diff --git a/atom/browser/lib/guest-window-manager.coffee b/atom/browser/lib/guest-window-manager.coffee
index f92c1a46c05..861b0d10aa3 100644
--- a/atom/browser/lib/guest-window-manager.coffee
+++ b/atom/browser/lib/guest-window-manager.coffee
@@ -57,13 +57,19 @@ ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', (event, guestId, me
   if guestContents?.getUrl().indexOf(targetOrigin) is 0 or targetOrigin is '*'
     guestContents.send 'ATOM_SHELL_GUEST_WINDOW_POSTMESSAGE', message, targetOrigin
 
-ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_OPENER_POSTMESSAGE', (event, message, targetOrigin) ->
+ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_OPENER_POSTMESSAGE', (event, guestId, message, targetOrigin, sourceOrigin) ->
   embedder = v8Util.getHiddenValue event.sender, 'embedder'
   if embedder?.getUrl().indexOf(targetOrigin) is 0 or targetOrigin is '*'
-    embedder.send 'ATOM_SHELL_GUEST_WINDOW_POSTMESSAGE', message, targetOrigin
+    embedder.send 'ATOM_SHELL_GUEST_WINDOW_POSTMESSAGE', guestId, message, sourceOrigin
 
 ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WEB_CONTENTS_METHOD', (event, guestId, method, args...) ->
   BrowserWindow.fromId(guestId)?.webContents?[method] args...
 
-ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_IS_GUEST_WINDOW', (event) ->
-  event.returnValue = v8Util.getHiddenValue(event.sender, 'embedder') isnt undefined
+ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_GET_GUEST_ID', (event) ->
+  embedder = v8Util.getHiddenValue event.sender, 'embedder'
+  if embedder?
+    guest = BrowserWindow.fromWebContents event.sender
+    if guest?
+      event.returnValue = guest.id
+      return
+  event.returnValue = null
diff --git a/atom/renderer/lib/override.coffee b/atom/renderer/lib/override.coffee
index a413178442f..b60374e0475 100644
--- a/atom/renderer/lib/override.coffee
+++ b/atom/renderer/lib/override.coffee
@@ -94,14 +94,22 @@ window.confirm = (message, title='') ->
 window.prompt = ->
   throw new Error('prompt() is and will not be supported.')
 
-# Simple implementation of postMessage.
-if ipc.sendSync 'ATOM_SHELL_GUEST_WINDOW_MANAGER_IS_GUEST_WINDOW'
+# Implement window.postMessage if current window is a guest window.
+guestId = ipc.sendSync 'ATOM_SHELL_GUEST_WINDOW_MANAGER_GET_GUEST_ID'
+if guestId?
   window.opener =
     postMessage: (message, targetOrigin='*') ->
-      ipc.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_OPENER_POSTMESSAGE', message, targetOrigin
+      ipc.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_OPENER_POSTMESSAGE', guestId, message, targetOrigin, location.origin
 
-ipc.on 'ATOM_SHELL_GUEST_WINDOW_POSTMESSAGE', (message, targetOrigin) ->
-  window.postMessage message, targetOrigin
+ipc.on 'ATOM_SHELL_GUEST_WINDOW_POSTMESSAGE', (guestId, message, sourceOrigin) ->
+  # Manually dispatch event instead of using postMessage because we also need to
+  # set event.source.
+  event = document.createEvent 'Event'
+  event.initEvent 'message', false, false
+  event.data = message
+  event.origin = sourceOrigin
+  event.source = new BrowserWindowProxy(guestId)
+  window.dispatchEvent event
 
 # Forward history operations to browser.
 sendHistoryOperation = (args...) ->