TikZ node positioning in Beamer ignores x coordinate

Question

I have the following frame in beamer:

\begin{frame}{Language}
\begin{block}{}
    \begin{align*}
    \onslide<1->{
        &\text{Types} & T &::= B | T \to T \\
    }
    \onslide<2->{    
        &\text{Terms} & E &::= A | E\ E | \ldots |
        \tikz[baseline]{
            \node (term-mon) [anchor=base,fill=blue!20]
            {$\mathsf{mon}^{f, g}_h(E)$};
        }
        \onslide<3> {
            \tikz[baseline]\node [fill=blue!20] (term-mon-expl) at ($(term-mon) + (1,2)$)
            {Comments};
        }
    }    
    \end{align*}
\end{block}
\end{frame}

Basically, I want to highlight a part of the formula and display an additional node nearby on the next slide. However, for some reason TikZ ignores my 1cm shift towards x-axis and only shifts the term-mon-expl node towards y-axis, placing it right next to the term-mon node. Why?

Answer 1

The following code shows to possibilities to use two nodes that should be placed relative to each other

• in one TikZ picture with
• with the remember picture-overlay combo.

Code

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\begin{frame}
  \begin{align*}
    & \text{Types} & T & ::= B | T \to T \\
   \onslide<2->{
    & \text{Terms} & E &::= A | E\ E | \ldots |
      \tikz[
        baseline=(term-mon.base),
        nodes={fill=blue!20}] {
          \node    (term-mon)                                    {$\mathsf{mon}^{f, g}_h(E)$};
          \begin{pgfinterruptboundingbox}
          \node<3> (term-mon-expl) at ([shift={(1,2)}] term-mon) {Comments};
          \end{pgfinterruptboundingbox}
       }
   }
  \end{align*}
\end{frame}
\begin{frame}
  \begin{align*}
   & \text{Types} & T & ::= B | T \to T \\
  \onslide<2->{
   & \text{Terms} & E &::= A | E\ E | \ldots |
     \tikz[baseline,remember picture]
       \node (term-mon) [anchor=base,fill=blue!20] {$\mathsf{mon}^{f, g}_h(E)$};
  }
  \onslide<3> {
     \tikz[baseline,remember picture,overlay]
       \node [fill=blue!20] (term-mon-expl) at ($(term-mon) + (1,2)$) {Comments};
  }
  \end{align*}
\end{frame}
\end{document}

Output