The current spec text is "Similar to table cells, flexbox items respond specially to the ‘collapse’ value of the ‘visibility’ property. When a flexbox item is set to ‘visibility: collapse;’, the item does not generate a box at all (identical to setting ‘display: none;’)."
There's a bit of controversy over whether this is the right thing to do: http://lists.w3.org/Archives/Public/www-style/2011Oct/0136.html. If we could get away with changing visibility:collapse across the board without compat problems, that would clearly be best.
The spec has changed to not mention visibility:collapse at all, which means that we should treat it as visibility:hidden. I expect this just works right now, but we should add some tests to verify.
The spec has changed to require special visibility:collapse behavior. http://dev.w3.org/csswg/css3-flexbox/#visibility-collapse
I don't especially love this behavior, but I don't care enough to argue it.
Here's a test case:
Basically, the middle div should have 0 width, but the height of the flexbox should be unchanged. Currently, it creates width.