update model positioning on offsetUpdated and zoomUpdated

the positioning and child-parent mechanism relies on position, bounding box and cursor but these data (except the last) all changes when the above actions are triggered, on panning and zooming (in/out).

When that happens all the nodes positioning and dimension need to be re-calculated.

Start by locating the offsetUpdated and zoomUpdated handlers and tracing the positioning, bounding-box, and child-parent logic they affect. Confirm how node positions and dimensions are currently calculated; the work is done when panning and zooming reliably trigger recalculation for all nodes.

Written by the indexing model from the issue text.