Content aligned with a figure changes positions when the figure is uncovered (beamer)

Question

When I align two minipages together, the contents location changes after a figure in one minipage is uncovered. The distance from the rule to the upper palettes illustrates this

\documentclass[aspectratio=169, xcolor={x11names}]{beamer}

    \setbeamercovered{transparent=30}

    \usecolortheme{seahorse}
    \useoutertheme{miniframes}
    \useinnertheme{inmargin}

    \usepackage{adjustbox}
    \usepackage[defaultmathsizes, subdued, italic, symbolre, symbolmisc]{mathastext}
% BEGIN_FOLD

\MTDeclareVersion[it]{charter}{T1}{bch}{m}{n}

\MTversion*{charter}


% END_FOLD


\begin{document}

\begin{frame}

    \uncover<+->{$\vphantom{1}$}
    \rule{\textwidth}{0.2mm}

    \begin{adjustbox}{minipage={0.45\textwidth}, valign=t}

        \includegraphics<+->[trim = 0mm 0mm 0mm 0mm, width=\columnwidth]{example-image-a}

    \end{adjustbox}
    \hfill
    \begin{adjustbox}{minipage={0.45\textwidth}, valign=t}

        \begin{itemize}
            \item<+-> Modeling the addition of branch $Z_{b}$ between two existing buses \mbox{($k$ and $p$)} in Brown's method
        \end{itemize}

    \end{adjustbox}

\end{frame}

\end{document}

Answer 1

Arguably using \visible for that is a simpler option.

\documentclass[aspectratio=169, xcolor={x11names}]{beamer}

    \setbeamercovered{transparent=30}

    \usecolortheme{seahorse}
    \useoutertheme{miniframes}
    \useinnertheme{inmargin}

    \usepackage{adjustbox}
    \usepackage[defaultmathsizes, subdued, italic, symbolre, symbolmisc]{mathastext}
% BEGIN_FOLD

\MTDeclareVersion[it]{charter}{T1}{bch}{m}{n}

\MTversion*{charter}


% END_FOLD


\begin{document}

\begin{frame}

    \uncover<+->{$\vphantom{1}$}
    \rule{\textwidth}{0.2mm}

    \begin{adjustbox}{minipage={0.45\textwidth}, valign=t}

        \visible<+->{\includegraphics[trim = 0mm 0mm 0mm 0mm,
        width=\columnwidth]{example-image-a}}

    \end{adjustbox}
    \hfill
    \begin{adjustbox}{minipage={0.45\textwidth}, valign=t}

        \begin{itemize}
            \item<+-> Modeling the addition of branch $Z_{b}$ between two existing buses \mbox{($k$ and $p$)} in Brown's method
        \end{itemize}

    \end{adjustbox}

\end{frame}

\end{document}

Answer 2

You can insert a vertical phantom of exactly the same figure; that is, place another figure with the original one and use phantom=v, with adjustbox. This will not typeset anything, but it will force tell LaTeX to make a vertical space equal to the one that the figure would have occupied if it was typeset. The syntax is

\adjustbox{phantom=v}{\includegraphics[trim = 0mm 0mm 0mm 0mm, width=\columnwidth]{example-image-a}}

Full code

\documentclass[aspectratio=169, xcolor={x11names}]{beamer}

    \setbeamercovered{transparent=30}

    \usecolortheme{seahorse}
    \useoutertheme{miniframes}
    \useinnertheme{inmargin}

    \usepackage{adjustbox}
    \usepackage[defaultmathsizes, subdued, italic, symbolre, symbolmisc]{mathastext}
% BEGIN_FOLD

\MTDeclareVersion[it]{charter}{T1}{bch}{m}{n}

\MTversion*{charter}


% END_FOLD


\begin{document}

\begin{frame}

    \uncover<+->{$\vphantom{1}$}
    \rule{\textwidth}{0.2mm}

    \begin{adjustbox}{minipage={0.45\textwidth}, valign=t}

        \includegraphics<+->[trim = 0mm 0mm 0mm 0mm, width=\columnwidth]{example-image-a}% do not remove this
        \adjustbox{phantom=v}{\includegraphics[trim = 0mm 0mm 0mm 0mm, width=\columnwidth]{example-image-a}}

    \end{adjustbox}
    \hfill
    \begin{adjustbox}{minipage={0.45\textwidth}, valign=t}

        \begin{itemize}
            \item<+-> Modeling the addition of branch $Z_{b}$ between two existing buses \mbox{($k$ and $p$)} in Brown's method
        \end{itemize}

    \end{adjustbox}

\end{frame}

\end{document}