Why are my boxes not the same height when specifying height as a function of width and content?

Question

• I save a box containing a \parbox.
• I set height of other \parboxes to the height of my saved box using TeX \the\ht\box

Code

For some reason, not all boxes are of the same size.

\documentclass{article}
\usepackage{fontspec}

\setlength\fboxsep{0pt}% perfectly size box around content
\setlength\fboxrule{.1pt}

\begin{document}

% Our goal is to specify the height dynamically as a function of the width and content of the box, which we set explicitly here:
\newsavebox{\tmpparbox}
\savebox{\tmpparbox}{\fbox{\parbox{40mm}{This parbox is top aligned with top-aligned content. And more text to fill it up.}}}

% I can use TeX or LaTeX to get the height of the saved box.
%\the\ht\tmpparbox % TeX

%\newlength{\tmpparboxht} % LaTeX
%\settoheight{\tmpparboxht}{\usebox{\tmpparbox}}

Here is the \string\savebox: \usebox{\tmpparbox}
\bigbreak

Target height: \the\ht\tmpparbox

\kern20pt\hrule\kern20pt

This is not a parbox. \fbox{\parbox[t][\the\ht\tmpparbox][t]{40mm}{This parbox is top aligned with top-aligned content. And more text to fill it up.}}
\bigbreak
This is not a parbox. \fbox{\parbox[b][\the\ht\tmpparbox][t]{40mm}{This parbox is bottom aligned with top-aligned content. And more text to fill it up.}}
\bigbreak
This is not a parbox. \fbox{\parbox[t][\the\ht\tmpparbox][c]{40mm}{This parbox is top aligned with center-aligned content. And more text to fill it up.}}
\bigbreak
This is not a parbox. \fbox{\parbox[t][\the\ht\tmpparbox][b]{40mm}{This parbox is top aligned with bottom-aligned content. And more text to fill it up.}}
\bigbreak
This is not a parbox. \fbox{\parbox[c][\the\ht\tmpparbox][t]{40mm}{This parbox is center aligned with top-aligned content. And more text to fill it up.}}
\bigbreak

\end{document}

Answer 1

The main error is that , as your image shows, the box you save is vertically centred not top aligned so by using \ht you are getting around half the required value. You could use \ht plus \dp but that would in fact be too much as what you need to specify for the height argument to \parbox is the target height of the text, not including the depth of the final row beyond the final baseline.

So best would be to use a \parbox[b] to save the box (and don't use \fbox as that would hide the last real row of text) and then use \ht of the saved box.