I am using the \documentclass option [fleqn] (which I think is identical to \usepackage[fleqn]{amsmath}). This seems to work fine, except when using an align* within the varwidth environment:
Notes:
- Problem does not occur with
align(non-starred) environment.
Code
\documentclass[fleqn]{article}
\usepackage{showframe}
\usepackage{varwidth}
\usepackage{mathtools}
\newcommand{\MyContent}{%
Using align:
\begin{align}
E &= mc^2
\shortintertext{and}
F &= ma
\end{align}%
Using align
\begin{align}
y = x + b
\end{align}%
}%
\begin{document}
\noindent
Outside of \verb|varwidth|:
\MyContent
\noindent
Inside \verb|varwidth|:
\noindent
\begin{varwidth}{\linewidth}
\MyContent
\end{varwidth}
\end{document}

