[Tex/LaTex] How to draw tikz node behind text

Question

I am trying to annotate an equation using tikz. I'd like to highlight a part of that equation using a tikz node, but the left part of the equation gets covered:

\documentclass[varwidth, border={0pt 4pt 0pt 4pt}]{standalone}
\usepackage{tikz}
\begin{document}
  $y(x) = g(x)f(x)h(x)$

  \bigskip
  $y(x) = g(x)
  \tikz[remember picture, overlay]{
      \node[fill=gray!20, inner sep=4pt, anchor=text, rectangle, rounded corners=1mm]{$f(x)$};
  }
  \phantom{f(x)}
  h(x)$
\end{document}

The first part of the equation, g(x), gets covered by the node, while the right hand side, h(x), doesn't, because it appears after the node in the LaTeX code:

Instead, I would like to have the node in the background, behind the rest of the equation, while preserving the original spacing within the equation:

Answer 1

TikZ may be an overkill for such simple tasks, you can try tcolorbox, mdframed, fancybox, etc. If you still like TikZ, your example can be easily modified.

\documentclass[12pt,a4paper]{book}
\usepackage{tikz,amsmath}
\usepackage{graphicx}
\begin{document}

\begin{equation*}
  y(x) = g(x)\,
  \tikz[remember picture, overlay]{
    \node[fill=gray!20, inner sep=1pt, anchor=text, rectangle, rounded corners=1mm]{$f(x)$};
  }
  \phantom{f(x)}\,
  h(x)
\end{equation*}

\end{document}