• i want an undo system
• first, figure out "go back to" points
  • drag
  • stretch (gigantic hierarchy ???)
  • rename
  • edit a cell
  • delete
  • add/insert child
  • rotate
  • straighten
  • not?
    • select
    • fit
    • 2D/3D
    • solid/xray

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Proposal: Snapshot-based Undo

Why snapshots over commands

Command pattern would require retrofitting every mutation site with execute/undo pairs — 12+ action types, each with its own reversal logic (delete is especially gnarly: subtree + formula cleanup). Snapshot approach: capture serialized scene state before each mutation, restore to undo.

Existing infrastructure

• scenes.save() serializes the full scene to Portable_Scene (all SOs, constants, camera, selection)
• Smart_Object.serialize() / deserialize() round-trips cleanly
• Engine's setup/load path rebuilds the live scene from Portable_Scene

Design

New file: src/lib/ts/managers/History.ts

class History {
    stack: Portable_Scene[]          // past states (capped, eg 50)
    redo_stack: Portable_Scene[]

    snapshot(): void                 // capture current state, push to stack, clear redo
    undo(): Portable_Scene | null    // pop stack, push current to redo, return scene to restore
    redo(): Portable_Scene | null    // pop redo, push current to stack, return scene to restore
    clear(): void                    // reset both stacks (on file load / new scene)
}

Snapshot sites (before the mutation, not after)

1. Drag start — drag.set_target()
2. Rotate start — rotate_object() first call per gesture
3. Dimension commit — dimensions.commit()
4. Angular commit — angulars.commit()
5. Delete — engine.delete_selected_so()
6. Add child — engine.add_child_so()
7. Insert from file — engine.insert_child_from_text()
8. Rename — SO name or constant name change
9. Reset angle — P_Angles.svelte reset handler
10. Formula edit — attribute cell commit in D_Attributes

Not undoable (view state, not model state)

select, fit, 2D/3D toggle, solid/xray

Trigger

Cmd+Z / Cmd+Shift+Z in Events.ts keydown handler.

Restore path

undo() returns a Portable_Scene. Caller (engine) runs the same path as file-load: clear scene, deserialize SOs, rewire hierarchy, rebuild constraints, restore selection + camera.

Tradeoffs

• Memory: 50 snapshots of a typical scene (10 SOs) ≈ ~500KB. Fine.
• Hierarchy concern ("stretch — gigantic hierarchy ???"): snapshot sidesteps this. No need to figure out which descendants changed. Restore the whole thing.
• Continuous drags: only pre-drag state saved, so undo jumps to before the drag started. Standard CAD behavior.

Build plan

1. History.ts — stack manager (~60 lines)
2. history.snapshot() calls at each mutation site (~10 one-liners)
3. engine.undo() / engine.redo() — call history then restore
4. Cmd+Z / Cmd+Shift+Z handling in Events.ts