Why does \vspace*{0pt} add vertical space?

Question

I'm trying to format the title of the table of contents using the tocloft package. It inserts code that looks like the following.

\par
\vspace*{\cftbeforetoctitleskip}%
% code to typeset \contentsname

The \vspace* is adding unwanted space. Here's a minimal example demonstrating the problem.

\documentclass{article}
\usepackage{showframe}
\begin{document}
\vspace*{0pt}%
ASDF
\end{document}

Compare that to \vspace{0pt} (or simply omitting the \vspace line altogether).

The difference between \vspace and \vspace* is clear. With the star, it expands to

\dimen@\prevdepth
\hrule \@height\z@
\nobreak
\vskip#1%
\vskip\z@skip
\prevdepth\dimen@

and without the star, it expands to just the two \vskips.

Why is there the extra space? (And bonus question: Doesn't this make \vspace* sort of worthless since the star only has a use when TeX is discarding items?)

Edit:
I was able to fix my real problem (with the table of contents) by using \patchcmd from etoolbox.

Answer 1

To ensure that white space is produced even at points in the document where page breaking takes place or at the top or bottom of a page one should replace \vspace by \vspace*.

At the beginning of a page \topskip is inserted, normally \topskip=10pt. Setting topskip=0pt and \offinterlineskip will produce identical results both for vspace and vscpace*.

\documentclass{article}
\usepackage{showframe}
\begin{document}
\topskip=0pt  \offinterlineskip 
\vspace*{0pt}%
ASDF

\end{document}

\offinterlineskip is macro to prevent interline glue globally. Try running both examples to see the difference.

\documentclass{article}
\usepackage{showframe}
\begin{document}
\topskip=0pt  %\offinterlineskip 
\vspace*{0pt}%
ASDF
\end{document}