What the host owns · Guide

The widget is stateless between frames and never mutates your data model. It renders the nodes and edges you pass in and reports intent through callbacks; your update applies the change and the next view reflects it. Which makes these yours to uphold:

Unique node ids. Lookups resolve by id, so a duplicate push is ignored (first wins) and debug builds assert on it. Prefer a stable id from your data - a database key, uuid::Uuid, a typed newtype - over a hand-managed counter.
Edge dedupe. on_connect fires on every snap during a drag, not on release, so one drag can report several connections. The default can_connect already rejects a second edge into an occupied input; for replace-on-drop instead, drop that rule (see connection) and remove the prior edge whose input matches - to is always the input pin.
Applying moves, deletes and clones. on_move / on_delete / on_clone report intent only.
Applying selection. Optional: the widget keeps a working selection, so clicks and the selection box work on their own. on_select reports it; to own it, mark the matching nodes with Node::selected on the next view and your value takes over whenever it changes. Selection is a node property, so there is no ordering to get right. The camera is the same story through on_camera and camera.