Put the shapes of a vector drawing on layers and in groups (#190)

A vector drawing was one flat list of shapes. With many shapes the
list got hard to read, and several shapes could not be kept or moved
together. Now the shapes stand on layers, as in pixel art, and can be
put into groups that nest.

- A drawing has layers, each with a name and an eye. A hidden layer
  is neither in the tab nor in the picture a sprite wears, and its
  shapes cannot be pressed. One layer is the one worked on: the pen
  and Add shape draw onto it, and SHAPES lists what is on it.
- The inspector has a list LAYERS over the shapes. Its plus adds a
  layer, the cross on a row removes it with all on it, the last one
  stays. A double-click renames, a drag sorts.
- Group (Ctrl+G) puts the selected shapes into a group, Ungroup
  (Ctrl+Shift+G) takes it away. A group stands over its rows in the
  list, the rows one step in, and a row is dragged into a group or
  out of it.
- In the tab a press on a shape takes its outermost group, and a drag
  moves the group as one undo step.
- The model has layers with a tree of shapes and groups instead of a
  flat shapes array. The rules are pure functions in
  core/model/vectorTree.ts. A drawing saved before opens with all its
  shapes on one layer.
- SortableList takes rows with a depth and drops into a tree. The
  flat lists of tilemap and pixel art run as before.

Decided here:

- A group has no transformation of its own and cannot be hidden or
  locked. Rotating and scaling is the follow-up #291.
- A group is never empty: the one that loses its last shape goes
  with it.
- Several selected shapes are dragged together, grouped or not, and
  a click on a shape of the selection keeps the selection. This is
  what #217 asked for.
- Shift+click selects like Ctrl+click. With Select, Shift pans only
  beside the shapes, as in the scene view. The middle button pans
  anywhere, and with the pen and Paint Shift pans as before.
- A row of the list picks one shape of a group alone.
- Shapes change their layer through a row Layer in the inspector,
  which is there from two layers on. Rows are not dragged between
  layers.
- A group cannot span two layers, the button is off then.
- The pen does not draw onto a hidden layer, the hint says so.
- Groups are always shown unfolded.
- The four order buttons work inside the own group or layer.

Tests:
- Gate: 204 files, 3067 tests (3008 + 59), exit 0
- e2e 47,48: 108 checks. e2e 39,40,47,48 before the Shift change:
  209 checks. No full run, the sum of 1645 (1639 + 6) is reckoned
  and CI measures it.
- 59 unit tests: 36 in tests/core/vector-tree.test.ts, 23 in
  tests/editor/vector-layers.test.ts
- Six e2e checks with a real mouse in block 47j
- 47 unit counter-probes for layers and groups and three for the
  Shift click all fail tests, run on stein2
- Three browser counter-probes fail their checks
- Two screenshots looked at (group in the list, hidden layer)
- Only unit tested: the row Layer, sorting and removing layers,
  copying a group, the Shift pan beside the shapes

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/vector-tree.test.ts

@@ -0,0 +1,505 @@
+/**
+ * Layers and groups of vector art (#190): the tree as it is read, every way it changes, the commands
+ * that change it with their undo, what a project file gives back and what is in the picture.
