As the demand for domain- and formalism-specific visual modelling environments grows, the need to rapidly construct complex User Interfaces (UIs) increases. In this paper, we propose a Multi-Paradigm Modelling (MPM) approach whereby structure, visual appearance and above all reactive behaviour of a UI are explicitly modelled. These models tructed at the most appropriate level of abstraction, using the most appropriate modelling formalisms. This allows for rapid application synthesis, easy adaptation to changing requirements, and simplified maintenance. In this paper, we introduce Scoped User Interfaces, and illustrate how one may model them using Hierarchically-linked Statecharts (HlS). The use of HlS is demonstrated through the rapid development of a DChart formalism-specific modelling environment.