In this document I tried to have wide figures hang in the outside margins.
\documentclass[twoside]{book}
\usepackage{graphicx}
\usepackage{lipsum}
\usepackage{floatrow}
\floatsetup[widefigure]{margins=hangoutside}
\begin{document}
~\newpage % just to get a spread
\lipsum[1]
\begin{figure*}[htbp]
\includegraphics[width=\linewidth]{example-image}
\end{figure*}
\lipsum[2]
\begin{figure*}[htbp]
\includegraphics[width=\linewidth]{example-image}
\end{figure*}
\lipsum[3]
\end{document}
the spread on pp. 2–3 looks like this:
So only on p. 2 is the outer margin used. With margins=hanginside I'll get the opposite. In my original document I assumed I got a problem because of interfering packages, but there is not much else in this file. Is this a problem with floatrow, or something I've misunderstood?


margins=hanginsideonly works on one page because its a two side document. So on each page the inner and outer margins are opposed to each other. Do you want the figure to hand always to the left? Or always to the outside? – G. Bay Jan 11 '19 at 15:54