Vertical space before and after hrule in tabular tables

Question

Consider this example:

\documentclass{article}
\begin{document}
\begin{tabular}{|c|}
  \downbracefill\\\hline
  text\\\hline
  \upbracefill
\end{tabular}
\end{document}

In the produced document, the vertical spacing between downbracefill and the first hline is not the same as the one between the second hline and upbracefill. This can also be reproduced with \hrulefill instead of \downbracefill and \upbracefill.

I suppose this has to do with the placement of the baselines.

How can I control the vertical space before and after hrule to make them equal in situations like in the given example?

Answer 1

The up/down-bracefill is only a component (it consists from three sub-components but it is not important now) used in the plain TeX macro \overbrace, \underbrace. This macro adds it over/under the given text without any vertical skips, the result is box over box. So, there is not important, how the up/down-bracefill is vertically positioned in respect to a general strut or to parentheses. The vertical position of these components chosen by D. Knuth is somewhat lower than you need. You can try to typeset:

(\downbracefill), (\upbracefill)

and you will see. D. Knuth didn't need to solve vertical position more accurately and we can only guess why the position was chosen as it was.

Edit: If you want to have horizontally stretchable \upbracefill and moreover to have vertical rules in the table, then you can try to apply affine transformation (shifting) before \upbracefill and then apply the inverse transformation:

\def\upbracecorr{3.5 }
\def\xupbracefill{\pdfliteral{1 0 0 1 0  \upbracecorr cm}%
   \upbracefill   \pdfliteral{1 0 0 1 0 -\upbracecorr cm}}

\begin{tabular}{|c|}
  \downbracefill\\\hline
  text text text\\\hline
  \xupbracefill
\end{tabular}