+ */
+import { describe, expect, it } from 'vitest';
+import { CreateAssetCommand } from '../../src/core/commands/assetCommands';
+import {
+  AddVectorLayerCommand,
+  AddVectorShapeCommand,
+  CombineVectorShapesCommand,
+  DuplicateVectorShapeCommand,
+  GroupVectorShapesCommand,
+  MoveVectorLayerCommand,
+  MoveVectorShapesCommand,
+  OrderVectorShapeCommand,
+  RemoveVectorLayerCommand,
+  RemoveVectorShapeCommand,
+  RemoveVectorShapesCommand,
+  RenameVectorLayerCommand,
+  RenameVectorShapeCommand,
+  SetVectorLayerHiddenCommand,
+  SetVectorShapeRingsCommand,
+  UngroupVectorShapesCommand,
+} from '../../src/core/commands/vectorArtCommands';
+import type { VectorArtAsset, VectorItem, VectorLayer, VectorShape } from '../../src/core/model/Asset';
+import { findAsset, type Project } from '../../src/core/model/Project';
+import { vectorArtRevision } from '../../src/core/model/vectorart';
+import { vectorArtDrawings } from '../../src/core/model/vectorPath';
+import {
+  allShapes,
+  drawnShapes,
+  outermostGroup,
+  placeOf,
+  selectedItems,
+  treeDrop,
+  treeDropDepth,
+  type TreeRow,
+  treeRows,
+  VectorTreeError,
+  withGroupDissolved,
+  withItemsGrouped,
+  withItemsMoved,
+  withItemsRemoved,
+  withLayerRemoved,
+} from '../../src/core/model/vectorTree';
+import { makeProject, roundTrip } from '../helpers';
+
+function shape(id: string, x = 0): VectorShape {
+  return {
+    id,
+    name: id.toUpperCase(),
+    fill: 0x112233,
+    opacity: 1,
+    rings: [
+      {
+        closed: true,
+        points: [
+          { x, y: 0 },
+          { x: x + 10, y: 0 },
+          { x, y: 10 },
+        ],
+      },
+    ],
+  };
+}
+
+const group = (id: string, ...items: VectorItem[]): VectorItem => ({ id, name: id.toUpperCase(), items });
+const layer = (id: string, ...items: VectorItem[]): VectorLayer => ({ id, name: id, items });
+
+/** How a tree reads, groups in brackets: `a g[b h[c]] d`. */
+function text(items: readonly VectorItem[]): string {
+  return items.map((item) => ('items' in item ? `${item.id}[${text(item.items)}]` : item.id)).join(' ');
+}
+const layersText = (layers: readonly VectorLayer[]) => layers.map((l) => `${l.id}: ${text(l.items)}`);
+
+/** a, then group g of b and the group h of c, then d – on one layer. */
+const nested = () => [layer('l1', shape('a'), group('g', shape('b'), group('h', shape('c'))), shape('d'))];
+
+const row = (id: string, depth: number, isGroup = false): TreeRow => ({
+  id,
+  name: id.toUpperCase(),
+  depth,
+  group: isGroup,
+});
+
+describe('reading the tree', () => {
+  it('hands the shapes out in drawing order, groups walked in place', () => {
+    const art = { layers: [...nested(), layer('l2', shape('e'))] } as VectorArtAsset;
+    expect(allShapes(art).map((s) => s.id)).toEqual(['a', 'b', 'c', 'd', 'e']);
+  });
+
+  it('leaves the shapes of a hidden layer out of what is drawn, and only there', () => {
+    const art = {
+      id: 'art',
+      type: 'vectorart',
+      name: 'Art',
+      props: {},
+      layers: [{ ...layer('l1', shape('a')), hidden: true }, layer('l2', shape('b'))],
+    } as VectorArtAsset;
+    expect(drawnShapes(art).map((s) => s.id)).toEqual(['b']);
+    expect(allShapes(art).map((s) => s.id)).toEqual(['a', 'b']);
+    expect(vectorArtDrawings(art).map((d) => d.id)).toEqual(['b']);
+  });
+
+  it('says where an item stands, and which outermost group it is in', () => {
+    const layers = nested();
+    expect(placeOf(layers, 'c')).toMatchObject({
+      index: 0,
+      depth: 2,
+      parent: { id: 'h' },
+      layer: { id: 'l1' },
+    });
+    expect(placeOf(layers, 'a')).toMatchObject({ index: 0, depth: 0, parent: null });
+    expect(placeOf(layers, 'nobody')).toBeNull();
+    expect(outermostGroup(layers, 'c')?.id).toBe('g');
+    expect(outermostGroup(layers, 'g')).toBeNull();
+    expect(outermostGroup(layers, 'a')).toBeNull();
+  });
+
+  it('reads a selection of shapes as the groups it covers completely', () => {
+    const layers = nested();
+    expect(selectedItems(layers, ['b', 'c'])).toEqual(['g']);
+    expect(selectedItems(layers, ['c'])).toEqual(['h']);
+    expect(selectedItems(layers, ['d', 'b', 'a'])).toEqual(['a', 'b', 'd']);
+  });
+
+  it('lists a layer top row first, a group over what is in it', () => {
+    expect(treeRows(nested()[0]!.items).map((r) => `${r.id}${r.depth}${r.group ? 'g' : ''}`)).toEqual([
+      'd0',
+      'g0g',
+      'h1g',
+      'c2',
+      'b1',
+      'a0',
+    ]);
+  });
+});
+
+describe('dropping a row', () => {
+  // d / g / h / c / b / a, as `treeRows` lists `nested()`.
