Let variables, fields, ports and list items be dragged into order (#310)

Variables of a behavior, the fields of a struct, the ports of a start
or end node and the items of a list could only be added and removed.
Changing their order meant removing and adding them again. Now each of
them is dragged by the mark in front of its row.

- sortBlocks (ui/blockDrag.ts) makes the rows of one column sortable.
  A block is every row it covers, so a struct moves with its fields and
  a list item with its nested list. The block is dimmed while it is
  carried, an accent line shows where it lands, and a scrolled panel
  follows the pointer at its edge. Less than 4 px stays a click.
- The arithmetic is pure and lives in ui/sortable.ts: blockGap,
  gapLineY, dropSlot and edgeScroll. dropIndex of the tile strip moved
  there too, so the reorder maths sits in one file.
- MoveVariableCommand, MoveStructFieldCommand and MovePortCommand
  build on the existing MoveCommand. Undo moves back.
- Lists need no command. InlineList replaces the list as a whole, as
  it does for every other edit of its items.
- The grip is the kind mark, for a list item its number. Moving only
  happens among siblings: a field stays in its struct.
- On a Struct asset the move runs through WithStructTypes. Typed
  variables, Pack/Expand ports and split outputs follow the new order.
- Values and edges hang on names, so only the order changes. Nodes
  running a behavior read their ports from the start node, the new
  order reaches them without a copy.
- Not sortable by design: the variables of a placed object, the fields
  of a typed struct (moved on the Struct asset) and the picked ports
  of Variables/Properties nodes.
- Test hook __gm.inspectorMark(label) finds the mark of a row.

Tests:
- Gate on melektaus-atx: 235 files, 3572 tests, CHECK EXIT 0 (+9).
- tests/core/reorder.test.ts: a variable, a field two levels deep, a
  Struct asset reaching a typed variable and Pack, and start ports
  reaching the calling node with edges and result kept.
- e2e 22: ALL PASSED, 39 checks (+4, sum 1739). 22f-iii drags a
  variable past a struct of several rows, then a field inside it.
- Counter-probes: dropSlot by top edge instead of middle and a field
  move that ignores its path fail 3 unit tests. With sortBlocks
  switched off, the three order checks of 22f-iii fail.

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

src/core/commands/portCommands.ts

@@ -23,8 +23,8 @@ import { isGraphAsset } from '../model/Asset';
 import type { PropertyValue } from '../model/Properties';
 import { kindLabel } from '../properties/kinds';
 import type { Project } from '../model/Project';
-import { type Removed, removeFirst, removeWhere, restoreAt } from '../util/list';
-import { type Command, CommandError, RenameCommand } from './Command';
+import { moveItem, type Removed, removeFirst, removeWhere, restoreAt } from '../util/list';
+import { type Command, CommandError, MoveCommand, RenameCommand } from './Command';
 import { nextFieldName } from './naming';
 import { requireGraph, requireNode, requireNodeDef } from './require';
 
@@ -303,6 +303,29 @@ export class RenamePortCommand extends RenameCommand {
   }
 }
 
+/**
+ * Edges hang on a port's name, so they stay connected; a running or calling node takes the order
+ * from here, as it takes the ports themselves (#310).
+ */
+export class MovePortCommand extends MoveCommand {
+  constructor(
+    private readonly assetId: string,
+    private readonly nodeId: string,
+    from: number,
+    to: number,
+  ) {
+    super('Reorder ports', from, to);
+  }
+
+  protected move(project: Project, from: number, to: number): void {
+    const node = requireNode(project, this.assetId, this.nodeId);
+    const ports = [...(node.ports ?? [])];
+    if (!moveItem(ports, from, to))
+      throw new CommandError(`There is no port ${from + 1} to move to place ${to + 1}`);
+    node.ports = ports;
+  }
+}
+
 /** Renames one declared port on one node, taking its edges and its typed-in value along. */
 function renamePortOn(graph: Graph, node: GraphNode, from: string, to: string): void {
   const ports = node.ports ?? [];

src/core/commands/variableCommands.ts

@@ -27,8 +27,8 @@ import {
   variableDef,
 } from '../properties/PropertySchema';
 import { valueAt, withValueAt } from '../properties/valuePath';
-import { type Removed, removeFirst, restoreAt } from '../util/list';
-import { type Command, CommandError, RenameCommand } from './Command';
+import { moveItem, type Removed, removeFirst, restoreAt } from '../util/list';
+import { type Command, CommandError, MoveCommand, RenameCommand } from './Command';
 import { renameVariablePorts } from './portCommands';
 import { requireBehavior, requireVariableOwner } from './require';
 
