What is the cause of this error adding align* environment in a forest tree node?

Question

For almost an hr I have been starring at this and can't figure where is my syntax error

\documentclass[crop,tikz]{standalone}
\usepackage{amsmath}

\usepackage{forest}
\useforestlibrary{edges}
\begin{document}

\begin{forest}
[   
  {\begin{align*}
      x &= 1
  \end{align*}}
]
\end{forest}    
\end{document}

Compiling gives

! Missing \endgroup inserted.
<inserted text> 
\endgroup 
l.16 \end{forest}

I know one needs to add {....} around all math to protect it and that is what I did. If I use normal math, it works

\documentclass[crop,tikz]{standalone}
\usepackage{amsmath}

\usepackage{forest}
\useforestlibrary{edges}
\begin{document}

\begin{forest}
[   
  {$x = 1$}
]
\end{forest}

\end{document}

No errors. I also found if I put the align environment in minipage the error goes away:

\documentclass[crop,tikz]{standalone}
\usepackage{amsmath}

\usepackage{forest}
\useforestlibrary{edges}
\begin{document}

\begin{forest}
[   
  \begin{minipage}{2cm} 
  {\begin{align*}
      x &= 1
  \end{align*}}
  \end{minipage}
]
\end{forest}

\end{document}

No errors.

So why does the first example above give an error? I know I must be making some syntax error, but I just do not see what it is.

TL 2022

Update

OMG! I find workaround, but have no clue why this makes it work. Simply adding ,text width=3cm after it, now it works

\documentclass[crop,tikz]{standalone}
\usepackage{amsmath}

\usepackage{forest}
\useforestlibrary{edges}
\begin{document}

\begin{forest}
[   
   {\begin{align*}
      x &= 1
    \end{align*}}
   ,text width=3cm% ADDING THIS MAKES IT WORK
]
\end{forest}    
\end{document}

But why??

Update

aligned does not really do what I want. Here is an example below

\documentclass[crop,tikz]{standalone}
\usepackage{amsmath}
\usepackage{forest}
\useforestlibrary{edges}
\begin{document}

\begin{forest}
[   
Write the ode as 
{$\begin{aligned}
y'+p(x)y&=q(x)\\
        &=f(x,y)
\end{aligned}$}
This is only possible if ode is linear in {$y$}
]
\end{forest}

\end{document}

I wanted it to look like this

\documentclass[crop,tikz]{standalone}
\usepackage{amsmath}
\usepackage{forest}
\useforestlibrary{edges}
\begin{document}

\begin{forest}
[   
Write the ode as 
{\begin{align*}
y'+p(x)y&=q(x)\\
        &=f(x,y)
\end{align*}
}
This is only possible if ode is linear in {$y$}
,text width=3cm
]
\end{forest}

\end{document}

But thanks to the answer, I know now that I needed to add specific value for the width for align* to work. So I am all OK now. I changed my code to make sure there is a specific text width given and the error now is gone.

Answer 1

• Display style math in node always require defined node sizes. For example, it doesn't work in:
  • pure standalone
  • in table with c, l or r type of columns.
  • etc
• I wonder, why you need display style in nodes? Isn't it simpler to use in-line math?

\documentclass[crop,tikz]{standalone}
\usepackage{amsmath}

\usepackage{forest}
\useforestlibrary{edges}

\begin{document}

\begin{forest}
[root
    [{$\begin{aligned}
          x &= 1 \\[-1ex]
      x + y &= 2
    \end{aligned}$}]
]
\end{forest}
\end{document}