+  const rows = [row('d', 0), row('g', 0, true), row('h', 1, true), row('c', 2), row('b', 1), row('a', 0)];
+
+  it('puts a row between two of the same list', () => {
+    expect(treeDrop(rows, 'a', 0, 0)).toEqual({ parentId: null, belowId: 'd' });
+    expect(treeDrop(rows, 'd', 6, 0)).toEqual({ parentId: null, belowId: null });
+  });
+
+  it('drops into a group right under its row, and deeper into the one inside', () => {
+    expect(treeDrop(rows, 'a', 2, 1)).toEqual({ parentId: 'g', belowId: 'h' });
+    expect(treeDrop(rows, 'a', 3, 2)).toEqual({ parentId: 'h', belowId: 'c' });
+    expect(treeDropDepth(rows, 'a', 3, 2)).toBe(2);
+  });
+
+  it('lets the pointer say which list the end of a group belongs to', () => {
+    // Between c (depth 2) and b (depth 1): the end of h, or g after h.
+    expect(treeDrop(rows, 'a', 4, 2)).toEqual({ parentId: 'h', belowId: null });
+    expect(treeDrop(rows, 'a', 4, 1)).toEqual({ parentId: 'g', belowId: 'b' });
+    // No shallower than the row below: g goes on under this gap.
+    expect(treeDrop(rows, 'a', 4, 0)).toEqual({ parentId: 'g', belowId: 'b' });
+    // Between b (depth 1) and a (depth 0), d dragged: the end of g, or the layer after g.
+    expect(treeDrop(rows, 'd', 5, 1)).toEqual({ parentId: 'g', belowId: null });
+    expect(treeDrop(rows, 'd', 5, 0)).toEqual({ parentId: null, belowId: 'a' });
+    expect(treeDropDepth(rows, 'd', 5, 7)).toBe(1);
+  });
+
+  it('takes a row out of its group when it is dropped where it stands, further left', () => {
+    // b is the last row of g; the gap under it, asked for at depth 0, is the layer.
+    expect(treeDrop(rows, 'b', 5, 0)).toEqual({ parentId: null, belowId: 'a' });
+    expect(treeDrop(rows, 'b', 5, 1)).toBeNull();
+  });
+
+  it('has no place where the row already is, inside itself, or for a row it does not know', () => {
+    expect(treeDrop(rows, 'd', 0, 0)).toBeNull();
+    expect(treeDrop(rows, 'd', 1, 0)).toBeNull();
+    expect(treeDrop(rows, 'g', 3, 2)).toBeNull();
+    expect(treeDrop(rows, 'g', 4, 1)).toBeNull();
+    expect(treeDropDepth(rows, 'g', 3, 2)).toBeNull();
+    expect(treeDrop(rows, 'nobody', 0, 0)).toBeNull();
+  });
+});
+
+describe('editing the tree', () => {
+  it('never touches the layers it was given', () => {
+    const layers = nested();
+    const before = JSON.stringify(layers);
+    withItemsRemoved(layers, ['c']);
+    withItemsGrouped(layers, ['a', 'd'], { id: 'n', name: 'N' });
+    withItemsMoved(layers, ['a'], 'l1', { parentId: 'h', belowId: null });
+    withGroupDissolved(layers, 'g');
+    expect(JSON.stringify(layers)).toBe(before);
+  });
+
+  it('takes a group away with its last item', () => {
+    expect(layersText(withItemsRemoved(nested(), ['c']))).toEqual(['l1: a g[b] d']);
+    expect(layersText(withItemsRemoved(nested(), ['c', 'b']))).toEqual(['l1: a d']);
+    expect(layersText(withItemsRemoved(nested(), ['g']))).toEqual(['l1: a d']);
+    expect(() => withItemsRemoved(nested(), ['nobody'])).toThrow(VectorTreeError);
+  });
+
+  it('groups items where the topmost of them stood, in the order they are drawn', () => {
+    const made = { id: 'n', name: 'N' };
+    expect(layersText(withItemsGrouped(nested(), ['d', 'a'], made))).toEqual(['l1: g[b h[c]] n[a d]']);
+    expect(layersText(withItemsGrouped(nested(), ['a', 'b'], made))).toEqual(['l1: g[n[a b] h[c]] d']);
+    expect(layersText(withItemsGrouped(nested(), ['g', 'a'], made))).toEqual(['l1: n[a g[b h[c]]] d']);