@@ -183,6 +183,22 @@ export class RenameVariableCommand extends RenameCommand {
   }
 }
 
+/** Only the order changes: values and the ports naming a variable go by its name (#310). */
+export class MoveVariableCommand extends MoveCommand {
+  constructor(
+    private readonly assetId: string,
+    from: number,
+    to: number,
+  ) {
+    super('Reorder variables', from, to);
+  }
+
+  protected move(project: Project, from: number, to: number): void {
+    if (!moveItem(requireVariableOwner(project, this.assetId).variables, from, to))
+      throw new CommandError(`There is no variable ${from + 1} to move to place ${to + 1}`);
+  }
+}
+
 /** The behavior, once it is known to declare `key`. */
 function requireBehaviorVariable(project: Project, assetId: string, key: string): BehaviorAsset {
   const asset = requireBehavior(project, assetId);
@@ -467,3 +483,25 @@ export class RenameStructFieldCommand extends RenameCommand {
       site.write(withFieldRenamed(site.read(), from, to));
   }
 }
+
+/** A struct value is read by field name, so only the declaration changes order (#310). */
+export class MoveStructFieldCommand extends MoveCommand {
+  /** `path` names the struct whose fields are reordered: none for the variable itself. */
+  constructor(
+    private readonly assetId: string,
+    private readonly key: string,
+    from: number,
+    to: number,
+    private readonly path: readonly string[] = [],
+  ) {
+    super('Reorder fields', from, to);
+  }
+
+  protected move(project: Project, from: number, to: number): void {
+    const owner = requireFieldOwner(project, this.assetId, this.key, this.path);
+    const fields = [...owner.fields()];
+    if (!moveItem(fields, from, to))
+      throw new CommandError(`There is no field ${from + 1} to move to place ${to + 1}`);
+    owner.setFields(fields);
+  }
+}

src/editor/debug/uiHooks.ts

@@ -54,7 +54,9 @@ export function uiHooks({ app, state, editor }: HookContext) {
     return control && { ...centreOf(control), left: control.getGlobalPosition().x };
   };
   /** The control right of the row label `label`, as `fieldOf` finds it. */
-  const controlOf = (label: string, last: boolean): Widget | null => {
+  const controlOf = (label: string, last: boolean): Widget | null => besideOf(label, last, 1);
+  /** The widget `step` places along from the row label `label`: +1 its control, -1 its mark. */
+  const besideOf = (label: string, last: boolean, step: number): Widget | null => {
     let found: Text | null = null;
     const walk = (node: Container): void => {
       if ((found !== null && !last) || !node.visible) return;
@@ -70,7 +72,7 @@ export function uiHooks({ app, state, editor }: HookContext) {
     while (owner && !(owner instanceof Widget)) owner = owner.parent;
     const row = owner?.parent;
     if (!row || !owner) return null;
-    const control = row.children[row.children.indexOf(owner) + 1];
+    const control = row.children[row.children.indexOf(owner) + step];
     if (!(control instanceof Widget)) return null;
     // Taller than the window since its groups have bands (#290): the row is scrolled into view.
     inspector.reveal(control);
@@ -164,6 +166,11 @@ export function uiHooks({ app, state, editor }: HookContext) {
       return (last ? hits.at(-1) : hits[0]) ?? null;
     },
     inspectorField: fieldOf,
+    /** The mark in front of the row named `label` – what a variable, a field or a port is dragged by (#310). */
+    inspectorMark: (label: string, last = false) => {
+      const mark = besideOf(label, last, -1);
+      return mark && centreOf(mark);
+    },
     /** The eyedropper beside the object field of a row (#276): where to press it, and whether it is lit. */
     inspectorEyedropper: (label: string, last = false) => {
       const button = controlOf(label, last)?.children.at(-1);

