A user interface control for scrolling a display of computer program data during retrieval of computer program data is provided. The control comprises a track, a bar overlying said track, and a thumb movable along the bar to scroll the displayed data; wherein the size of the track is indicative of the total amount of data being retrieved; and the size of the bar varies to indicate the progress of the data retrieval. Thus, the ratio of the size of the bar to that of the track indicates the ratio of available to total data. The advantage of this technique is that the user can see the progress towards completion of the data retrieval. The computer program data is displayed in a display area allocated to the computer program, and a size of the thumb (compared to a corresponding size of the track) indicates the ratio of the amount of data displayed in the display area to the total amount of data being retrieved. This has the further advantage that the scrollbar thumb does not change size over time nor move without user input, making it easier for a user to interact with the scroll bar control.