You don't need control over Window B, you can use something like this:
Code:
<html>
<head>
<script type="text/javascript">
var googleWin;
var googleTimeout;
var googleWait = 500;
function openGoogle() {
googleWin = window.open("http://www.google.com","googleWindow");
checkGoogleOpen();
}
function checkGoogleOpen() {
if (googleWin && googleWin.closed) {
alert("Window was closed, do something");
window.clearTimeout(googleTimeout);
} else {
window.setTimeout(checkGoogleOpen, googleWait);
}
}
</script>
</head>
<body>
<button onclick="openGoogle();">Google</button>
</body>
</html>
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