\hphantom behaving like \mbox, not like the counterpart of \vphantom

Question

I have the following figure as part of a slideshow I'm writing with beamer. If it is relevant, I've used pst-jtree (which is essentially a bunch of macros over pstricks) to produce the tree branches.

The code that produces the right-hand node is:

{multiline Sluicing Elicitation \cr $\underbracket{\textrm{Protocol}}_{typology}$ \endmultiline}

Note how the underbracket typology only goes the width of Protocol, which is visually unpleasant. Ideally, I'd like it to be width of the upper line Sluicing Elicitation. The following figure is what I'm aiming at (I've produced this figure by adding an mbox to either side of textrm{Protocol} in the line above and then manually adjusting the width of both mboxes until it looked good; clearly a very inefficient method).

I've tried to do this with hphantom{}, which should be the horizontal counterpart of vphantom{} ---i.e., it produces a box of zero height and the width of whatever is inside the brackets, and doesn't disturb the actual text. But if I write this

{multiline Sluicing Elicitation \cr $\underbracket{\hphantom{Sluicing Elicitation}\textrm{Protocol}}_{typology}$ \endmultiline}

I get this

which is effectively the equivalent of adding an mbox with the width of Sluicing Elicitation. The question I have is: am I using hphantom{} in the wrong way? And if not (i.e., if it is producing the result it is meant to produce), how can I get the result I want?

Because I've been in SE long enough to know that someone will say this: I guess I can get the result I want by putting the multiline text in a minipage and then underbracketing the minipage itself, or whatever. I'm asking this question despite knowing that this type of solutions exist because I don't just want to get this particular result; I also want to understand why I'm not getting there with hphantom.

Answer 1

Add \usepackage{calc} to your document preamble and use

\makebox[\widthof{Ellipsis Consortium}]{Protocol}

when you want the phantom. This makes a box as wide as “Ellipsis Consortium”, with “Protocol” centered in the available space.