src/editor/inspector/assetSection.ts

@@ -12,6 +12,8 @@ import { nextFieldName, nextVariableName } from '../../core/commands/naming';
 import {
   AddStructFieldCommand,
   AddVariableCommand,
+  MoveStructFieldCommand,
+  MoveVariableCommand,
   RemoveStructFieldCommand,
   RemoveVariableCommand,
   RenameStructFieldCommand,
@@ -51,7 +53,7 @@ import {
 } from '../../core/properties/PropertySchema';
 import { markedAssetSchema } from '../../core/properties/deviation';
 import { isModified, resolveAssetProps } from '../../core/properties/resolve';
-import { Button, Dropdown, Label, Menu, type MenuEntry } from '../../ui';
+import { Button, Dropdown, Label, Menu, type MenuEntry, type SortBlock, sortBlocks } from '../../ui';
 import { FontPreview } from '../font/FontPreview';
 import { clearSaved, hasSaved } from '../saves';
 import { SoundPreview } from '../sound/SoundPreview';
@@ -59,7 +61,7 @@ import type { VariablesOwner } from '../state';
 import { aspectActions, sizeCommand } from './aspectAction';
 import { buildCutRows, CUT_ROW_KEYS, refreshCutRows } from './cutSection';
 import { actionIf, createFieldRow, type FieldHandlers } from './fieldFactory';
-import { addNameField, addNote, addTitle, type InspectorHost, sectionHeader } from './host';
+import { addedRows, addNameField, addNote, addTitle, type InspectorHost, sectionHeader } from './host';
 import { buildShapeListSection, shapeListSignature } from './shapeSection';
 import { buildVariable, kindMenu, type StructTyping, type VariableEditing } from './variableRows';
 import { listSource, previewingHandlers, structFields } from './previewHandlers';
@@ -289,6 +291,7 @@ export function buildVariables(host: InspectorHost, asset: VariableOwnerAsset, i
   const values = shownValues(host, asset);
   const renamed = (path: readonly string[], from: string, to: string): FieldRename | undefined =>
     isType ? { structType: asset.id, path, from, to } : undefined;
+  const blocks: SortBlock[] = [];
   for (const def of asset.variables) {
     const edit: VariableEditing = {
       rename: (next) => run(new RenameVariableCommand(asset.id, def.key, next), renamed([], def.key, next)),
@@ -308,6 +311,7 @@ export function buildVariables(host: InspectorHost, asset: VariableOwnerAsset, i
             path,
           ),
         ),
+      moveField: (path, from, to) => run(new MoveStructFieldCommand(asset.id, def.key, from, to, path)),
     };
     const behavior = asset.type === 'behavior';
     const keep = behavior
@@ -324,11 +328,13 @@ export function buildVariables(host: InspectorHost, asset: VariableOwnerAsset, i
       : undefined;
     const value = values[def.key] ?? def.default;
     const typing = structTyping(host, asset, def.key);
-    host.fields.set(
-      def.key,
+    const { rows, built } = addedRows(host, () =>
       buildVariable(host, def, value, assetHandlers(host, asset.id, def), { edit, typing, keep, hidden }),
     );
+    host.fields.set(def.key, built);
+    blocks.push({ grip: built.grip, rows });
   }
+  sortBlocks(blocks, (from, to) => run(new MoveVariableCommand(asset.id, from, to)));
 }
 
 /**

src/editor/inspector/host.ts

@@ -8,6 +8,7 @@ import {
   type TextFieldOptions,
   Theme,
   type VStack,
+  Widget,
 } from '../../ui';
 import { foldKey, isFolded, setFolded } from '../sectionFolds';
 import type { EditorState } from '../state';
@@ -70,6 +71,14 @@ export function addNote(
   host.body.add(new Label(text, { style }));
 }
 
+/** The rows `build` adds to the body – what one block of a sortable group covers (#310). */
+export function addedRows<T>(host: InspectorHost, build: () => T): { rows: Widget[]; built: T } {
+  const before = host.body.children.length;
+  const built = build();
+  const rows = host.body.children.slice(before).filter((child): child is Widget => child instanceof Widget);
+  return { rows, built };
+}
+
 export function addGap(host: InspectorHost): void {
   host.body.add(new Label(''));
 }

src/editor/inspector/portSection.ts

@@ -1,6 +1,7 @@
 import {
   AddPortCommand,
   AddPortsCommand,
+  MovePortCommand,
   nextPortName,
   RemovePortCommand,
   RenamePortCommand,
@@ -41,6 +42,8 @@ import {
   Menu,
   type MenuItem,
   type SectionHeader,
+  type SortBlock,
+  sortBlocks,
   Spacer,
   TextField,
   Theme,
@@ -261,7 +264,10 @@ export function buildPortSection(
             : 'No outputs yet. Every output added here becomes a result of running this behavior; one of the kind Flow is a further way out.',
     );
 
