Position option in jQuery UI menu: y axis can only be centered (top/bottom don't work). (Bug?)

Position option in jQuery UI menu: y axis can only be centered (top/bottom don't work). (Bug?)

I'm using jQuery-ui 1.11.4.  I've set up a menu to run horizontally across the top of the screen (by floating top-level li's to the left).  The submenu default position is "my top left at top right".  I want it to be "my top right at bottom right", so it won't interfere with movement in the top-level menu.  When I specify this (or any other value), the y-axis is always "center".  I've tried using various pixel adjustments as well, with no change. 

Sounds suspiciously like a value is being set to zero when it shouldn't be, so I smell a bug.  Here's a  jsFiddle and here's my code:
[code]
    $('#topMenu').menu({
        select: function( event, ui ) { alert('selected ' + ui.item.text);
        },        
        position: {
            my: 'right-top',
            at: 'right-bottom'
        },
        icons: {
            submenu: 'ui-icon-triangle-1-s'
        }
    });
[/code]
I've also tried using previous versions with the same result, as far back as 1.8.0 (although that one had other problems as well).  Can anyone show me why this isn't a bug?