Yup. After opening the window, you can set a callback on the window's `load` event. But since there's no way to separate window construction and `window.open`, there's a race -- the event may fire *before* you set a callback.

Here's the best solution I could find: Set a timer to `postMessage`, but cancel it if you receive the `load` event:
http://stackoverflow.com/questions/896083/how-to-tell-whether-the-window-load-window-onload-event-has-already-fired