-  for (const port of ports) portRow(host, assetId, node, port, portNameField(host, assetId, node, port));
+  const blocks = ports.map((port) =>
+    portRow(host, assetId, node, port, portNameField(host, assetId, node, port)),
+  );
+  sortBlocks(blocks, (from, to) => host.state.execute(new MovePortCommand(assetId, node.id, from, to)));
 }
 
 const plural = (noun: string) => `${noun.toLowerCase()}s`;
@@ -286,15 +292,17 @@ function portRow(
   port: DeclaredPort,
   name: Widget,
   extras: readonly Widget[] = [],
-): void {
+): SortBlock {
   const row = new HStack({ gap: Theme.gap, align: 'center' });
   row.minHeight = Theme.rowHeight;
-  row.add(markLabel(port.kind));
+  const mark = markLabel(port.kind);
+  row.add(mark);
   row.add(name, 1);
   for (const extra of extras) row.add(extra);
   row.add(new Spacer());
   row.add(removeButton(() => host.state.execute(new RemovePortCommand(assetId, node.id, port.key))));
   host.body.add(row);
+  return { grip: mark, rows: [row] };
 }
 
 /**

src/editor/inspector/variableRows.ts

@@ -10,6 +10,8 @@ import {
   Menu,
   type MenuEntry,
   type MenuItem,
+  type SortBlock,
+  sortBlocks,
   Spacer,
   TextField,
   Theme,
@@ -33,7 +35,7 @@ import {
   structFieldValue,
 } from './fieldFactory';
 import { KIND_ICON_SVG, VARIABLE_SWITCH_ICON_SVG } from '../panels/markIcons';
-import { addNote, type InspectorHost } from './host';
+import { addedRows, addNote, type InspectorHost } from './host';
 
 /** The menu every "add a kind" button opens: one entry per kind, named as the kind table names it. */
 export function kindMenu<K extends string>(kinds: readonly K[], pick: (kind: K) => void): MenuItem[] {
@@ -50,6 +52,8 @@ export interface VariableEditing {
   renameField: (path: readonly string[], field: StructFieldDef, next: string) => void;
   removeField: (path: readonly string[], field: StructFieldDef) => void;
   addField: (path: readonly string[], kind: NestedKind) => void;
+  /** A field dragged to another place among the fields of its struct (#310). */
+  moveField: (path: readonly string[], from: number, to: number) => void;
 }
 
 /**
@@ -82,6 +86,11 @@ export interface VariableOptions {
   actions?: RowActions;
 }
 
+/** A variable's first row, and its mark – where the asset lets it be dragged to another place (#310). */
+export interface VariableRow extends FieldRow {
+  grip: Widget;
+}
+
 /**
  * One variable, drawn the same on its asset and on a placed object – minus `edit` there, since the
  * shape belongs to the asset. Adds its rows to the body itself and returns the first.
@@ -92,10 +101,11 @@ export function buildVariable(
   value: PropertyValue,
   handlers: FieldHandlers,
   options: VariableOptions = {},
-): FieldRow {
+): VariableRow {
   const { edit, typing, keep, hidden, actions } = options;
   const name = nameCell(def.key, 0, edit && ((next) => edit.rename(next)));
-  const head = gridRow(0, markLabel(def.kind), name);
+  const grip = markLabel(def.kind);
+  const head = gridRow(0, grip, name);
   let setValue: (next: PropertyValue) => void = () => {};
   let below: Widget | undefined;
   // A struct with a type has the type's fields: they are changed on the Struct asset, not here.
@@ -138,6 +148,7 @@ export function buildVariable(
 
