There might be some disparity between onMouseMove which is called every frame and Tick which is also called every frame. Using stale data during Tick could be causing it, hard to tell.
Could you test it by setting position of the grabbed widget in the onMouseMove rather than during Tick?
jitter
Is this jitter closer to 1px jitter / crawl or it seem to want to snap to the upper left corner of the screen kind of jitter?