Overfull \vbox (too high) warning on title page: fixed-height minipage uses \paperheight

[wenje-ma]

Bug description

Compiling the theme's minimal example with a recent Beamer produces a persistent

Overfull \vbox (15.63992pt too high) detected at line 8

warning on the title page. The warning is content- and font-independent (identical value for English and Chinese / ctexbeamer input), which points to the template structure rather than the user's document.

Root cause

In source/beamerinnerthememetropolis.dtx the title page template opens a fixed-height minipage:

\begin{minipage}[b][\paperheight]{\textwidth}

The actual usable height of the assembled title frame is ~15.6 pt smaller than \paperheight, so the vbox always overflows by that fixed amount.

Reproduction

\documentclass{beamer}
\usetheme{metropolis}
\title{A minimal example}
\date{\today}
\author{Matthias Vogelgesang}
\institute{Centre for Modern Beamer Themes}
\begin{document}
\maketitle
\end{document}

Compile with xelatex / lualatex, recent TeX Live or MiKTeX + current Beamer.

Suggested fix

Reserve the fixed margin by shrinking the minipage height:

\begin{minipage}[b][\dimexpr\paperheight-16pt\relax]{\textwidth}

Verified to remove the warning for both the English example and a ctexbeamer Chinese example, with no visible change to the rendered title page.

Environment: Windows, MiKTeX, xelatex, current Beamer.