Milestone 26 — Snap & Pin

Concept

While dragging an SO (constraining movement to a plane), when one edge comes close to the edge of another SO, snap it (bring the separation to zero). Then offer to pin the edges together with cross-referencing formulas.

Tasks

• Phase 1: snap detection during face drag
• Phase 2: attached flag on Attribute + propagation logic
• Phase 3: pin-offer button near snapped edge
• resolve overlap
• dado/piercing -> slider?
• tests with library items:
  • pin stringer diagonal to the end points
  • location of the big beams in basement
  • cabinet carcasse

Decision

Pin via cross-reference with attached flag

Both attributes get formulas pointing at each other. Both are marked attached. During deserialization, the first encountered keeps its value as seed; the second evaluates its formula (resolving to the first's value). During propagation, both use the formula.

No givens needed. No ownership. The relationship is direct and self-documenting.

Proposal

Phase 1: Snap during drag

Where: Drag.apply_face_drag_absolute

After computing new bounds from the delta, scan sibling SOs (children of same parent). For each axis on the drag plane, compare dragged SO's min/max against each sibling's min/max. If within snap threshold, clamp the bound to match. Store which bounds snapped and to which sibling.

Phase 2: attached flag on Attribute

Where: Attribute.ts, Constraints.ts

Add attached: boolean to Attribute. Serialize/deserialize it. In deserialize, when encountering an attached attribute with a compiled formula: if it's the first attached attribute in the current pass, skip evaluation (keep value as seed). The second evaluates normally.

Phase 3: Pin offer on drag end

Where: Drag.ts, Graph.svelte

After drag ends with a snap active, show a small button near the snapped edge. On accept:

1. Set formula on dragged SO's snapped attribute → references sibling's corresponding bound
2. Set formula on sibling's corresponding attribute → references dragged SO's bound
3. Mark both attributes as attached

On decline or ignore: snap position holds but no formula link.