   return {
     row: head,
+    grip,
     setValue: (next) => setValue(next),
     setModified: (modified) => {
       if (name instanceof Label) name.setStyle(modified ? 'override' : 'muted');
@@ -175,7 +186,8 @@ function lockValue(field: Field, locked: boolean): void {
 /**
  * One row per field, each with its own value; a field that is a struct again is a head row with its
  * fields indented below it (#239). `path` names the struct these fields belong to. Returns the
- * setter that pushes a bundle into them.
+ * setter that pushes a bundle into them. Where the fields are edited, each one – its own fields
+ * and items included – is dragged by its mark among its siblings (#310).
  */
 function structFieldRows(
   host: InspectorHost,
@@ -188,58 +200,69 @@ function structFieldRows(
   locked: boolean,
 ): (next: PropertyValue) => void {
   const setters = new Map<string, (item: PropertyValue) => void>();
+  const blocks: SortBlock[] = [];
   for (const field of fields) {
-    const row = fieldRow(field, path, edit);
+    const { row, mark } = fieldRow(field, path, edit);
     const item = structFieldValue(value, field);
-    if (field.kind === 'struct') {
-      const inner = [...path, field.key];
-      const typed = typing?.typeName(field.structType) ?? null;
-      row.add(new Spacer(), 1);
-      if (edit && typed === null) row.add(addFieldMenu(edit, inner));
+    const { rows } = addedRows(host, () => {
+      if (field.kind === 'struct') {
+        const inner = [...path, field.key];
+        const typed = typing?.typeName(field.structType) ?? null;
+        row.add(new Spacer(), 1);
+        if (edit && typed === null) row.add(addFieldMenu(edit, inner));
+        if (edit) row.add(removeButton(() => edit.removeField(path, field)));
+        host.body.add(row);
+        if (typing)
+          host.body.add(typeRow(inner.length + 1, typed, () => typing.menu(inner, field.structType)));
+        const own = typed === null;
+        setters.set(
+          field.key,
+          structFieldRows(
+            host,
+            field.fields,
+            inner,
+            item,
+            handlers,
+            own ? edit : undefined,
+            own ? typing : undefined,
+            locked,
+          ),
+        );
+        return;
+      }
+      const own = handlers.structFieldHandlers?.([...path, field.key], field) ?? READ_ONLY;
+      const sub = createField(host, fieldDef(field), item, own);
+      setters.set(field.key, addValue(row, sub));
+      lockValue(sub, locked);
       if (edit) row.add(removeButton(() => edit.removeField(path, field)));
       host.body.add(row);
-      if (typing) host.body.add(typeRow(inner.length + 1, typed, () => typing.menu(inner, field.structType)));
-      const own = typed === null;
-      setters.set(
-        field.key,
-        structFieldRows(
-          host,
-          field.fields,
-          inner,
-          item,
-          handlers,
-          own ? edit : undefined,
-          own ? typing : undefined,
-          locked,
-        ),
-      );
-      continue;
-    }
-    const own = handlers.structFieldHandlers?.([...path, field.key], field) ?? READ_ONLY;
-    const sub = createField(host, fieldDef(field), item, own);
-    setters.set(field.key, addValue(row, sub));
-    lockValue(sub, locked);
-    if (edit) row.add(removeButton(() => edit.removeField(path, field)));
-    host.body.add(row);
-    if (field.kind === 'list' && typing) {
-      const inner = [...path, field.key];
-      host.body.add(
-        itemsRow(path.length + 2, typing.typeName(field.itemType), () =>
-          typing.listMenu(inner, field.itemType),
-        ),
-      );
-    }
-    if (sub.below) host.body.add(indentedBy(sub.below, path.length + 2));
+      if (field.kind === 'list' && typing) {
+        const inner = [...path, field.key];
+        host.body.add(
+          itemsRow(path.length + 2, typing.typeName(field.itemType), () =>
+            typing.listMenu(inner, field.itemType),
+          ),
+        );
+      }
+      if (sub.below) host.body.add(indentedBy(sub.below, path.length + 2));
+    });
+    blocks.push({ grip: mark, rows });
   }
+  if (edit) sortBlocks(blocks, (from, to) => edit.moveField(path, from, to));
   return (next) => {
     for (const field of fields) setters.get(field.key)?.(structFieldValue(next, field));
   };
 }
 
-function fieldRow(field: StructFieldDef, path: readonly string[], edit: VariableEditing | undefined): HStack {
+function fieldRow(
+  field: StructFieldDef,
+  path: readonly string[],
+  edit: VariableEditing | undefined,
+): { row: HStack; mark: Widget } {
   const depth = path.length + 1;
   const name = nameCell(field.key, depth, edit && ((next) => edit.renameField(path, field, next)));
-  return gridRow(depth, markLabel(field.kind), name);
+  const mark = markLabel(field.kind);
+  return { row: gridRow(depth, mark, name), mark };
 }
 
 /** Which Struct asset a struct follows, as a row of its own under the struct's head. */

src/editor/list/InlineList.ts

@@ -17,6 +17,8 @@ import {
   Menu,
   type MenuEntry,
   ScrollContainer,
+  type SortBlock,
+  sortBlocks,
   Spacer,
   TextField,
   Theme,
@@ -40,6 +42,7 @@ import {
 import type { Project } from '../../core/model/Project';
 import { structAssetOf, typedItem, typeFields } from '../../core/model/structTypes';
 import { nextFieldName } from '../../core/commands/naming';
+import { moveItem } from '../../core/util/list';
 import { type FieldKind, NESTED_KINDS, type NestedKind } from '../../core/properties/kinds';
 import {
   type ListItem,
@@ -312,7 +315,8 @@ export class InlineList extends VStack {
     this.more = null;
     if (access) {
       const { shown, more } = shownItems(access.value.length, this.options.limit);
-      access.value.slice(0, shown).forEach((item, index) => this.itemRow(access, item, index));
+      const blocks = access.value.slice(0, shown).map((item, index) => this.itemRow(access, item, index));
+      sortBlocks(blocks, (from, to) => this.moveItem(from, to));
       if (more > 0) this.add(this.moreLine(access, more));
     }
     this.relayout();
@@ -332,7 +336,16 @@ export class InlineList extends VStack {
     return line;
   }
 
