Online modular adaptation and self-adaptation techniques have demonstrated significant benefits in coarse-grained software, enabling agile and high-performance deployments. We are studying the same kinds of runtime adaptation applied to fine-grained software such as graphical user interfaces and web server implementations. However, this kind of software is defined by pervasive use of behaviourally-driven structure. Existing runtime component models fail to capture this necessity due to their exclusive reliance on externally-driven structural composition. In this paper we present a novel runtime component model that both satisfies the need to externally manage software structure, enabling runtime adaptation and self-adaptation, while also satisfying the need for fine-grained software to create elements of its own structure based on application-specific system behaviour. We present the key details of our model along with an initial evaluation.