That appears to be a draggable and resizable rectangular shape. (Presumably only resizable on the left and right sides -- customize the Part.ResizeAdornmentTemplate to only have the handles on the left and right sides. The Sequential Function sample demonstrates that, but you might want to use taller handles.)
Then you just need to implement Diagram.SelectionMoved and Diagram.NodeResized event handlers to update the main diagram's timeline. I guess the former just needs to scroll the diagram; the latter needs to change the distance between the tick marks and move all of the nodes as well as change the Diagram.Panel.Position.