-  private itemRow(access: ListAccess, item: ListValue[number], index: number): void {
+  /** An item dragged by its number to another place (#310): the order is part of the value. */
+  private moveItem(from: number, to: number): void {
+    const access = this.access();
+    if (!access) return;
+    const next = [...access.value];
+    if (moveItem(next, from, to)) access.replace(next);
+  }
+
+  /** The item's rows, its number being where it is dragged from. */
+  private itemRow(access: ListAccess, item: ListValue[number], index: number): SortBlock {
     const kind = itemKindOf(item);
     // A struct item is a stack of field rows: its row grows with it and aligns to the top.
     const tall = kind === 'struct';
@@ -365,7 +378,10 @@ export class InlineList extends VStack {
     const widgets = this.rowWidgets[index];
     if (widgets) widgets.remove = remove;
     this.add(row);
-    if (below) this.add(indented(below, INDEX_WIDTH + Theme.gap));
+    if (!below) return { grip: label, rows: [row] };
+    const items = indented(below, INDEX_WIDTH + Theme.gap);
+    this.add(items);
+    return { grip: label, rows: [row, items] };
   }
 
   /** A list inside this one: its head goes where a value would, its items below, one step further in. */

src/editor/tile/PictureStrip.ts

@@ -8,6 +8,7 @@ import type { TileAsset } from '../../core/model/Asset';
 import { pictureChoices, textureRefLabel } from '../../core/properties/textureRef';
 import {
   Button,
+  dropIndex,
   type DropSpot,
   HStack,
   Label,
@@ -22,24 +23,6 @@ import type { EditorState } from '../state';
 
 const WIDTH = 210;
 
-/**
- * Where a dragged row lands, counted in the list it has *left* – which is what the move command
- * takes. A drop on the middle of a row takes that row's place: the list is flat, so there is no
- * "inside" for it to mean, and without this a drag downwards onto the next row would cancel itself
- * out. The gaps above and below a row are insert points, and one below the dragged row shifts up
- * by one once the row is out. `-1` is "nothing to do".
- */
-export function dropIndex(from: number, spot: DropSpot, length: number): number {
-  const last = length - 1;
-  if (from < 0 || from > last) return -1;
-  if (spot.at === 'end') return last;
-  const target = Number(spot.targetId);
-  if (!Number.isInteger(target) || target < 0 || target > last) return -1;
-  if (spot.at === 'onto') return target;
-  const insert = target + (spot.at === 'after' ? 1 : 0);
-  return Math.min(last, insert > from ? insert - 1 : insert);
-}
-
 /**
  * The picture list beside the board (#118): one row per animation step, in the order they run.
  * Rows are addressed by their place rather than by the picture they name – the same block may be

