Emerging ultra-small wearables like smartwatches pose a design challenge for touch-based text entry. This is due to the "fat-finger problem," wherein users struggle to select elements much smaller than their fingers. To address this challenge, we developed DriftBoard, a panning-based text entry technique where the user types by positioning a movable qwerty keyboard on an interactive area with respect to a fixed cursor point. In this paper, we describe the design and implementation of DriftBoard and report results of a user study on a watch-size touchscreen. The study compared DriftBoard to two ultra-small keyboards, ZoomBoard (tapping-based) and Swipeboard (swiping-based). DriftBoard performed comparably (no significant difference) to ZoomBoard in the major metrics of text entry speed and error rate, and outperformed Swipeboard, which suggests that panning-based typing is a promising input method for text entry on ultra-small touchscreens.