[Tex/LaTex] Why \nointerlineskip if background of Beamer-color frametitle is empty

beamercolorsource

I'm trying to read the source of Beamer default outer theme (the file beamerouterthemedefault.sty). In line 144 there is this code:

\defbeamertemplate*{frametitle}{default}[1][left]
{
  \ifbeamercolorempty[bg]{frametitle}{}{\nointerlineskip}%
  % more things here
}

My question is simple: what is the purpose of this \nointerlineskip, and how is this connected to the background of the frametitle Beamer-color?

Edit: I checked on the repository on Bitbucket, and couldn't find any hints there (I hoped for some enlightening commit message found through "blame" – but no, nothing.)

Best Answer

In the definition of beamer@frameslide environment in beamerbaseframe.sty, there is an empty vbox above the frametitle:

\ifx\beamer@frametitle\@empty%
  \setbox\beamer@frametitlebox=\box\voidb@x%
\else%
  \setbox\beamer@frametitlebox=\vbox{
    \vbox{}%
    {\parskip0pt\usebeamertemplate***{frametitle}\vskip0.25em}%
  }%
\fi%

Now assume we remove \nointerlineskip line from the template code for frametitle, then there will be vertical space between the empty vbox and frametitle:

.\vbox(0.0+0.0)x0.0
.\glue(\lineskip) 1.0
.\hbox(19.136+0.0)x324.79259

The vertical space comes from \lineskip, since by default the height of the frametitle box is larger than \baselineskip.

When the backgoround color of the frametitle is empty, this extra vertical space is usually unobtrusive. But if the background color exists, and furthermore if the headline also exists, this space looks redundant：

enter image description hereenter image description here

To get rid of this extra space, we may also patch beamer@frameslide, removing the extra vbox{}.

enter image description here

\documentclass{beamer}
\usepackage{xpatch}
\usetheme{Copenhagen}
\begin{document}

\section{First Section}

\begin{frame}
\frametitle{Default frame title}
\end{frame}

\makeatletter
% remove \nointerlineskip in frametitle template
\xpatchcmd{\beamer@@tmpl@frametitle}{\nointerlineskip}{\relax}{}{}
\makeatother

\begin{frame}
\frametitle{Frame title without nointerlineskip}
\end{frame}

\makeatletter
% undo the above patch
%\xpatchcmd{\beamer@@tmpl@frametitle}{\relax}{\nointerlineskip}{}{}
% patch in another way: modify beamer@frameslide environment
\xpatchcmd{\endbeamer@frameslide}{\vbox{}}{}{}{}
\makeatother

\begin{frame}
\frametitle{Frame title without empty vbox}
\end{frame}

\end{document}
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