src/ui/blockDrag.ts

@@ -0,0 +1,152 @@
+import { type Container, type FederatedPointerEvent, Graphics, Point } from 'pixi.js';
+import { cancelFrameWork, onFrame } from './render';
+import { blockGap, type BlockSpan, dropSlot, edgeScroll, gapLineY } from './sortable';
+import { Theme } from './Theme';
+import type { Widget } from './Widget';
+import { ScrollContainer } from './widgets/ScrollContainer';
+
+/** One thing to reorder: where it is taken hold of, and the rows it fills in its column. */
+export interface SortBlock {
+  grip: Widget;
+  /** Siblings in one column, top to bottom. */
+  rows: readonly Widget[];
+}
+
+/** How far the pointer goes before a press on a grip becomes a drag; less stays a click. */
+const DRAG_THRESHOLD = 4;
+const FRAME_KEY = 'block-drag';
+const DIMMED = 0.4;
+
+/**
+ * Lets the blocks of one column be reordered by dragging their grips (#310). Unlike a
+ * `SortableList` the rows are ordinary widgets of any height – a struct is its head with its fields
+ * below – so a block is told by the rows it covers. While dragging, the block is dimmed, a line
+ * shows where it lands, and a scrolled column follows the pointer to its edge. `move` gets the
+ * indices the model takes, the block already taken out.
+ */
+export function sortBlocks(blocks: readonly SortBlock[], move: (from: number, to: number) => void): void {
+  if (blocks.length < 2) return;
+  blocks.forEach((block, index) => {
+    block.grip.eventMode = 'static';
+    block.grip.cursor = 'grab';
+    block.grip.on('pointerdown', (e: FederatedPointerEvent) => {
+      if (e.button === 0) new BlockDrag(blocks, index, e.global.y, move);
+    });
+  });
+}
+
+class BlockDrag {
+  private readonly column: Container | null;
+  private readonly scroll: ScrollContainer | null;
+  private readonly line = new Graphics();
+  private readonly alphas: number[] = [];
+  private readonly pointer = new Point();
+  private active = false;
+  private readonly unbind: () => void;
+
+  constructor(
+    private readonly blocks: readonly SortBlock[],
+    private readonly from: number,
+    private readonly startY: number,
+    private readonly move: (from: number, to: number) => void,
+  ) {
+    const block = blocks[from]!;
+    this.column = block.rows[0]?.parent ?? null;
+    this.scroll = scrollOf(this.column);
+    const grip = block.grip;
+    const onMove = (e: FederatedPointerEvent) => this.onMove(e);
+    const onUp = () => this.finish(true);
+    const onGone = () => this.finish(false);
+    grip.on('globalpointermove', onMove);
+    grip.on('pointerup', onUp);
+    grip.on('pointerupoutside', onUp);
+    // A rebuild of the rows mid-drag takes the grip away – and with it the release.
+    grip.once('destroyed', onGone);
+    this.unbind = () => {
+      grip.off('globalpointermove', onMove);
+      grip.off('pointerup', onUp);
+      grip.off('pointerupoutside', onUp);
+      grip.off('destroyed', onGone);
+    };
+  }
+
+  private get rows(): readonly Widget[] {
+    return this.blocks[this.from]!.rows;
+  }
+
+  private onMove(e: FederatedPointerEvent): void {
+    this.pointer.copyFrom(e.global);
+    if (!this.active && Math.abs(e.global.y - this.startY) < DRAG_THRESHOLD) return;
+    if (!this.column) return;
+    if (!this.active) {
+      this.active = true;
+      for (const row of this.rows) {
+        this.alphas.push(row.alpha);
+        row.alpha = DIMMED;
+      }
+      this.column.addChild(this.line);
+    }
+    this.drawLine();
+    this.followEdge();
+  }
+
+  /** The blocks' extents in the column, read now: rows may have moved since the drag began. */
+  private spans(): BlockSpan[] {
+    return this.blocks.map(({ rows }) => ({
+      top: Math.min(...rows.map((row) => row.y)),
+      bottom: Math.max(...rows.map((row) => row.y + row.h)),
+    }));
+  }
+
+  private localY(): number {
+    return this.column ? this.column.toLocal(this.pointer).y : 0;
+  }
+
+  /** Only where a drop would move the block: a line where it already stands promises nothing. */
+  private drawLine(): void {
+    const column = this.column;
+    this.line.clear();
+    if (!column) return;
+    const spans = this.spans();
+    const y = this.localY();
+    if (dropSlot(y, spans, this.from) === null) return;
+    // From the grip's own indent, so a field's line shows it stays inside its struct.
+    const left = column.toLocal(this.blocks[this.from]!.grip.getGlobalPosition()).x;
+    const right = Math.max(...this.rows.map((row) => row.x + row.w));
+    const at = Math.round(gapLineY(blockGap(y, spans), spans));
+    this.line.rect(left, at - 1, Math.max(0, right - left), 2).fill(Theme.colors.accent);
+  }
+
+  /** Scrolls a frame's worth while the pointer stays at the column's edge, and asks for the next. */
+  private followEdge(): void {
+    const scroll = this.scroll;
+    if (!scroll || !this.active) return;
+    onFrame(FRAME_KEY, () => {
+      if (!this.active) return;
+      const before = scroll.offset;
+      scroll.scrollTo(before + edgeScroll(this.pointer.y, scroll.getGlobalPosition().y, scroll.h));
+      if (scroll.offset === before) return;
+      this.drawLine();
+      this.followEdge();
+    });
+  }
+
+  private finish(drop: boolean): void {
+    this.unbind();
+    cancelFrameWork(FRAME_KEY);
+    if (!this.active) return;
+    this.active = false;
+    const to = drop ? dropSlot(this.localY(), this.spans(), this.from) : null;
+    this.rows.forEach((row, i) => {
+      if (!row.destroyed) row.alpha = this.alphas[i] ?? 1;
+    });
+    if (!this.line.destroyed) this.line.destroy();
+    if (to !== null) this.move(this.from, to);
+  }
+}
+
+function scrollOf(node: Container | null): ScrollContainer | null {
+  for (let parent = node; parent; parent = parent.parent)
+    if (parent instanceof ScrollContainer) return parent;
+  return null;
+}

