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Selection Partition

A multi-node selection is not always one thing. When the selected nodes share a parent, they present and act as a single unit. When they span parents, the editor partitions the selection by parent and treats each partition as its own host: the surface draws N overlay boxes rather than one, and a class of commands is applied once per partition. This document defines the partition function, the presentation it drives, and the rule that decides which commands respect it — the shared substrate under grouping, auto-layout, flatten, and boolean.

The partition function​

The partition key is each node's direct parent. Nodes at the scene/root level share one partition — the scene partition. Because selection is normalized (a parent and its own descendant are never selected together — see selection), every selected node is a leaf of the partition set, and the partition is a clean cover: every member lands in exactly one partition.

Formally: partition(selection) = group members by (parent(m) ?? scene). Within a partition, members keep their document sibling order — the order a structural command will preserve when it adopts them.

The partition is derived, never stored. It is recomputed from the current selection and the current tree on every read, so it follows re-parenting, deletion, and selection change without its own state.

Presentation — one overlay per partition​

The surface renders the partition directly:

  • A single node → the single-node overlay (its own bounds, resize, rotate).
  • A multi-node selection under one parent (one partition) → one group overlay over their union bounds.
  • A multi-node selection spanning M parents (M partitions) → M group overlays, each over its own partition's union bounds, each carrying that partition's bounds-relative affordances: resize handles, the gap/distribution overlay, and a click/drag target scoped to that partition's members.

The editor never draws a single box spanning two partitions. This is the observable heart of the concept: a cross-parent selection has no uniform bounding box, because there is no single parent frame such a box could belong to.

Relation to the union drag. The plain move gesture still translates the whole selection together — translation is a union command (below), owned by translate. The per-partition overlays own the bounds-relative operations (resize, gap, distribute) that only make sense within one frame. What selection calls the "selection group overlay" — the draggable multi-selection unit — is the single-partition case of this presentation; the general case is N of them.

The command taxonomy — per-partition vs union​

The split between the two kinds of command is not a per-command flag; it follows one rule:

A command is applied per partition if and only if it inserts a new adopting parent — a node that becomes the parent of the selected members. Every other command acts on the whole-selection union, parent-blind.

Per-partition (structural / containment). Each produces one new node per partition, adopting that partition's members in place with world position preserved:

  • grouping — wrap into a Group or a Container.
  • auto-layout — wrap into a laid-out flex container.
  • flatten — union the partition's shapes into one vector node.
  • boolean — wrap the partition into one boolean-operation node.

Union (spatial). Each reads the whole selection as one set — either its union bounds or a uniform delta — regardless of parent:

  • align & distribute — one shared world frame; each member still commits in its own parent's coordinates, but the frame is the union, not the partition.
  • translate / nudge — a uniform delta to every member.
  • resize-nudge, scale — a uniform transform.

Why the split is principled: a new parent can only adopt the children of one existing parent context without re-parenting, and the structural commands deliberately do not re-parent across partitions (they preserve each member's place in the tree). So a structural command must run once per partition. A spatial command moves nodes in place and has no parent to respect, so it has nothing to partition.

Root, scene, and refusal​

  • Scene/root-level nodes form the scene partition (their parent is the scene).
  • A partition whose parent is the scene may still be wrapped — unless the scene constrains its children to a single child, in which case the wrap is refused for that partition rather than violating the constraint.
  • A single-node selection needs no partition (one host): structural commands still run (one wrapper), the presentation is the single-node overlay, and align special-cases to the node's parent frame (its N = 1 rule).

Contracts​

  • PART-1 Partition function: a selection partitions by each member's direct parent; scene/root-level members share the scene partition; within a partition members keep document sibling order. The partition is derived from the current selection and tree on every read, never stored.
  • PART-2 Presentation mirror: a single node renders the single-node overlay; a multi-node selection renders exactly one group overlay per partition — never a single box spanning two partitions — and each overlay carries its partition's union bounds and bounds-relative affordances (resize, gap, distribute, click).
  • PART-3 Command taxonomy: a command applies per partition iff it inserts an adopting parent (grouping, auto-layout, flatten, boolean); every spatial command (align, distribute, translate, nudge, resize-nudge, scale) acts on the whole-selection union, parent-blind.
  • PART-4 Per-partition independence: a per-partition command produces one result node per partition, each adopting that partition's members with world position preserved; partitions never merge across parents into one result.
  • PART-5 Scene partition & refusal: scene/root-level members form the scene partition; a wrap targeting the scene is refused when the scene constrains its children to one.

Deferred, named: the move-gesture routing over multi-partition overlays (the whole-selection translate, and which overlay's drag starts it) is owned by translate; this document owns the partition and its presentation, not the move mechanics.