+  });
+
+  it('refuses a group across two layers', () => {
+    const layers = [...nested(), layer('l2', shape('e'))];
+    expect(() => withItemsGrouped(layers, ['a', 'e'], { id: 'n', name: 'N' })).toThrow(
+      'Shapes of different layers cannot be grouped',
+    );
+  });
+
+  it('dissolves a group into the place it stood in', () => {
+    expect(layersText(withGroupDissolved(nested(), 'g'))).toEqual(['l1: a b h[c] d']);
+    expect(layersText(withGroupDissolved(nested(), 'h'))).toEqual(['l1: a g[b c] d']);
+    expect(() => withGroupDissolved(nested(), 'a')).toThrow(VectorTreeError);
+  });
+
+  it('moves items into a group, out of one, and onto another layer', () => {
+    const layers = [...nested(), layer('l2', shape('e'))];
+    expect(layersText(withItemsMoved(layers, ['a'], 'l1', { parentId: 'g', belowId: 'b' }))).toEqual([
+      'l1: g[b a h[c]] d',
+      'l2: e',
+    ]);
+    expect(layersText(withItemsMoved(layers, ['c'], 'l1', { parentId: null, belowId: null }))).toEqual([
+      'l1: c a g[b] d',
+      'l2: e',
+    ]);
+    // Without a place it lands on top of the layer; a group goes as a whole.
+    expect(layersText(withItemsMoved(layers, ['g', 'a'], 'l2'))).toEqual(['l1: d', 'l2: e a g[b h[c]]']);
+  });
+
+  it('keeps a group whose only item is put back into it', () => {
+    expect(layersText(withItemsMoved(nested(), ['c'], 'l1', { parentId: 'h', belowId: null }))).toEqual([
+      'l1: a g[b h[c]] d',
+    ]);
+  });
+
+  it('refuses to move a group into itself or into a group that is not there', () => {
+    expect(() => withItemsMoved(nested(), ['g'], 'l1', { parentId: 'h', belowId: null })).toThrow(
+      'A group cannot be moved into itself',
+    );
+    expect(() => withItemsMoved(nested(), ['a'], 'l1', { parentId: 'nobody', belowId: null })).toThrow(
+      VectorTreeError,
+    );
+    expect(() => withItemsMoved(nested(), ['a'], 'l1', { parentId: 'd', belowId: null })).toThrow(
+      VectorTreeError,
+    );
+  });
+
+  it('keeps the last layer', () => {
+    expect(() => withLayerRemoved(nested(), 'l1')).toThrow('A drawing needs one layer');
+  });
+});
+
+function drawing() {
+  const fixture = makeProject();
+  const create = new CreateAssetCommand('vectorart', 'Logo');
+  fixture.stack.execute(create);
+  const assetId = create.asset.id;
+  const art = (project: Project = fixture.project): VectorArtAsset =>
+    findAsset(project, assetId) as VectorArtAsset;
+  const add = (layerId = ''): string => {
+    const command = new AddVectorShapeCommand(assetId, shape('x').rings, undefined, layerId);
+    fixture.stack.execute(command);
+    return command.shapeId;
+  };
+  const tree = () => layersText(art().layers).map((line) => line.replace(/^[^:]+: /, ''));
+  return { ...fixture, assetId, art, add, tree };
+}
+
+describe('the commands', () => {
+  it('starts a drawing with one layer and draws onto the topmost one unless told', () => {
+    const f = drawing();
+    expect(f.art().layers.map((l) => l.name)).toEqual(['Layer 1']);
+    const first = f.add();
+    const addLayer = new AddVectorLayerCommand(f.assetId);
+    f.stack.execute(addLayer);
+    expect(f.art().layers.map((l) => l.name)).toEqual(['Layer 1', 'Layer 2']);
+    const second = f.add();
+    const third = f.add(f.art().layers[0]!.id);
+    expect(f.tree()).toEqual([`${first} ${third}`, second]);
+    // Numbered across the layers: no two shapes of a drawing share a name.
