Position: Documentation for flip collision says it falls back to center

From the documentation of position/options/collision:

flip: to the opposite side and the collision detection is run again to see if it will fit.
If it won't fit in either position, the center option should be used as a fall back.

While I am not entirely sure what "the center option" is, I assume it means that it should be positioned in the middle of the element instead of default position. This doesn't seem to work. Another option would be that "fit" is the fallback for "flip", since this would make sure that the elements is visible if possible.

Testcase:

http://jqueryui.com/demos/position/default.html

my: left,bottom

at: left,top

collision: flip

Now try resizing the browser window so it can't fit over OR under. There seems to be no fallback at all.