How can I visually group sections in a Beamer TOC?

Question

This is essentially a follow-up question to Can I control the position of the column break within a two-column TOC?.

I have the following slide:

As you can see, the slide is a little hard to digest at-a-glance. Is there any way to group sections with their subsections and introduce a good amount of vertical space between each section-group?

Answer 1

Throw this in your preamble:

\usepackage{xpatch}
\makeatletter
\xpretocmd\beamer@sectionintoc{\bigskip}{}{} % customize your skip here
\makeatother

The *.toc file Beamer generates uses that command to format \sections; prepending a smart vertical space to it will introduce the necessary padding.

Answer 2

Not really sure why your solution does not work for a manually split toc, but based on https://tex.stackexchange.com/a/170438/36296 the following seems to work:

\documentclass{beamer}
\usetheme{default}

\usepackage{etoolbox}

\makeatletter
\patchcmd{\beamer@sectionintoc}
  {\vfill}
  {\vskip2\itemsep}
  {}
  {}
\makeatother

\begin{document}
\section{Problem Definition}
\subsection{Terminology}
\subsection{Naive Workflow}
\subsection{Fatal Problem}
\section{Current Solution}
\subsection{What we have}
\subsection{How we use it}
\section{How it Works}
\section{How to Build}
\section{Troubleshooting}
\subsection{No defaulting}
\subsection{Inconsistent defaulting}
\subsection{Most other problems}

\begin{frame}{Questions}
    \begin{columns}[onlytextwidth,T]
        \begin{column}{.45\textwidth}
            \tableofcontents[sections=1-2]
        \end{column}
        \begin{column}{.45\textwidth}
            \tableofcontents[sections=3-5]
        \end{column}
    \end{columns}
\end{frame}
\end{document}