+    expect(allShapes(f.art()).map((s) => s.name)).toEqual(['Shape 1', 'Shape 3', 'Shape 2']);
+  });
+
+  it('groups and ungroups in one undo step each, under the same id on redo', () => {
+    const f = drawing();
+    const [a, b, c] = [f.add(), f.add(), f.add()];
+    const made = new GroupVectorShapesCommand(f.assetId, [a, c]);
+    f.stack.execute(made);
+    expect(f.tree()).toEqual([`${b} ${made.groupId}[${a} ${c}]`]);
+    expect(placeOf(f.art().layers, made.groupId)?.item.name).toBe('Group 1');
+    f.stack.undo();
+    expect(f.tree()).toEqual([`${a} ${b} ${c}`]);
+    f.stack.redo();
+    expect(f.tree()).toEqual([`${b} ${made.groupId}[${a} ${c}]`]);
+    f.stack.execute(new UngroupVectorShapesCommand(f.assetId, [made.groupId]));
+    expect(f.tree()).toEqual([`${b} ${a} ${c}`]);
+    f.stack.undo();
+    expect(f.tree()).toEqual([`${b} ${made.groupId}[${a} ${c}]`]);
+  });
+
+  it('brings a removed group back with a shape edited before it went', () => {
+    const f = drawing();
+    const [a, b] = [f.add(), f.add()];
+    const made = new GroupVectorShapesCommand(f.assetId, [a, b]);
+    f.stack.execute(made);
+    f.stack.execute(new SetVectorShapeRingsCommand(f.assetId, a, shape('y', 50).rings));
+    f.stack.execute(new RemoveVectorShapeCommand(f.assetId, made.groupId));
+    expect(allShapes(f.art())).toEqual([]);
+    f.stack.undo();
+    expect(f.tree()).toEqual([`${made.groupId}[${a} ${b}]`]);
+    expect(allShapes(f.art())[0]!.rings[0]!.points[0]).toEqual({ x: 50, y: 0 });
+    f.stack.undo();
+    expect(allShapes(f.art())[0]!.rings[0]!.points[0]).toEqual({ x: 0, y: 0 });
+  });
+
+  it('removes shapes out of a group and takes the emptied group with them', () => {
+    const f = drawing();
+    const [a, b, c] = [f.add(), f.add(), f.add()];
+    const made = new GroupVectorShapesCommand(f.assetId, [a, b]);
+    f.stack.execute(made);
+    f.stack.execute(new RemoveVectorShapesCommand(f.assetId, [a, b]));
+    expect(f.tree()).toEqual([c]);
+    f.stack.undo();
+    expect(f.tree()).toEqual([`${made.groupId}[${a} ${b}] ${c}`]);
+  });
+
+  it('orders an item among those of its own list only', () => {
+    const f = drawing();
+    const [a, b, c] = [f.add(), f.add(), f.add()];
+    const made = new GroupVectorShapesCommand(f.assetId, [a, b]);
+    f.stack.execute(made);
+    f.stack.execute(new OrderVectorShapeCommand(f.assetId, a, 'front'));
+    expect(f.tree()).toEqual([`${made.groupId}[${b} ${a}] ${c}`]);
+    // Already on top of its group: the shape outside is no step further.
