I see. _HasDropDown is calling Calendar.focus(), but it isn't working. Rather, focus is going to the top of the page, to do text selection. Actually you can see a similar problem in test_Calendar.html. After pressing F7 you cannot tab into the Calendar widget, or to any other field for that matter.
I'm not sure why programmatic focus isn't working for the DateTextBox case when it works in a simple test case like running:
setTimeout(function(){ dojo.byId("after").focus(); }, 5000);
in test_Calendar.html. But regardless, I doubt there's anything that dijit can practically do to workaround this behavior. One obvious choice would be to not try to focus the drop down Calendar to begin with, or to not close the drop down when focus is shifted somewhere else, but that would completely break the accessibility of the page.
