Why does adding `\par` here add vertical space in a different place?

Question

I've been playing around with LaTeX trying to really learn how spacing works in-depth. Here's a silly bit of code that I've been experimenting with:

\documentclass[]{article}

% Draw a raised \hrulefill
\newcommand{\hmidrule}[1]
    {\leavevmode \leaders
     \hbox{\rule[#1]{1pt}{.4pt}}
     \hfill \kern0pt }

% Draw a divider with a given ornament in the middle
\newcommand\divider[2][0.33em]
    {\par\noindent\hmidrule{#1}{#2}\hmidrule{#1}\par}

\begin{document}
\divider{\large\ A\ }
%%
{\hspace*{\fill}
    \huge {This is the First and Last Line.}
\hspace*{\fill}}  % <--- Keep this line in mind!
%%
\divider{\large\ $\Omega$\ }
\end{document}

This produces exactly the result I'd expect from this: minimal spacing on the top and bottom of the centered text.

Then I put \par on this line:

% ...
    \huge {This is the First and Last Line.}
\hspace*{\fill}\par}  % <--- Added `\par` here.
%%
% ...

And I expected that no change would occur, since ending this paragraph would cause nothing; the paragraph was already ending there, but I also thought that it could feasibly add some vertical space below the centered text.

However, this was the result: vertical space before the centered text!

I really don't understand why this is happening. Why would adding this command after the text cause spacing before it? Could it be that LaTeX trying to format this as paragraphed text? If so, why didn't it add space there before I added \par?

Any other feedback would be greatly appreciated.

I'm compiling my code on Overleaf, with pdfLaTeX. The same result occurs with XeTeX.

Answer 1

The difference is that with \par the paragraph is set with the baselineskip relative to \huge, whereas it isn't without it.

If you want to control precisely the spacing between the decorated rules and the title, the best is to disable the interline spacing at the transition, with \nointerlineskip.

I provided a different definition of \divider that avoids the need to guess a dimension.

\documentclass[]{article}

% Draw a divider with a given ornament in the middle
\newlength{\dividerht}
\newcommand{\hmidrule}{%
  \leaders\hrule
    height \dimexpr0.5\dividerht+0.2pt 
    depth  \dimexpr-0.5\dividerht+0.2pt
  \hfill\kern0pt
}
\newcommand\divider[1]{%
  \par\noindent
  \settoheight{\dividerht}{#1}%
  \par\noindent\hmidrule{#1}\hmidrule\par
}

\begin{document}

\divider{\large A}\nointerlineskip\vspace{6pt}
{\centering\huge This is the First and Last Line.\par}
\nointerlineskip\vspace{6pt}
\divider{\large $\Omega$}

\bigskip

\divider{\large A}\nointerlineskip\vspace{6pt}
{\centering\huge This is the First line and this title is so long that 
  it splits across lines.\par}
\nointerlineskip\vspace{6pt}
\divider{\large $\Omega$}

\end{document}

If you want that the bottom rule is spaced with respect to the baseline of the bottom line in the title, you need to remove the depth:

\divider{\large A}\nointerlineskip\vspace{6pt}
{\centering\huge This is the First line and this title is so long that 
  it splits across lines.\par}
\vspace{-\prevdepth}\nointerlineskip\vspace{6pt}
\divider{\large $\Omega$}