+    expect(() => f.stack.execute(new OrderVectorShapeCommand(f.assetId, a, 'forward'))).toThrow();
+    f.stack.execute(new OrderVectorShapeCommand(f.assetId, made.groupId, 'front'));
+    expect(f.tree()).toEqual([`${c} ${made.groupId}[${b} ${a}]`]);
+  });
+
+  it('duplicates a group with all that is in it, right over it, under new ids', () => {
+    const f = drawing();
+    const [a, b] = [f.add(), f.add()];
+    const made = new GroupVectorShapesCommand(f.assetId, [a, b]);
+    f.stack.execute(made);
+    const copy = new DuplicateVectorShapeCommand(f.assetId, made.groupId);
+    f.stack.execute(copy);
+    const place = placeOf(f.art().layers, copy.shapeId)!;
+    expect(place.index).toBe(1);
+    expect(place.item.name).toBe('Group 1 copy');
+    const ids = allShapes(f.art()).map((s) => s.id);
+    expect(ids).toHaveLength(4);
+    expect(new Set(ids).size).toBe(4);
+    const copied = ids[2]!;
+    f.stack.undo();
+    f.stack.redo();
+    expect(allShapes(f.art())[2]!.id).toBe(copied);
+  });
+
+  it('duplicates a shape inside its group', () => {
+    const f = drawing();
+    const [a, b] = [f.add(), f.add()];
+    const made = new GroupVectorShapesCommand(f.assetId, [a, b]);
+    f.stack.execute(made);
+    const copy = new DuplicateVectorShapeCommand(f.assetId, a);
+    f.stack.execute(copy);
+    expect(f.tree()).toEqual([`${made.groupId}[${a} ${copy.shapeId} ${b}]`]);
+  });
+
+  it('combines shapes across groups into the lowest and undoes it with the groups', () => {
+    const f = drawing();
+    const [a, b] = [f.add(), f.add()];
+    const made = new GroupVectorShapesCommand(f.assetId, [b]);
+    f.stack.execute(made);
+    const combine = new CombineVectorShapesCommand(f.assetId, [a, b], 'union');
+    f.stack.execute(combine);
+    expect(combine.shapeId).toBe(a);
+    expect(f.tree()).toEqual([a]);
+    f.stack.undo();
+    expect(f.tree()).toEqual([`${a} ${made.groupId}[${b}]`]);
+  });
+
+  it('renames a group and refuses a blank name', () => {
+    const f = drawing();
+    const made = new GroupVectorShapesCommand(f.assetId, [f.add()]);
+    f.stack.execute(made);
+    f.stack.execute(new RenameVectorShapeCommand(f.assetId, made.groupId, ' Wheels '));
+    expect(placeOf(f.art().layers, made.groupId)?.item.name).toBe('Wheels');
+    expect(() => f.stack.execute(new RenameVectorShapeCommand(f.assetId, made.groupId, ' '))).toThrow(
+      'A group needs a name',
+    );
+    f.stack.undo();
+    expect(placeOf(f.art().layers, made.groupId)?.item.name).toBe('Group 1');
+  });
+
+  it('moves shapes to another layer and back with an undo', () => {
+    const f = drawing();
+    const [a, b] = [f.add(), f.add()];
+    const upper = new AddVectorLayerCommand(f.assetId);
+    f.stack.execute(upper);
+    f.stack.execute(new MoveVectorShapesCommand(f.assetId, [a], upper.layerId));
+    expect(f.tree()).toEqual([b, a]);
+    f.stack.undo();
+    expect(f.tree()).toEqual([`${a} ${b}`, '']);
+  });
+
+  it('adds a layer over the one named, reorders, renames and removes with all on it', () => {
+    const f = drawing();
+    const base = f.art().layers[0]!.id;
+    const top = new AddVectorLayerCommand(f.assetId);
+    f.stack.execute(top);
+    const between = new AddVectorLayerCommand(f.assetId, base);
+    f.stack.execute(between);
+    expect(f.art().layers.map((l) => l.id)).toEqual([base, between.layerId, top.layerId]);
+    f.stack.execute(new MoveVectorLayerCommand(f.assetId, 0, 2));
+    expect(f.art().layers.map((l) => l.id)).toEqual([between.layerId, top.layerId, base]);
