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Semantically speaking, although the <div> may contain elements that are present, unless an explicit height has been set in CSS for the <div> you wouldn't be able to see it nor any of the items (ie. they wouldn't be visible on screen).
Were we to not check for height being defined, we risk breaking standard CSS behavior by making toggle display content even when a height hasn't been defined.
In my opinion this is behavior which may be okay to define in a user/developer's own application but shouldn't be generalized to the core.
Please also remember to submit a working code example replicating the behavior you are submitting a bug fix or feature request for as this makes it easier for us to verify the behavior you're describing.