Cluster categories for completed infinity-gons I: Categorifying triangulations

Paquette and Y{\i}ld{\i}r{\i}m recently introduced triangulated categories of arcs in completed infinity-gons, which are discs with an infinite closed set of marked points on their boundary. These categories have many features in common with the cluster categories associated to discs with different sets of marked points. In particular, they have (weak) cluster-tilting subcategories, which Paquette and Y{\i}ld{\i}r{\i}m show are in bijection with very special triangulations of the disc. This is in contrast to Igusa and Todorov's earlier work in the uncompleted case, in which every triangulation corresponds to a weak cluster-tilting subcategory. In this paper, we replace the triangulated structure of Paquette and Y{\i}ld{\i}r{\i}m's category by an extriangulated substructure and prove that, with this structure, the weak cluster-tilting subcategories are once again in bijection with triangulations. We further show that functorial finiteness of a weak cluster-tilting subcategory is equivalent to a very mild condition on the triangulation, which also appears in \c{C}anak\c{c}{\i} and Felikson's study of infinite rank cluster algebras from Teichm\"uller theory. By comparison with the combinatorics of triangulations, we are also able to characterise when weak cluster-tilting subcategories can be mutated in this new extriangulated category.