+    f.stack.execute(new RenameVectorLayerCommand(f.assetId, base, 'Ink'));
+    expect(f.art().layers[2]!.name).toBe('Ink');
+    expect(() => f.stack.execute(new RenameVectorLayerCommand(f.assetId, base, '  '))).toThrow(
+      'A layer needs a name',
+    );
+    const a = f.add(base);
+    f.stack.execute(new RemoveVectorLayerCommand(f.assetId, base));
+    expect(allShapes(f.art())).toEqual([]);
+    f.stack.undo();
+    expect(allShapes(f.art()).map((s) => s.id)).toEqual([a]);
+  });
+
+  it('makes a new picture of a layer hidden, and the old one of it shown again', () => {
+    const f = drawing();
+    f.add();
+    const layerId = f.art().layers[0]!.id;
+    const shown = vectorArtRevision(f.art());
+    f.stack.execute(new SetVectorLayerHiddenCommand(f.assetId, layerId, true));
+    expect(f.art().layers[0]!.hidden).toBe(true);
+    expect(vectorArtRevision(f.art())).not.toBe(shown);
+    expect(vectorArtDrawings(f.art())).toEqual([]);
+    f.stack.execute(new SetVectorLayerHiddenCommand(f.assetId, layerId, false));
+    expect('hidden' in f.art().layers[0]!).toBe(false);
+    expect(vectorArtRevision(f.art())).toBe(shown);
+  });
+
+  it('draws the same picture whatever groups the shapes stand in', () => {
+    const f = drawing();
+    const [a, b] = [f.add(), f.add()];
+    const flat = vectorArtRevision(f.art());
+    f.stack.execute(new GroupVectorShapesCommand(f.assetId, [a, b]));
+    expect(vectorArtRevision(f.art())).toBe(flat);
+  });
+});
+
+describe('the project file', () => {
+  it('gives layers, groups and the hidden mark back as they were written', () => {
+    const f = drawing();
+    const [a, b] = [f.add(), f.add()];
+    f.stack.execute(new GroupVectorShapesCommand(f.assetId, [a, b]));
+    const upper = new AddVectorLayerCommand(f.assetId);
+    f.stack.execute(upper);
+    f.add(upper.layerId);
+    f.stack.execute(new SetVectorLayerHiddenCommand(f.assetId, upper.layerId, true));
+    expect(f.art(roundTrip(f.project)).layers).toEqual(f.art().layers);
+  });
+
+  it('reads the flat shapes of an older drawing as its one layer', () => {
+    const f = drawing();
+    const art = f.art() as unknown as Record<string, unknown>;
+    delete art.layers;
+    art.shapes = [shape('a'), shape('b')];
+    const loaded = f.art(roundTrip(f.project));
+    expect(loaded.layers).toHaveLength(1);
+    expect(loaded.layers[0]).toMatchObject({ name: 'Layer 1' });
+    expect(text(loaded.layers[0]!.items)).toBe('a b');
+    expect('shapes' in loaded).toBe(false);
+  });
+
+  it('drops an empty group, a doubled id anywhere in the drawing and a layer without an id', () => {
+    const f = drawing();
+    const art = f.art() as unknown as Record<string, unknown>;
+    art.layers = [
+      {
+        id: 'l1',
+        name: 'One',
+        items: [shape('a'), { id: 'g', name: 'G', items: [] }, group('h', shape('a'))],
+      },
+      { id: 'l1', name: 'Twin', items: [shape('b')] },
+      { name: 'Nameless', items: [shape('c')] },
+      { id: 'l2', hidden: 'yes', items: [shape('a'), shape('d')] },
+    ];
+    const loaded = f.art(roundTrip(f.project));
+    expect(layersText(loaded.layers)).toEqual(['l1: a', 'l2: d']);
+    expect(loaded.layers[1]).toMatchObject({ name: 'Layer' });
+    expect('hidden' in loaded.layers[1]!).toBe(false);
+  });
+
+  it('gives a drawing left without a layer one to draw on', () => {
+    const f = drawing();
+    (f.art() as unknown as Record<string, unknown>).layers = [];
+    const loaded = f.art(roundTrip(f.project));
+    expect(loaded.layers).toHaveLength(1);
+    expect(loaded.layers[0]!.items).toEqual([]);
+  });
+});