If I go to https://secure.phabricator.com/D13363?vs=32339&id=32359 on a mobile device, I am presented with a side-by-side diff. Going to https://secure.phabricator.com/D13363 shows a unified diff by default.
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At HEAD, I can't reproduce this:
The decision to default to unified is based only on the window size. I think we fixed some bugs with this a while ago, so D15135 may have resolved this. Yell if you're still seeing issues.