Tooltip-only updates

In tooltip-only scenarios (target is unspecified, cannot be found, etc) the tooltip's position defaults to the center of the screen, but only at the time that step is initiated. Resizing the window would not cause an update, and the result was a tooltip that was off-center or even off screen entirely. Now, in tooltip-only cases, a quick recalculation of the tooltip's position is performed; if this calculation yields a different value from the current tooltip position, an update is scheduled (shouldUpdate returns true).