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I don't think we should leave something as important as selection behavior completely undefined. On the other hand, the selection behavior is currently underspec'ed and defining the selection's behavior in general is outside the scope of the shadow DOM specification. However, the non-normative text that follows "here's one possible, admittedly naive way" is actively harmful since it's contradicting the discussion following http://lists.w3.org/Archives/Public/public-webapps/2013OctDec/0106.html
I think it makes sense to leave selection handling undefined given that effectively it's undefined on the web. I.e. the specs that we have for selection handling doesn't actually reflect reality, and it's not clear that there's consensus to implement them. I do tend to agree though that including a non-normative suggestion which I believe two vendors have stated they do not think is a good suggestion doesn't seem terribly useful though, and will likely cause more confusion than have any positive impact. I can't say I feel super strongly though.
*** This bug has been marked as a duplicate of bug 15444 ***