How can I get scrollTop in iframe of tiny mce 3.4.3.1

When I us开发者_StackOverflowe tiny_mce_3_2_2. I can't get scrolltop of iframe in tiny by

document.getElementById("body_ifr").contentWindow.document.body.scrollTop;

When I updated to tiny_mce_3_4_3_1 I met a problem when getting scrollTop in IE(7,8,9), but it runs normally in Chrome and FF.

Please help repair this problem.

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A little late but:

tinymce.DOM.getViewPort(tinymce.activeEditor.getWin()).y