  We initiate the study of the $C^0$ symplectic mapping class group, i.e. the
group of isotopy classes of symplectic homeomorphisms. We prove that none of
the different powers of the square of the Dehn-Seidel twist belong to the same
connected component of the group of symplectic homeomorphisms of certain
Liouville domains. This generalizes to the $C^0$ setting a celebrated result of
Seidel. In other words, we obtain the non-triviality of the $C^0$ symplectic
mapping class group in these domains and in fact an element of infinite order.
For that purpose, we develop a method coming from Floer theory and the theory
of barcodes. This builds on recent developments of $C^0$-symplectic topology.
In particular, we adapt and generalize to our context results by
Buhovsky-Humili\`ere-Seyfaddini and Kislev-Shelukhin.
