Presentation state is not a fact

Some of what a screen shows is known only to the DOM: where an element is, what the pointer is over, where the caret sits in a field. Putting it in the Model costs a Message and a render per pointer move or scroll, for a value no transition reads.

The test for whether something belongs in the Model: is it replayed, stored, sent, or does it decide a Message? If any of those, it is a fact, and the Model owns it. If none, it is presentation, and it lives in a Behavior or a Mount on the DOM, where it is measured and drawn without a transition.

• The page builder's selection is a fact: it decides what a Delete removes, and a link can name it. The box drawn over the selected node is not: where that node is on screen is measured by Measure (foldkit-primitives/dom) and written as CSS custom properties, with no Message.
• A form's draft is a fact, even text that does not parse yet, because a submit is decided by it. The caret inside the field is not.
• A hover sits on the line: the builder keeps its hovered node in the Model, because a layer row's hover must mark the node on the canvas and a Message is the only path between the two. It is still presentation, so a stored Builder shown again drops it (settle) rather than showing a stale hover.