src/ui/index.ts

@@ -65,7 +65,8 @@ export {
   type SortableValue,
   type ThumbClip,
 } from './widgets/SortableList';
-export { dropGap, dropMove, rowOrder } from './sortable';
+export { dropGap, dropIndex, dropMove, rowOrder } from './sortable';
+export { type SortBlock, sortBlocks } from './blockDrag';
 export { Popup, type PopupOptions } from './widgets/Popup';
 export {
   currentMenu,

src/ui/sortable.ts

@@ -1,10 +1,12 @@
+import type { DropSpot } from './widgets/ListView';
+
 /**
  * How a sortable list maps between what is on screen and what its owner holds. A layer list shows the
  * drawing order upside down – the item drawn last is the topmost row – so every index has two
  * readings, and dropping a dragged row has a third: the place it lands in is counted with the row
  * still in the list, while the model takes it out first.
  *
- * Kept apart from the widget so the arithmetic can be measured without a canvas.
+ * Kept apart from the widgets so the arithmetic can be measured without a canvas.
  */
 
 /** The ids as the list shows them, top first. `reversed` puts the model's last item on top. */
@@ -36,3 +38,59 @@ export function dropMove(
   const last = count - 1;
   return { from: last - fromRow, to: last - toRow };
 }
+
+/**
+ * Where a dragged row lands, counted in the list it has *left* – which is what the move command
+ * takes. A drop on the middle of a row takes that row's place: the list is flat, so there is no
+ * "inside" for it to mean, and without this a drag downwards onto the next row would cancel itself
+ * out. The gaps above and below a row are insert points, and one below the dragged row shifts up
+ * by one once the row is out. `-1` is "nothing to do".
+ */
+export function dropIndex(from: number, spot: DropSpot, length: number): number {
+  const last = length - 1;
+  if (from < 0 || from > last) return -1;
+  if (spot.at === 'end') return last;
+  const target = Number(spot.targetId);
+  if (!Number.isInteger(target) || target < 0 || target > last) return -1;
+  if (spot.at === 'onto') return target;
+  const insert = target + (spot.at === 'after' ? 1 : 0);
+  return Math.min(last, insert > from ? insert - 1 : insert);
+}
+
+/** The rows of one block, top to bottom – a struct with its fields, an item with its nested list. */
+export interface BlockSpan {
+  top: number;
+  bottom: number;
+}
+
+/**
+ * The gap a block dropped at `y` falls into, 0…count: past every block whose middle is above `y`.
+ * Blocks differ in height, so the middle of each is what the pointer has to cross.
+ */
+export function blockGap(y: number, spans: readonly BlockSpan[]): number {
+  return spans.filter((span) => (span.top + span.bottom) / 2 < y).length;
+}
+
+/** Where the line for `gap` is drawn: halfway between the blocks around it, on the edge at either end. */
+export function gapLineY(gap: number, spans: readonly BlockSpan[]): number {
+  const above = spans[gap - 1];
+  const below = spans[gap];
+  if (above && below) return (above.bottom + below.top) / 2;
+  return above?.bottom ?? below?.top ?? 0;
+}
+
+/** The place block `from` moves to when dropped at `y`, or null where it would stay. */
+export function dropSlot(y: number, spans: readonly BlockSpan[], from: number): number | null {
+  return dropMove(spans.length, from, blockGap(y, spans), false)?.to ?? null;
+}
+
+/**
+ * How far a scrolled column moves this frame while a drag holds the pointer near its top or bottom:
+ * the deeper into the `edge` band, the faster, up to `step`. Positive scrolls down.
+ */
+export function edgeScroll(y: number, top: number, height: number, edge = 24, step = 12): number {
+  const into = (depth: number) => Math.ceil((Math.min(edge, depth) / edge) * step);
+  if (y < top + edge) return -into(top + edge - y);
+  if (y > top + height - edge) return into(y - (top + height - edge));
+  return 0;
+}