[css-transitions] interpolating animation of the 'order' property

The flex spec introduced the 'order' property as an animatable property.
This is supported and implemented in Gecko, but the implementation doesn't
interpolate the position of the element that is moving.

In the CSS Grid implementation, elements moved via a transition on the
'order' property jump from grid-area to grid-area. It would be ideal if we
could "slide" the elements instead of jumping them.

See https://bugzilla.mozilla.org/attachment.cgi?id=8734793 for a reduced
testcase.

What are some complicated cases that make this a fundamentally hard problem?

Thanks,
Jared