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Two borders next to each other are ugly, and border-collapse does not work for divs. You can hide one border, but when the border changes on hover for instance that is fugly. See the following fiddle for the trick to solve it.
Joining controls, Bulma style
See this fiddle.
In DomUI I want something else: controls like Text2<> and DateInput2 have "internal" buttons. These must be inside the encapsulating div of the control so that the whole thing stays together. These buttons must join like has-addons. But the whole thing itself must also be joinable outside. See this fiddle for the experiment with two levels of joining (incomplete).