A mobile computing device runs multiple activities concurrently and provides at least two modes for interacting with the activities. The user may toggle between the modes as desired. In a full-screen mode, one activity occupies substantially an entire display screen. In a windowed mode, the activity is visible within a window, and a portion of at least one other window is also visible. In the windowed mode, the user can cause windows to move, thereby changing focus from one activity to another. For example, the window having focus can be moved off the screen, to be replaced by a new window that is then given focus. The windows are ordered in a sequence and move in concert with one another; pushing one window off one edge of the screen causes a new window to appear at the opposite edge. A persistent positional relationship can be established among windows.