in bug 457587 we had to workaround an issue with OS X native menus, practically the binding is attached when a popup is opened, but we have to access to binding fields in the capturing phase of the popupshowing event. In the handler the binding is not attached yet, so the fields we try to access are undefined. This works correctly on other OSes so most likely due to lazy binding attachment on native OS X menus.
Suggestion from Josh: Is this related to, or maybe a dup of, bug 365405?
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