Beamer Metropolis theme overfull vbox error on title page

Question

I'm trying to make slides for a course I'm teaching this coming semester using beamer and I have a nagging issue with an overfull vbox that I can't diagnose when using the metropolis theme. Here is the minimal example. Note, I'm using pdflatex (for microtype features):

% !TEX program = pdflatex
% !TEX spellcheck = en_US

\documentclass[aspectratio=169]{beamer}

% Font Settings

\usepackage[english]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[autostyle,english=american]{csquotes}
\usepackage[sfdefault,scaled=0.85,lf]{FiraSans}
\usepackage[scaled=0.85]{FiraMono}
\usepackage{newtxsf}
\usepackage[protrusion,expansion,kerning,spacing,final]{microtype}
\microtypecontext{spacing=nonfrench}

% PDF Bookmarks

\usepackage{bookmark}

% Beamer Setup

\usetheme{metropolis}
\usecolortheme{owl}

% AMS Math Packages

\usepackage{amsmath}

\title{A minimal example}
\date{\today}
\author{Matthias Vogelgesang}
\institute{Centre for Modern Beamer Themes}
\begin{document}
    \maketitle %<-- here is where i get an "Overfull \vbox (15.63992pt too high)"
    \section{First Section}
    \begin{frame}{First Frame}
        Hello, world!
    \end{frame}
\end{document}

I basically pulled this example from the package documentation but included my own preamble. To my knowledge I have the current version of all packages except for babel (which is at 3.18 on my system).

Answer 1

The theme is not very careful regarding spurious spaces. If one removes the one from the titlepage definition the warning disappears (but there are more ...):

\documentclass[aspectratio=169]{beamer}

% Font Settings

\usepackage[english]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[autostyle,english=american]{csquotes}
\usepackage[sfdefault,scaled=0.85,lf]{FiraSans}
\usepackage[scaled=0.85]{FiraMono}
\usepackage{newtxsf}
\usepackage[protrusion,expansion,kerning,spacing,final]{microtype}
\microtypecontext{spacing=nonfrench}

% PDF Bookmarks

\usepackage{bookmark}

% Beamer Setup

\usetheme{metropolis}
\usecolortheme{owl}

% AMS Math Packages

\usepackage{amsmath}

\title{A minimal example}
\date{\today}
\author{Matthias Vogelgesang}
\institute{Centre for Modern Beamer Themes}
\def\titlepage{%
  \usebeamertemplate{title page}%<---
}
\begin{document}
    \maketitle %<-- here is where i get an "Overfull \vbox (15.63992pt too high)"

    \section{First Section}
    \begin{frame}{First Frame}
        Hello, world!
    \end{frame}
\end{document}