Undo and the edit model

Undo and redo

Undo and Redo in the Edit tab, or ⌘/Ctrl + Z and ⇧⌘Z / Ctrl + Y. Up to 100 steps.

One step is one thing you meant to do: a whole node drag, one session in a colour picker, one mirror. Adding a background and then choosing its colour are two steps.

What survives a re-trace

The document is three things (apps/app/src/lib/doc.ts):

Part	Holds	On re-trace
inks	Colour replacements and removals, keyed by traced colour	Kept
background	The background colour, or none	Kept
doc	Your edited shapes, or nothing	See below

doc stays empty, meaning “whatever the tracer produces”, until your first shape edit (a node drag, a delete, a mirror). That edit copies the trace into an editable document, and the Edit tab gets a dot to show it.

From then on, changing a trace setting or opening another image would throw those edits away, so it asks first:

• Changing a setting: Discard edits and re-trace or Keep editing. If you discard, Undo brings the edited shapes back.
• Opening another image: this one can’t be undone, and the prompt says so.

How it works

Undo stores whole snapshots rather than inverse operations. Every edit already makes a new immutable document, unchanged shapes are shared between snapshots, and there’s no inverse of a boolean union to get wrong. A key on each edit coalesces bursts (a drag sends dozens of updates) into one step, and every edit is computed from the latest state, so an edit that finishes late (a mirror waiting for paper.js to load) never reverts changes made in the meantime.