Profinite isomorphisms and fixed-point properties

Abstract: We describe a flexible construction that produces triples of finitely generated, residually finite groups $M\hookrightarrow P \hookrightarrow \Gamma$, where the maps induce isomorphisms of profinite completions $\widehat{M}\cong\widehat{P}\cong\widehat{\Gamma}$, but $M$ and $\Gamma$ have Serre's property FA while $P$ does not. In this construction, $P$ is finitely presented and $\Gamma$ is of type ${\rm{F}}_\infty$. More generally, given any positive integer $d$, one can demand that $M$ and $\Gamma$ have a fixed point whenever they act by semisimple isometries on a complete CAT$(0)$ space of dimension at most $d$, while $P$ acts without a fixed point on a tree.