  for (const key in parent) {
     if (!hasProp.call(parent, key)) continue
-    value = parent[key]
-    if (!(key in child)) {
-      if (typeof value === 'object') {
-        child[key] = mergeOptions({}, value)
-      } else {
-        child[key] = value
-      }
+    if (key in child) continue
+
+    const value = parent[key]
+    if (typeof value === 'object') {
+      child[key] = mergeOptions({}, value, visited)
+    } else {
+      child[key] = value
     }
   }
+  visited.delete(parent)
+
   return child
 }