on OSX the topLevelMenuIndex seems to be wrong when using native mac menus. The index seems to be the index of the menu item within the pulldown menu, not the index of its parent menu in the menu bar.
Happens when OSX platform-specific code calls the MenuBarModelcallback :
MenuBarModel::menuItemSelected (int menuItemID, int topLevelMenuIndex)
Apart from this there were amazingly few other problems when porting my first windows app to OSX. I love juce!