onbeforeunload is fired whenever dialog tabs are being switched

[martinkou]

To reproduce:

1. Open replacebyclass.html in IE.
2. Execute

   javascript: void(window.onbeforeunload=function(){alert('a');});
   

   in the URL bar.
3. Open any dialog with tabs (e.g. Find/Replace dialog).
4. Change to another tab in the dialog.
5. The onbeforeunload event is fired, incorrectly.

This bug is related to #3114 and is a known browser bug: ​http://msdn.microsoft.com/en-us/library/ms536907%28VS.85%29.aspx

[martinkou]

Fixed with [3506].

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