Good news.  As of e39b48e, this bug has gone away.

On Tue, Apr 30, 2013 at 04:03:41AM -0700, Jörn Zaefferer wrote:
>> @@ -445,25 +392,29 @@ $.widget( "ui.menubar", {
>>  			.removeAttr("aria-hidden")
>>  			.attr("aria-expanded", "true")
>>  			.menu("focus", event, menu.children(".ui-menu-item").first() )
>> -			// TODO need a comment here why both events are triggered
>> -			.focus()
>> -			.focusin();
>> +			.focus() // Establish focus on the submenu item
>> +			.focusin(); // Move focus within the submenu containing the above item
>
>I still don't understand why we need both calls. Seems wrong.
>
>---
>Reply to this email directly or view it on GitHub:
>https://github.com/jquery/jquery-ui/pull/953/files#r4016315