The spec has been recently updated as you can see in the following discussion:
The main issue here is that we shouldn't take into account the collapsed tracks for gutters when we do the Content Distribution.
This has been already fixed in Blink:
Created attachment 311060 [details]
Comment on attachment 311060 [details]
Clearing flags on attachment: 311060
Committed r217345: <http://trac.webkit.org/changeset/217345>
All reviewed patches have been landed. Closing bug.