[Tex/LaTex] Tikz figure inside tikz overlay

Question

I would like to create a callout inside my slides. I'm not an expert of tikz, however I find something online.

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{shapes,positioning,calc,shadows,decorations.pathmorphing,decorations.pathreplacing,tikzmark}
\newcommand{\tikzmarkinside}[1]{\tikz[overlay,remember picture,baseline=-0.5ex] \node (#1) {};}
\begin{document}
 \begin{frame}
 \begin{itemize}
  \item a\tikzmarkinside{first}\pause
  \item b\pause
  \item c\pause
 \end{itemize}

\tikz[remember picture, overlay]\node[align=center, fill=cyan!20,opacity=0.85, text width=4.2cm, rounded corners,
  draw,rectangle callout,anchor=pointer,callout absolute pointer=(first.south),
      below right= 1 and 1]
  at (first) {
   \begin{minipage}[t][2cm][t]{4cm}
    \begin{tikzpicture}
     \draw [fill=orange] (1,1) rectangle (2,2);
    \end{tikzpicture}
    \end{minipage}
  };
 \end{frame}
\end{document}

there are two problems here
1) there is a rectangle inside the callout but it has rounded corner, like it has inherited them from the callout shape. I would like to have "square" corners
2) that definition of \tikzmarkinside works but maybe is not so elegant

can you help me?

P.S.: I know that I could use \includegraphics and include a pre-compiled image of my rectangle but I would like to do all at once.

Answer 1

You should avoid nesting Tikz pictures. Instead you can draw the callout and the rectangle in the same (overlay) picture. That will also fix the problem with rounded corners.

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{shapes,positioning,calc,shadows,decorations.pathmorphing,decorations.pathreplacing,tikzmark}
\newcommand{\tikzmarkinside}[1]{\tikz[overlay,remember picture,baseline=-0.5ex] \node (#1) {};}
\begin{document}
\begin{frame}
  \begin{itemize}
  \item a\tikzmarkinside{first}\pause
  \item b\pause
  \item c\pause
  \end{itemize}

  \begin{tikzpicture}[remember picture, overlay]
    \node[align=center, fill=cyan!20,opacity=0.85, text width=4.2cm, rounded corners,
    draw,rectangle callout,anchor=pointer,callout absolute pointer=(first.south),
    below right= 1 and 1,
    minimum height=5em]
    (TheBubble) at (first) {};
    \draw[fill=orange] ($(TheBubble.north west)+(0.2,-0.2)$) rectangle +(1,-1);
  \end{tikzpicture}
\end{frame}
\end{document}

EDIT

With a more complicated structure inside the callout I would use a scope to separate it from the rest. The reference point inside the scope is the lower left corner of the callout.

\begin{tikzpicture}[remember picture, overlay]
    \node[align=center, fill=cyan!20,opacity=0.85, text width=4.2cm, rounded corners,
    draw,rectangle callout,anchor=pointer,callout absolute pointer=(first.south),
    below right= 1 and 1,
    minimum height=7em]
    (TheBubble) at (first) {text inside the callout. It behaves as normal text.
      \begin{itemize}
      \item Test
      \item New test
      \end{itemize}
    };
    %\draw[fill=orange] ($(TheBubble.north west)+(0.2,-0.2)$) rectangle +(1,-1);
    \begin{scope}[shift={($(TheBubble.south west) +(0.2,0.2)$)}]
      \draw[fill=orange] (0,1) rectangle +(1,1);
      \draw[->] (2,0) -- +(0,2) node[left]{$y$};
      \draw[->] (2,0) -- +(1.5,0) node[right]{$x$};
      \draw[smooth,xshift=2cm] plot coordinates {(0,0)(0.5,1)(1,1.5)(1.5,0.5)};
    \end{scope}
  \end{tikzpicture}