Why longtable adds vertical skip after head on subsequent pages and how to fix this?

Question

Why longtable adds vertical skip after head on subsequent pages and how to fix this?

\documentclass{article}
\usepackage[a6paper,margin=4mm]{geometry}
\usepackage{booktabs,caption,longtable}
\usepackage{lipsum}
\usepackage{tikz}
\pagestyle{empty}
\parindent=0pt
\begin{document}

\begin{longtable}{rp{8cm}}
\caption{Example table}
\\\toprule
\textbf{P} & \textbf{Text}
\\\toprule
\endfirsthead

\caption[]{Example table (continued)}
\\\toprule
\textbf{P} & \textbf{Text}
\\\toprule
\endhead

\multicolumn{2}{r}{\footnotesize continued on the next page}
\endfoot

\bottomrule
\endlastfoot

1 & \lipsum[1]\vspace{-\baselineskip}
\\*\midrule
2 & \lipsum[2]\vspace{-\baselineskip}
\\*\midrule
3 & \lipsum[3]\vspace{-\baselineskip}
\\*\midrule
4 & \lipsum[4]\vspace{-\baselineskip}
\end{longtable}

\begin{tikzpicture}[remember picture, overlay, color=red, line width=0.2pt]
\draw ([yshift=-18.9mm]current page.north west) -- ([yshift=-18.9mm]current page.north east);
\draw ([yshift=-19.9mm]current page.north west) -- ([yshift=-19.9mm]current page.north east);
\end{tikzpicture}%

\end{document}

Above document and images of its pages clearly show the problem (I've used TikZ to stress it).

Answer 1

It's the space after a \midrule; you can cancel it by introducing an explicit \addlinespace:

\begin{longtable}{rp{8cm}}
\caption{Example table}
\\\toprule
\textbf{P} & \textbf{Text}
\\\midrule[\heavyrulewidth]
\endfirsthead

\caption[]{Example table (continued)}
\\\toprule
\textbf{P} & \textbf{Text}
\\\midrule[\heavyrulewidth]
\addlinespace[0pt]
\endhead

\multicolumn{2}{r}{\footnotesize continued on the next page}
\endfoot

\bottomrule
\endlastfoot

1 & \lipsum*[1]
\\\midrule
2 & \lipsum*[2]
\\\midrule
3 & \lipsum*[3]
\\\midrule
4 & \lipsum*[4]
\end{longtable}

Notice that \midrule[\heavyrulewidth] is preferable to a \toprule in that position; \lipsum* avoids a final \par command, so that it's not necessary to backspace.