With the continued proliferation of mobile devices, apps will increasingly become multi-surface, running seamlessly across multiple user devices (e.g., phone, tablet, etc.). Yet general systems support for multi-surface app is limited to (1) screencasting, which relies on a single master device's computing power and battery life or (2) cloud backing, which is unsuitable in the face of disconnected operation or untrusted cloud providers. We present an alternative approach: Flux, an Android-based system that enables any app to become multi-surface through app migration. Flux overcomes device heterogeneity and residual dependencies through two key mechanisms. Selective Record/Adaptive Replay records just those device-agnostic app calls that lead to the generation of app-specific device-dependent state in system services and replays them on the target. Checkpoint/Restore in Android (CRIA) transitions an app into a state in which device-specific information can be safely discarded before checkpointing and restoring the app. Our implementation of Flux can migrate many popular, unmodified Android apps-including those with extensive device interactions like 3D accelerated graphics-across heterogeneous devices and is fast enough for interactive use.