A user interface apparatus includes a display device, display application, and processor. The display device includes a display area, and the display area includes an active area. The processor and display application are configured to display application stacks, each of which may be moved into and out of the active area in response to user input, and each of which contains one or more stack cards. Each application stack is associated with a computer application, and each stack card represents a content entity capable of being operated on by the computer application corresponding to its application stack. When an application stack is in the active area the application stack is activated such that its stack cards are displayed in sequence and at least one of them may be selected. Selecting a stack card causes the corresponding computer application to be executed and to operate on the corresponding content entity.