Absolute positioning within the viewport, with no scrolling (or panning) or zooming allowed.
There is a special colored background Part that shows the fixed area where Parts may be.
It is in the "Grid" Layer so that it is not selectable and is always behind the regular Parts.
Parts may not be dragged outside of the fixed document area of the diagram.
This is implemented by a custom Part.dragComputation function.
Note that the user may still scroll or zoom the whole page.
The model data, automatically updated after each change or undo or redo: