diff --git a/src/editor/viewport/SceneViewport.ts b/src/editor/viewport/SceneViewport.ts
index 019f644e..70fee04f 100644
--- a/src/editor/viewport/SceneViewport.ts
+++ b/src/editor/viewport/SceneViewport.ts
@@ -5,13 +5,7 @@
  */
 import { type FederatedPointerEvent, Graphics, Text } from 'pixi.js';
 import { oneStep } from '../../core/commands/CompoundCommand';
-import {
-  MoveSceneObjectCommand,
-  PlaceAssetCommand,
-  SetOverrideCommand,
-} from '../../core/commands/sceneObjectCommands';
-import { type CellEdit, paintTiles } from '../../core/commands/tilemapCommands';
-import type { TilemapAsset, TilemapLayer } from '../../core/model/Asset';
+import { MoveSceneObjectCommand, PlaceAssetCommand } from '../../core/commands/sceneObjectCommands';
 import { hasSceneIcon } from '../../core/model/AssetTypes';
 import { findObject, subtreeRoots } from '../../core/model/hierarchy';
 import { objectRef, resolvedHolder } from '../../core/model/instance';
@@ -22,65 +16,35 @@ import {
   invert,
   linearOf,
   type Mat,
-  scaleOf,
 } from '../../core/model/matrix';
 import { placementRejection } from '../../core/model/placement';
-import type { ListValue, Position, PropertyValue, Vec2 } from '../../core/model/Properties';
-import { findAsset, findAssetOf, type ObjectHolder } from '../../core/model/Project';
-import {
-  cellAt,
-  cellAtPoint,
-  cellKey,
-  cellOrigin,
-  cellsAlong,
-  cellsInRect,
-  findLayer,
-  floodFill,
-  inGrid,
-  tilemapColumns,
-  tilemapRows,
-  tilemapTileSize,
-} from '../../core/model/tilemap';
-import type { BoundaryLine } from '../../core/model/polygon';
-import { type CellPolygon, type CellRect, solidBoundary } from '../../core/model/tilemapCollision';
+import type { Position, Vec2 } from '../../core/model/Properties';
+import { findAsset, type ObjectHolder } from '../../core/model/Project';
+import { cellAtPoint } from '../../core/model/tilemap';
 import { footprintCorners } from '../../core/properties/bounds';
-import { colliderPoints, COLLIDER_POINTS_KEY } from '../../core/properties/collider';
 import { localMatrixOf, parentTransform, transformOf } from '../../core/properties/transform';
-import { type EditorState, objectSelection, type TilemapTool } from '../state';
-import { paintedLayerId, paintedTilemap, pickTile } from '../panels/TilemapPanel';
-import {
-  activeDrag,
-  drawBand,
-  drawGrid,
-  endDrag,
-  onDragChange,
-  pressCount,
-  Theme,
-  textStyle,
-} from '../../ui';
-import { gripAt, withCornerAfter, withCornerMoved, withoutCorner } from '../polygonEdit';
+import { type EditorState, objectSelection } from '../state';
+import { paintedLayerId, paintedTilemap } from '../panels/TilemapPanel';
+import { activeDrag, drawBand, drawGrid, endDrag, onDragChange, Theme, textStyle } from '../../ui';
 import { collectHolderItems, type DrawItem } from '../../viewport/items';
 import { menuTargetOf, type PointerHit, pressOn, stackAt } from '../../viewport/pick';
 import { openObjectMenu } from '../objectMenu';
 import { type LayerFocus, layerAlpha } from '../../viewport/tilemap';
-import { CONE_LENGTH, dashedLine, markBlockingSide, placeNode } from '../../viewport/symbols';
+import { dashedLine } from '../../viewport/symbols';
 import { drawSymbolIcon } from '../../viewport/symbolIcons';
 import { screenToWorld, type View, worldToScreen } from '../../viewport/view';
 import { type CastRay, RAY_SHOWN, type World } from '../../runtime/World';
 import { FreeViewport } from './FreeViewport';
+import { PolygonCornerEditor } from './polygonCorners';
+import { MIN_GRID_STEP, type SceneViewHost } from './sceneViewHost';
+import { type PaintTarget, TilemapPainter } from './tilemapPainting';
+import { TilemapSolidOutlines } from './tilemapSolids';
 import type { Frame } from './WorldViewport';
 
-/** Screen pixels around a polygon's corner or edge that grab it. */
-const CORNER_GRIP = 7;
-/** Side of a corner handle in screen pixels. */
-const CORNER_SIZE = 8;
-
 /** Pixels of travel before a press turns into a drag. */
 const MARQUEE_THRESHOLD = 3;
 const DRAG_THRESHOLD = 2;
 const GRID_STEP = 100;
-/** Below this on-screen spacing the grid is noise rather than orientation, so it is dropped. */
-const MIN_GRID_STEP = 12;
 
 interface ObjectDrag {
   holderId: string;
@@ -92,24 +56,6 @@ interface ObjectDrag {
   onClick: string | null;
 }
 
-/**
- * A corner of a polygon collider being dragged (#151) – one just put on an edge, too. Like a move
- * drag it writes the model as a preview; `before` is the override that was there, put back on
- * release so the command records the edit as one undo step.
- */
-interface CornerDrag {
-  holderId: string;
-  objectId: string;
-  before: PropertyValue | undefined;
-  points: Vec2[];
-  index: number;
-  /** From the world into the collider's own units. */
-  toLocal: Mat;
-  pointer: Vec2;
-  inserted: boolean;
-  moved: boolean;
-}
-
 /** An asset hovering over the view on its way in: where it would land, and whether it may. */
 interface DropPreview {
   assetId: string;
@@ -117,26 +63,6 @@ interface DropPreview {
   rejection: string;
 }
 
-/**
- * A stroke on a placed tilemap. `previous` is what the cells held before the live preview wrote
- * into them – the same trick a move drag uses: the model shows the stroke at once, and on release
- * it is put back so the command records it as one undo step.
- */
-interface PaintStroke {
-  holderId: string;
-  objectId: string;
-  assetId: string;
-  layerId: string;
-  tool: TilemapTool;
-  tileId: string;
-  edits: CellEdit[];
-  previous: Map<string, string>;
-  from: Vec2;
-  last: Vec2 | null;
-  /** Where the pointer is now, in cells – what the rectangle preview is drawn from. */
-  to: Vec2;
-}
-
 /** A rubber-band selection in progress, in viewport-local pixels. */
 interface Marquee {
   start: Vec2;
@@ -160,51 +86,29 @@ const LINK_COLORS: Partial<Record<DrawItem['type'], number>> = {
   rigidbody: Theme.colors.collider,
 };
 
-/** Where a resolved tilemap blocks: what its outline in the scene view is worked out from. */
-type TilemapSolids = Pick<NonNullable<DrawItem['tilemap']>, 'solids' | 'outlines' | 'oneWay'>;
-
-function sameSolids(a: TilemapSolids, b: TilemapSolids): boolean {
-  const sameRects = (p: readonly CellRect[], q: readonly CellRect[]) =>
-    p.length === q.length &&
-    p.every((r, i) => r.x === q[i]!.x && r.y === q[i]!.y && r.w === q[i]!.w && r.h === q[i]!.h);
-  const samePolygons = (p: readonly CellPolygon[], q: readonly CellPolygon[]) =>
-    p.length === q.length &&
-    p.every(
-      (polygon, i) =>
-        polygon.outline.length === q[i]!.outline.length &&
-        polygon.outline.every((c, j) => c.x === q[i]!.outline[j]!.x && c.y === q[i]!.outline[j]!.y),
-    );
-  return (
-    sameRects(a.solids, b.solids) &&
-    samePolygons(a.outlines, b.outlines) &&
-    sameRects(a.oneWay.solids, b.oneWay.solids) &&
-    samePolygons(a.oneWay.outlines, b.oneWay.outlines)
-  );
-}
-
 export class SceneViewport extends FreeViewport {
   private readonly grid = new Graphics();
   private readonly links = new Graphics();
   private readonly gizmos = new Graphics();
   private readonly labels = new Map<string, Text>();
   private readonly icons = new Map<string, Graphics>();
-  /** Where each placed tilemap blocks, outlined like a collider (#132). */
-  private readonly solids = new Map<string, Graphics>();
-  /**
-   * The outer edges of each tilemap's solids and what they were worked out from (#173). A map is
-   * resolved afresh on every draw, so they are kept by what blocks rather than by the resolved map.
-   */
-  private readonly boundaries = new Map<
-    string,
-    { from: TilemapSolids; solid: BoundaryLine[]; oneWay: BoundaryLine[] }
-  >();
+  private readonly solids = new TilemapSolidOutlines(this.overlay);
+  private readonly host: SceneViewHost = {
+    state: this.state,
+    holder: () => this.holder(),
+    items: () => this.lastItems,
+    worldAt: (global) => {
+      const local = this.toLocal(global);
+      return screenToWorld(this.currentView(), local.x, local.y);
+    },
+    scale: () => this.currentView().scale,
+    redraw: () => this.redraw(),
+  };
+  private readonly painter = new TilemapPainter(this.host);
+  private readonly corners = new PolygonCornerEditor(this.host);
   private editorWorld: World | null = null;
   private lastRays: CastRay[] = [];
   private dragging: ObjectDrag | null = null;
-  private corner: CornerDrag | null = null;
-  /** The corners drawn for the edited polygon, in global pixels (test hook). */
-  private lastCorners: { objectId: string; corners: Vec2[] } | null = null;
-  private stroke: PaintStroke | null = null;
   private marquee: Marquee | null = null;
   private dropPreview: DropPreview | null = null;
   private lastGizmos: GizmoInfo[] = [];
@@ -286,7 +190,7 @@ export class SceneViewport extends FreeViewport {
       this.startPan(e);
       return;
     }
-    if (e.button === 0 && this.startCornerEdit(e)) return;
+    if (e.button === 0 && this.corners.press(e)) return;
     if (e.button === 0 && e.target === this) this.pressEmpty(e);
   }
 
@@ -311,24 +215,7 @@ export class SceneViewport extends FreeViewport {
       this.trackIncomingAsset(incoming.kind === 'asset' ? incoming.id : '', e);
       return;
     }
-    if (this.panTo(e)) return;
-    if (this.corner) {
-      this.dragCorner(e);
-      return;
-    }
-    if (this.stroke) {
-      const item = this.lastItems.find((i) => i.id === this.stroke!.objectId);
-      if (!item) return;
-      const at = this.cellUnder(item, e);
-      if (this.stroke.last && at.x === this.stroke.last.x && at.y === this.stroke.last.y) return;
-      const from = this.stroke.last ?? this.stroke.from;
-      this.stroke.last = at;
-      this.stroke.to = at;
-      // A pointer jumps between events, so the cells in between are filled in as well.
-      if (this.stroke.tool !== 'rect') this.paintCells(item, cellsAlong(from, at));
-      this.redraw();
-      return;
-    }
+    if (this.panTo(e) || this.corners.move(e.global) || this.painter.move(e.global)) return;
     if (this.dragging) {
       const local = this.toLocal(e.global);
       const dx = (local.x - this.dragging.pointer.x) / this.view.zoom;
@@ -406,14 +293,7 @@ export class SceneViewport extends FreeViewport {
       return;
     }
     this.endPan();
-    if (this.corner) {
-      this.endCorner();
-      return;
-    }
-    if (this.stroke) {
-      this.endStroke();
-      return;
-    }
+    if (this.corners.release() || this.painter.release()) return;
     const marquee = this.marquee;
     this.marquee = null;
     if (marquee) {
@@ -440,253 +320,9 @@ export class SceneViewport extends FreeViewport {
     if (moves.length > 0) this.state.execute(oneStep('Move objects', moves));
   }
 
-  /**
-   * The tilemap a click paints on: the one selected (see `paintedTilemap`), with a painting tool
-   * chosen. Anything else – another object, the select tool, a modifier held – leaves the view
-   * doing what it always did.
-   */
-  private paintTargetOf(objectId: string): { item: DrawItem; asset: TilemapAsset; layerId: string } | null {
-    if (this.state.tilemapPaint.tool === 'select') return null;
-    const target = paintedTilemap(this.state);
-    if (!target || target.objectId !== objectId) return null;
-    const item = this.lastItems.find((i) => i.id === objectId);
-    if (!item || item.type !== 'tilemap') return null;
-    const layerId = paintedLayerId(this.state, target.asset);
-    return layerId ? { item, asset: target.asset, layerId } : null;
-  }
-
-  /** The cell under a pointer position, in the tilemap's own grid. */
-  private cellUnder(item: DrawItem, e: FederatedPointerEvent): Vec2 {
-    const local = this.toLocal(e.global);
-    const world = screenToWorld(this.currentView(), local.x, local.y);
-    const inMap = applyPoint(invert(item.m), world);
-    return cellAtPoint(item.props, inMap.x, inMap.y);
-  }
-
-  /**
-   * What a press with a painting tool does. Fill and pick are done with the click – a drag adds
-   * nothing to either – so only the other three start a stroke. True when the press was used.
-   */
-  private usePaintTool(
-    target: { item: DrawItem; asset: TilemapAsset; layerId: string },
-    holderId: string,
-    e: FederatedPointerEvent,
-  ): boolean {
-    const tool = this.state.tilemapPaint.tool;
-    if (tool !== 'fill' && tool !== 'pick') return this.startStroke(target, holderId, e);
-    const layer = findLayer(target.asset, target.layerId);
-    const at = this.cellUnder(target.item, e);
-    if (!layer || !inGrid(target.item.props, at.x, at.y)) return true;
-    if (tool === 'pick') this.pickCell(layer, at);
-    else this.fillFrom(target, layer, at);
-    return true;
-  }
-
-  /** Takes the tile of a cell on the painted layer – the one painted on next (#133). */
-  private pickCell(layer: TilemapLayer, at: Vec2): void {
-    const tileId = cellAt(layer, at.x, at.y);
-    if (!tileId) this.state.setNotice('This cell is empty on the active layer.');
-    else if (!findAssetOf(this.state.project, tileId, 'tile'))
-      this.state.setNotice('This cell names a tile that no longer exists.');
-    else pickTile(this.state, tileId);
-  }
-
-  /** Fills the connected cells of one tile in a single command, so one click is one undo step. */
-  private fillFrom(
-    target: { item: DrawItem; asset: TilemapAsset; layerId: string },
-    layer: TilemapLayer,
-    at: Vec2,
-  ): void {
-    const tileId = this.state.tilemapPaint.tileId;
-    if (!tileId) {
-      this.state.setNotice('Choose a tile to paint with.');
-      return;
-    }
-    const cells = floodFill(target.item.props, layer, at, tileId);
-    const command = paintTiles(
-      layer,
-      target.asset.id,
-      cells.map((cell) => ({ ...cell, tileId })),
-      'Fill tiles',
-    );
-    if (command) this.state.execute(command);
-  }
-
-  private startStroke(
-    target: { item: DrawItem; asset: TilemapAsset; layerId: string },
-    holderId: string,
-    e: FederatedPointerEvent,
-  ): boolean {
-    const paint = this.state.tilemapPaint;
-    const tileId = paint.tool === 'erase' ? '' : paint.tileId;
-    if (paint.tool !== 'erase' && !tileId) {
-      this.state.setNotice('Choose a tile to paint with.');
-      return false;
-    }
-    const at = this.cellUnder(target.item, e);
-    this.stroke = {
-      holderId,
-      objectId: target.item.id,
-      assetId: target.asset.id,
-      layerId: target.layerId,
-      tool: paint.tool,
-      tileId,
-      edits: [],
-      previous: new Map(),
-      from: at,
-      last: null,
-      to: at,
-    };
-    // A rectangle only shows where it would go until the button comes up; the other two draw at once.
-    if (paint.tool !== 'rect') this.paintCells(target.item, [at]);
-    this.redraw();
-    return true;
-  }
-
-  private strokeLayer(stroke: PaintStroke): TilemapLayer | undefined {
-    const asset = findAssetOf(this.state.project, stroke.assetId, 'tilemap');
-    return asset && findLayer(asset, stroke.layerId);
-  }
-
-  /** Writes cells into the model as a preview, remembering what they held. */
-  private paintCells(item: DrawItem, cells: readonly Vec2[]): void {
-    const stroke = this.stroke;
-    const layer = stroke && this.strokeLayer(stroke);
-    if (!stroke || !layer) return;
-    let changed = false;
-    for (const at of cells) {
-      if (!inGrid(item.props, at.x, at.y)) continue;
-      const key = cellKey(at.x, at.y);
-      if (!stroke.previous.has(key)) stroke.previous.set(key, layer.cells[key] ?? '');
-      if ((layer.cells[key] ?? '') === stroke.tileId) continue;
-      stroke.edits.push({ x: at.x, y: at.y, tileId: stroke.tileId });
-      if (stroke.tileId) layer.cells[key] = stroke.tileId;
-      else delete layer.cells[key];
-      changed = true;
-    }
-    if (changed) this.state.preview();
-  }
-
-  /** Puts the preview back and lets the command do the edit, so one stroke is one undo step. */
-  private endStroke(): void {
-    const stroke = this.stroke;
-    this.stroke = null;
-    if (!stroke) return;
-    const layer = this.strokeLayer(stroke);
-    if (layer)
-      for (const [key, value] of stroke.previous) {
-        if (value) layer.cells[key] = value;
-        else delete layer.cells[key];
-      }
-    const item = this.lastItems.find((i) => i.id === stroke.objectId);
-    const edits =
-      stroke.tool === 'rect' && item
-        ? cellsInRect(item.props, stroke.from, stroke.to).map((at) => ({
-            ...at,
-            tileId: stroke.tileId,
-          }))
-        : stroke.edits;
-    const command =
-      layer &&
-      paintTiles(layer, stroke.assetId, edits, stroke.tool === 'erase' ? 'Erase tiles' : 'Paint tiles');
-    if (command) this.state.execute(command);
-    else this.state.preview();
-    this.redraw();
-  }
-
-  /**
-   * The polygon collider whose corners can be edited: the primary selection, if it is one and
-   * belongs to this view's holder – a placed container's content is edited in its template.
-   */
-  private editedPolygon(): DrawItem | null {
-    const s = this.state.selection;
-    const holder = this.holder();
-    if (s?.kind !== 'object' || !holder || !objectRef(this.state.project, holder, s.objectId)) return null;
-    const item = this.lastItems.find((i) => i.id === s.objectId);
-    return item?.type === 'collider' && item.props.shape === 'polygon' ? item : null;
-  }
-
-  /**
-   * A press on a corner or an edge of the edited polygon: a double-click takes the corner away,
-   * a press on an edge puts a corner there, and either can then be dragged. True when it was one.
-   */
-  private startCornerEdit(e: FederatedPointerEvent): boolean {
-    const item = this.editedPolygon();
-    const holder = this.holder();
-    const ref = item && holder && objectRef(this.state.project, holder, item.id);
-    if (!item || !holder || !ref) return false;
-    const toLocal = invert(item.m);
-    const pointer = this.toLocal(e.global);
-    const at = applyPoint(toLocal, screenToWorld(this.currentView(), pointer.x, pointer.y));
-    const points = colliderPoints(item.props);
-    const tolerance = CORNER_GRIP / (this.currentView().scale * (scaleOf(item.m) || 1));
-    const grip = gripAt(points, at, tolerance);
-    if (!grip) return false;
-    e.stopPropagation();
-    if (grip.kind === 'corner' && pressCount(e, `corner ${item.id} ${grip.index}`) === 2) {
-      const next = withoutCorner(points, grip.index);
-      if (next)
-        this.state.execute(new SetOverrideCommand(holder.id, item.id, COLLIDER_POINTS_KEY, listOf(next)));
-      else this.state.setNotice('A polygon keeps at least three corners.');
-      return true;
-    }
-    const inserted = grip.kind === 'edge';
-    this.corner = {
-      holderId: holder.id,
-      objectId: item.id,
-      before: ref.object.overrides[COLLIDER_POINTS_KEY],
-      points: inserted ? withCornerAfter(points, grip.index, rounded(grip.at)) : points,
-      index: inserted ? grip.index + 1 : grip.index,
-      toLocal,
-      pointer: { x: pointer.x, y: pointer.y },
-      inserted,
-      moved: false,
-    };
-    if (inserted) this.previewCorners();
-    return true;
-  }
-
-  private dragCorner(e: FederatedPointerEvent): void {
-    const corner = this.corner!;
-    const pointer = this.toLocal(e.global);
-    if (
-      !corner.moved &&
-      Math.hypot(pointer.x - corner.pointer.x, pointer.y - corner.pointer.y) < DRAG_THRESHOLD
-    )
-      return;
-    corner.moved = true;
-    const at = applyPoint(corner.toLocal, screenToWorld(this.currentView(), pointer.x, pointer.y));
-    corner.points = withCornerMoved(corner.points, corner.index, rounded(at));
-    this.previewCorners();
-  }
-
-  private previewCorners(): void {
-    const corner = this.corner;
-    const holder = this.holder();
-    const ref = corner && holder && objectRef(this.state.project, holder, corner.objectId);
-    if (!corner || !ref) return;
-    ref.setOverride(COLLIDER_POINTS_KEY, listOf(corner.points));
-    this.state.preview();
-  }
-
-  /** Puts the preview back and lets the command do the edit, so one drag is one undo step. */
-  private endCorner(): void {
-    const corner = this.corner!;
-    this.corner = null;
-    const holder = this.holder();
-    const ref = holder && objectRef(this.state.project, holder, corner.objectId);
-    if (!ref) return;
-    ref.setOverride(COLLIDER_POINTS_KEY, corner.before);
-    if (corner.moved || corner.inserted)
-      this.state.execute(
-        new SetOverrideCommand(corner.holderId, corner.objectId, COLLIDER_POINTS_KEY, listOf(corner.points)),
-      );
-    else this.state.preview();
-  }
-
   /** The corners of the polygon being edited, where they are drawn (test hook). */
   cornerInfo(): { objectId: string; corners: Vec2[] } | null {
-    return this.lastCorners;
+    return this.corners.info();
   }
 
   /**
@@ -700,7 +336,7 @@ export class SceneViewport extends FreeViewport {
       return;
     }
     if (e.button !== 0) return;
-    if (this.startCornerEdit(e)) return;
+    if (this.corners.press(e)) return;
     const holder = this.resolved();
     if (!holder) return;
     const stack = this.stackUnder(e);
@@ -715,7 +351,7 @@ export class SceneViewport extends FreeViewport {
     e.stopPropagation();
     // A painting tool on the selected tilemap paints anywhere on its grid, empty cells included.
     const target = this.paintTargetUnder(e);
-    if (target && this.usePaintTool(target, holder.id, e)) return;
+    if (target && this.painter.press(target, e.global)) return;
     const press = pressOn(stack, this.state.selectedObjectIds, this.primaryObjectId());
     if (!press) {
       this.pressEmpty(e);
@@ -745,7 +381,8 @@ export class SceneViewport extends FreeViewport {
   private openMenuUnder(e: FederatedPointerEvent): void {
     const source = this.holder();
     const holder = this.resolved();
-    if (!source || !holder || this.dragging || this.stroke || this.corner || this.marquee) return;
+    if (!source || !holder || this.dragging || this.painter.active || this.corners.active || this.marquee)
+      return;
     const target = menuTargetOf(this.stackUnder(e), this.state.selectedObjectIds, this.primaryObjectId());
     if (!target) return;
     e.stopPropagation();
@@ -790,11 +427,11 @@ export class SceneViewport extends FreeViewport {
   }
 
   /** The painted tilemap when the pointer is on its grid, whatever lies above it there. */
-  private paintTargetUnder(e: FederatedPointerEvent): ReturnType<SceneViewport['paintTargetOf']> {
+  private paintTargetUnder(e: FederatedPointerEvent): PaintTarget | null {
     const target = paintedTilemap(this.state);
     if (!target) return null;
     const onGrid = this.hitsUnder(e.global).some((hit) => hit.id === target.objectId);
-    return onGrid ? this.paintTargetOf(target.objectId) : null;
+    return onGrid ? this.painter.targetOf(target.objectId) : null;
   }
 
   /** What lies under a point of the screen, front to back – what a click there picks from (test hook). */
@@ -863,7 +500,7 @@ export class SceneViewport extends FreeViewport {
 
   protected override syncDecorations(item: DrawItem, onScreen: boolean, scale: number, extent: Extent): void {
     this.syncIcon(item, onScreen);
-    this.syncSolids(item, onScreen, scale);
+    this.solids.sync(item, onScreen, scale);
     this.syncLabel(item, onScreen, scale, extent);
   }
 
@@ -872,118 +509,12 @@ export class SceneViewport extends FreeViewport {
     this.labels.delete(id);
     this.icons.get(id)?.destroy();
     this.icons.delete(id);
-    this.solids.get(id)?.destroy();
-    this.solids.delete(id);
-    this.boundaries.delete(id);
+    this.solids.dispose(id);
   }
 
   /** The blocking rectangles and outlined cells of a placed tilemap, and the outer edges drawn round them, in cells (test hook). */
-  solidInfo(id: string): {
-    visible: boolean;
-    rects: CellRect[];
-    outlines: Vec2[][];
-    oneWay: CellRect[];
-    oneWayOutlines: Vec2[][];
-    boundary: BoundaryLine[];
-  } | null {
-    const node = this.solids.get(id);
-    const item = this.lastItems.find((i) => i.id === id);
-    if (!node || !item?.tilemap) return null;
-    return {
-      visible: node.visible,
-      rects: item.tilemap.solids.map((rect) => ({ ...rect })),
-      outlines: item.tilemap.outlines.map((polygon) => polygon.outline.map((p) => ({ ...p }))),
-      oneWay: item.tilemap.oneWay.solids.map((rect) => ({ ...rect })),
-      oneWayOutlines: item.tilemap.oneWay.outlines.map((polygon) => polygon.outline.map((p) => ({ ...p }))),
-      boundary: this.boundaryOf(item).solid.map((line) => ({
-        points: line.points.map((p) => ({ ...p })),
-        closed: line.closed,
-      })),
-    };
-  }
-
-  private boundaryOf(item: DrawItem): { solid: BoundaryLine[]; oneWay: BoundaryLine[] } {
-    const from: TilemapSolids = item.tilemap!;
-    let boundary = this.boundaries.get(item.id);
-    if (!boundary || !sameSolids(boundary.from, from)) {
-      boundary = {
-        from,
-        solid: solidBoundary(from.solids, from.outlines),
-        oneWay: solidBoundary(from.oneWay.solids, from.oneWay.outlines),
-      };
-      this.boundaries.set(item.id, boundary);
-    }
-    return boundary;
-  }
-
-  /**
-   * The solid cells of a tilemap, drawn the way a collider is: tinted and outlined, above the
-   * content so a painted wall cannot hide them. Editor-only for the same reason as the icon: the
-   * game view never syncs decorations.
-   */
-  private syncSolids(item: DrawItem, show: boolean, scale: number): void {
-    let node = this.solids.get(item.id);
-    const rects = item.tilemap?.solids ?? [];
-    const outlines = item.tilemap?.outlines ?? [];
-    const oneWay = item.tilemap?.oneWay ?? { solids: [], outlines: [] };
-    if (!show || rects.length + outlines.length + oneWay.solids.length + oneWay.outlines.length === 0) {
-      if (node) node.visible = false;
-      return;
-    }
-    if (!node) {
-      node = new Graphics();
-      node.eventMode = 'none';
-      this.solids.set(item.id, node);
-      this.overlay.addChild(node);
-    }
-    node.visible = true;
-    placeNode(node, item.m);
-    const tile = tilemapTileSize(item.props);
-    const width = 2 / (scale * scaleOf(item.m));
-    node.clear();
-    const color = Theme.colors.collider;
-    const boundary = this.boundaryOf(item);
-    // Filled piece by piece, but stroked only along the outer edges, so no seam shows (#173).
-    const fill = (rects: readonly CellRect[], outlines: readonly CellPolygon[], alpha: number) => {
-      if (rects.length + outlines.length === 0) return;
-      for (const rect of rects) {
-        const corner = cellOrigin(item.props, rect.x, rect.y);
-        node.rect(corner.x, corner.y, rect.w * tile, rect.h * tile);
-      }
-      for (const polygon of outlines)
-        node.poly(
-          polygon.outline.map((p) => cellOrigin(item.props, p.x, p.y)),
-          true,
-        );
-      node.fill({ color, alpha });
-    };
-    const stroke = (lines: readonly BoundaryLine[], lineWidth: number) => {
-      if (lines.length === 0) return;
-      for (const line of lines)
-        node.poly(
-          line.points.map((p) => cellOrigin(item.props, p.x, p.y)),
-          line.closed,
-        );
-      node.stroke({ width: lineWidth, color });
-    };
-    fill(rects, outlines, 0.12);
-    stroke(boundary.solid, width);
-    fill(oneWay.solids, oneWay.outlines, 0.06);
-    stroke(boundary.oneWay, width / 2);
-    // A platform's top is what blocks, so that is what stands out, with the cone it lands within.
-    const length = CONE_LENGTH / (scale * scaleOf(item.m));
-    for (const rect of oneWay.solids) {
-      const a = cellOrigin(item.props, rect.x, rect.y);
-      const b = cellOrigin(item.props, rect.x + rect.w, rect.y + rect.h);
-      markBlockingSide(
-        node,
-        { left: a.x, top: a.y, right: b.x, bottom: b.y },
-        { x: 0, y: -1 },
-        width * 2,
-        color,
-        { angle: rect.angle, length },
-      );
-    }
+  solidInfo(id: string): ReturnType<TilemapSolidOutlines['info']> {
+    return this.solids.info(this.lastItems.find((i) => i.id === id));
   }
 
   /** The icon of a particle source, the name beside it and whether it hangs off its owner (test hook). */
@@ -1100,76 +631,6 @@ export class SceneViewport extends FreeViewport {
       .stroke({ width: 1, color });
   }
 
-  /**
-   * While a painting tool is chosen, the selected tilemap shows its cells – without the grid one
-   * paints blind – and a rectangle drag shows the box it would fill.
-   */
-  private drawPaintOverlay(items: DrawItem[], view: View): void {
-    const target = this.state.tilemapPaint.tool === 'select' ? null : paintedTilemap(this.state);
-    if (!target) return;
-    const item = items.find((i) => i.id === target.objectId && i.type === 'tilemap');
-    if (!item) return;
-    const tile = tilemapTileSize(item.props);
-    const columns = tilemapColumns(item.props);
-    const rows = tilemapRows(item.props);
-    const corner = (x: number, y: number): Vec2 => {
-      const at = cellOrigin(item.props, x, y);
-      return worldToScreen(
-        view,
-        item.m.tx + at.x * item.m.a + at.y * item.m.c,
-        item.m.ty + at.x * item.m.b + at.y * item.m.d,
-      );
-    };
-    // Below this the lines are noise, exactly as the scene grid is (see `syncGrid`).
-    if (tile * view.scale >= MIN_GRID_STEP) {
-      for (let x = 0; x <= columns; x++) {
-        const a = corner(x, 0);
-        const b = corner(x, rows);
-        this.gizmos.moveTo(a.x, a.y).lineTo(b.x, b.y);
-      }
-      for (let y = 0; y <= rows; y++) {
-        const a = corner(0, y);
-        const b = corner(columns, y);
-        this.gizmos.moveTo(a.x, a.y).lineTo(b.x, b.y);
-      }
-      this.gizmos.stroke({ width: 1, color: Theme.colors.gizmo, alpha: 0.25 });
-    }
-    const stroke = this.stroke;
-    if (!stroke || stroke.tool !== 'rect') return;
-    const x0 = Math.min(stroke.from.x, stroke.to.x);
-    const y0 = Math.min(stroke.from.y, stroke.to.y);
-    const x1 = Math.max(stroke.from.x, stroke.to.x) + 1;
-    const y1 = Math.max(stroke.from.y, stroke.to.y) + 1;
-    this.gizmos
-      .poly([corner(x0, y0), corner(x1, y0), corner(x1, y1), corner(x0, y1)], true)
-      .fill({ color: Theme.colors.accent, alpha: 0.2 })
-      .stroke({ width: 2, color: Theme.colors.accent });
-  }
-
-  /** A handle on every corner of the polygon collider being edited (#151). */
-  private drawCorners(view: View): void {
-    const item = this.editedPolygon();
-    this.lastCorners = null;
-    if (!item) return;
-    const half = CORNER_SIZE / 2;
-    const corners = colliderPoints(item.props).map((p) => {
-      const world = applyPoint(item.m, p);
-      return worldToScreen(view, world.x, world.y);
-    });
-    for (const c of corners)
-      this.gizmos
-        .rect(c.x - half, c.y - half, CORNER_SIZE, CORNER_SIZE)
-        .fill(Theme.colors.accentText)
-        .stroke({ width: 1, color: Theme.colors.accent });
-    this.lastCorners = {
-      objectId: item.id,
-      corners: corners.map((c) => {
-        const g = this.gizmos.toGlobal(c);
-        return { x: g.x, y: g.y };
-      }),
-    };
-  }
-
   private screenCorners(corners: readonly Vec2[], view: View): Vec2[] {
     return corners.map((c) => worldToScreen(view, c.x, c.y));
   }
@@ -1198,19 +659,10 @@ export class SceneViewport extends FreeViewport {
           this.gizmos.rect(c.x - 3, c.y - 3, 6, 6).fill({ color: Theme.colors.gizmo, alpha });
       }
     }
-    this.drawCorners(view);
+    this.corners.draw(this.gizmos, view);
     this.drawDropPreview(view);
-    this.drawPaintOverlay(items, view);
+    this.painter.draw(this.gizmos, items, view);
     const m = this.marquee;
     if (m?.active) drawBand(this.gizmos, m.start, m.current);
   }
 }
-
-/** A corner lands on whole units, as a dragged object does. */
-function rounded(p: Vec2): Vec2 {
-  return { x: Math.round(p.x), y: Math.round(p.y) };
-}
-
-function listOf(points: readonly Vec2[]): ListValue {
-  return points.map((p) => ({ x: p.x, y: p.y }));
-}
diff --git a/src/editor/viewport/polygonCorners.ts b/src/editor/viewport/polygonCorners.ts
new file mode 100644
index 00000000..e0647058
--- /dev/null
+++ b/src/editor/viewport/polygonCorners.ts
@@ -0,0 +1,186 @@
+import type { FederatedPointerEvent, Graphics } from 'pixi.js';
+import { SetOverrideCommand } from '../../core/commands/sceneObjectCommands';
+import { objectRef } from '../../core/model/instance';
+import { apply as applyPoint, invert, type Mat, scaleOf } from '../../core/model/matrix';
+import type { ListValue, PropertyValue, Vec2 } from '../../core/model/Properties';
+import { colliderPoints, COLLIDER_POINTS_KEY } from '../../core/properties/collider';
+import { pressCount, Theme } from '../../ui';
+import type { DrawItem } from '../../viewport/items';
+import { type View, worldToScreen } from '../../viewport/view';
+import { gripAt, withCornerAfter, withCornerMoved, withoutCorner } from '../polygonEdit';
+import type { SceneViewHost } from './sceneViewHost';
+
+/** Screen pixels around a polygon's corner or edge that grab it. */
+const CORNER_GRIP = 7;
+/** Side of a corner handle in screen pixels. */
+const CORNER_SIZE = 8;
+/** Pixels of travel before a press turns into a drag. */
+const DRAG_THRESHOLD = 2;
+
+/**
+ * A corner of a polygon collider being dragged (#151) – one just put on an edge, too. Like a move
+ * drag it writes the model as a preview; `before` is the override that was there, put back on
+ * release so the command records the edit as one undo step.
+ */
+interface CornerDrag {
+  holderId: string;
+  objectId: string;
+  before: PropertyValue | undefined;
+  points: Vec2[];
+  index: number;
+  /** From the world into the collider's own units. */
+  toLocal: Mat;
+  /** Global pointer position of the press. */
+  pointer: Vec2;
+  inserted: boolean;
+  moved: boolean;
+}
+
+/** Handles on the corners of the selected polygon collider, to move, add and take away (#151). */
+export class PolygonCornerEditor {
+  private corner: CornerDrag | null = null;
+  /** The corners drawn for the edited polygon, in global pixels (test hook). */
+  private lastCorners: { objectId: string; corners: Vec2[] } | null = null;
+
+  constructor(private readonly host: SceneViewHost) {}
+
+  /** A corner is being dragged. */
+  get active(): boolean {
+    return this.corner !== null;
+  }
+
+  /**
+   * The polygon collider whose corners can be edited: the primary selection, if it is one and
+   * belongs to the view's holder – a placed container's content is edited in its template.
+   */
+  private editedPolygon(): DrawItem | null {
+    const state = this.host.state;
+    const s = state.selection;
+    const holder = this.host.holder();
+    if (s?.kind !== 'object' || !holder || !objectRef(state.project, holder, s.objectId)) return null;
+    const item = this.host.items().find((i) => i.id === s.objectId);
+    return item?.type === 'collider' && item.props.shape === 'polygon' ? item : null;
+  }
+
+  /**
+   * A press on a corner or an edge of the edited polygon: a double-click takes the corner away,
+   * a press on an edge puts a corner there, and either can then be dragged. True when it was one.
+   */
+  press(e: FederatedPointerEvent): boolean {
+    const state = this.host.state;
+    const item = this.editedPolygon();
+    const holder = this.host.holder();
+    const ref = item && holder && objectRef(state.project, holder, item.id);
+    if (!item || !holder || !ref) return false;
+    const toLocal = invert(item.m);
+    const at = applyPoint(toLocal, this.host.worldAt(e.global));
+    const points = colliderPoints(item.props);
+    const tolerance = CORNER_GRIP / (this.host.scale() * (scaleOf(item.m) || 1));
+    const grip = gripAt(points, at, tolerance);
+    if (!grip) return false;
+    e.stopPropagation();
+    if (grip.kind === 'corner' && pressCount(e, `corner ${item.id} ${grip.index}`) === 2) {
+      const next = withoutCorner(points, grip.index);
+      if (next) state.execute(new SetOverrideCommand(holder.id, item.id, COLLIDER_POINTS_KEY, listOf(next)));
+      else state.setNotice('A polygon keeps at least three corners.');
+      return true;
+    }
+    const inserted = grip.kind === 'edge';
+    this.corner = {
+      holderId: holder.id,
+      objectId: item.id,
+      before: ref.object.overrides[COLLIDER_POINTS_KEY],
+      points: inserted ? withCornerAfter(points, grip.index, rounded(grip.at)) : points,
+      index: inserted ? grip.index + 1 : grip.index,
+      toLocal,
+      pointer: { x: e.global.x, y: e.global.y },
+      inserted,
+      moved: false,
+    };
+    if (inserted) this.preview();
+    return true;
+  }
+
+  /** Follows the pointer with the corner under way; false when there is none. */
+  move(global: Vec2): boolean {
+    const corner = this.corner;
+    if (!corner) return false;
+    if (
+      !corner.moved &&
+      Math.hypot(global.x - corner.pointer.x, global.y - corner.pointer.y) < DRAG_THRESHOLD
+    )
+      return true;
+    corner.moved = true;
+    const at = applyPoint(corner.toLocal, this.host.worldAt(global));
+    corner.points = withCornerMoved(corner.points, corner.index, rounded(at));
+    this.preview();
+    return true;
+  }
+
+  private preview(): void {
+    const corner = this.corner;
+    const holder = this.host.holder();
+    const ref = corner && holder && objectRef(this.host.state.project, holder, corner.objectId);
+    if (!corner || !ref) return;
+    ref.setOverride(COLLIDER_POINTS_KEY, listOf(corner.points));
+    this.host.state.preview();
+  }
+
+  /**
+   * Puts the preview back and lets the command do the edit, so one drag is one undo step. False
+   * when no corner was under way.
+   */
+  release(): boolean {
+    const corner = this.corner;
+    if (!corner) return false;
+    this.corner = null;
+    const state = this.host.state;
+    const holder = this.host.holder();
+    const ref = holder && objectRef(state.project, holder, corner.objectId);
+    if (!ref) return true;
+    ref.setOverride(COLLIDER_POINTS_KEY, corner.before);
+    if (corner.moved || corner.inserted)
+      state.execute(
+        new SetOverrideCommand(corner.holderId, corner.objectId, COLLIDER_POINTS_KEY, listOf(corner.points)),
+      );
+    else state.preview();
+    return true;
+  }
+
+  /** The corners of the polygon being edited, where they are drawn (test hook). */
+  info(): { objectId: string; corners: Vec2[] } | null {
+    return this.lastCorners;
+  }
+
+  /** A handle on every corner of the polygon collider being edited. */
+  draw(g: Graphics, view: View): void {
+    const item = this.editedPolygon();
+    this.lastCorners = null;
+    if (!item) return;
+    const half = CORNER_SIZE / 2;
+    const corners = colliderPoints(item.props).map((p) => {
+      const world = applyPoint(item.m, p);
+      return worldToScreen(view, world.x, world.y);
+    });
+    for (const c of corners)
+      g.rect(c.x - half, c.y - half, CORNER_SIZE, CORNER_SIZE)
+        .fill(Theme.colors.accentText)
+        .stroke({ width: 1, color: Theme.colors.accent });
+    this.lastCorners = {
+      objectId: item.id,
+      corners: corners.map((c) => {
+        const p = g.toGlobal(c);
+        return { x: p.x, y: p.y };
+      }),
+    };
+  }
+}
+
+/** A corner lands on whole units, as a dragged object does. */
+function rounded(p: Vec2): Vec2 {
+  return { x: Math.round(p.x), y: Math.round(p.y) };
+}
+
+function listOf(points: readonly Vec2[]): ListValue {
+  return points.map((p) => ({ x: p.x, y: p.y }));
+}
diff --git a/src/editor/viewport/sceneViewHost.ts b/src/editor/viewport/sceneViewHost.ts
new file mode 100644
index 00000000..84a9bbd4
--- /dev/null
+++ b/src/editor/viewport/sceneViewHost.ts
@@ -0,0 +1,21 @@
+import type { ObjectHolder } from '../../core/model/Project';
+import type { Vec2 } from '../../core/model/Properties';
+import type { DrawItem } from '../../viewport/items';
+import type { EditorState } from '../state';
+
+/** Below this on-screen spacing a grid is noise rather than orientation, so it is dropped. */
+export const MIN_GRID_STEP = 12;
+
+/** What the scene view lends the tools that work inside it. */
+export interface SceneViewHost {
+  readonly state: EditorState;
+  /** What the view shows and edits. */
+  holder(): ObjectHolder | undefined;
+  /** What the view drew last. */
+  items(): readonly DrawItem[];
+  /** The world point under a global pointer position. */
+  worldAt(global: Vec2): Vec2;
+  /** Screen pixels per world unit. */
+  scale(): number;
+  redraw(): void;
+}
diff --git a/src/editor/viewport/tilemapPainting.ts b/src/editor/viewport/tilemapPainting.ts
new file mode 100644
index 00000000..529fe6a8
--- /dev/null
+++ b/src/editor/viewport/tilemapPainting.ts
@@ -0,0 +1,275 @@
+import type { Graphics } from 'pixi.js';
+import { type CellEdit, paintTiles } from '../../core/commands/tilemapCommands';
+import type { TilemapAsset, TilemapLayer } from '../../core/model/Asset';
+import { apply as applyPoint, invert } from '../../core/model/matrix';
+import type { Vec2 } from '../../core/model/Properties';
+import { findAssetOf } from '../../core/model/Project';
+import {
+  cellAt,
+  cellAtPoint,
+  cellKey,
+  cellOrigin,
+  cellsAlong,
+  cellsInRect,
+  findLayer,
+  floodFill,
+  inGrid,
+  tilemapColumns,
+  tilemapRows,
+  tilemapTileSize,
+} from '../../core/model/tilemap';
+import { Theme } from '../../ui';
+import type { DrawItem } from '../../viewport/items';
+import { type View, worldToScreen } from '../../viewport/view';
+import { paintedLayerId, paintedTilemap, pickTile } from '../panels/TilemapPanel';
+import type { TilemapTool } from '../state';
+import { MIN_GRID_STEP, type SceneViewHost } from './sceneViewHost';
+
+/** The selected tilemap a painting tool works on, and the layer it paints. */
+export interface PaintTarget {
+  item: DrawItem;
+  asset: TilemapAsset;
+  layerId: string;
+}
+
+/**
+ * A stroke on a placed tilemap. `previous` is what the cells held before the live preview wrote
+ * into them – the same trick a move drag uses: the model shows the stroke at once, and on release
+ * it is put back so the command records it as one undo step.
+ */
+interface PaintStroke {
+  objectId: string;
+  assetId: string;
+  layerId: string;
+  tool: TilemapTool;
+  tileId: string;
+  edits: CellEdit[];
+  previous: Map<string, string>;
+  from: Vec2;
+  last: Vec2 | null;
+  /** Where the pointer is now, in cells – what the rectangle preview is drawn from. */
+  to: Vec2;
+}
+
+/** The tilemap tools of the scene view: brush, eraser, rectangle, fill and pick. */
+export class TilemapPainter {
+  private stroke: PaintStroke | null = null;
+
+  constructor(private readonly host: SceneViewHost) {}
+
+  /** A stroke is under way. */
+  get active(): boolean {
+    return this.stroke !== null;
+  }
+
+  /**
+   * The tilemap a click paints on: the one selected (see `paintedTilemap`), with a painting tool
+   * chosen. Anything else – another object, the select tool, a modifier held – leaves the view
+   * doing what it always did.
+   */
+  targetOf(objectId: string): PaintTarget | null {
+    const state = this.host.state;
+    if (state.tilemapPaint.tool === 'select') return null;
+    const target = paintedTilemap(state);
+    if (!target || target.objectId !== objectId) return null;
+    const item = this.host.items().find((i) => i.id === objectId);
+    if (!item || item.type !== 'tilemap') return null;
+    const layerId = paintedLayerId(state, target.asset);
+    return layerId ? { item, asset: target.asset, layerId } : null;
+  }
+
+  /**
+   * What a press with a painting tool does. Fill and pick are done with the click – a drag adds
+   * nothing to either – so only the other three start a stroke. True when the press was used.
+   */
+  press(target: PaintTarget, global: Vec2): boolean {
+    const tool = this.host.state.tilemapPaint.tool;
+    if (tool !== 'fill' && tool !== 'pick') return this.startStroke(target, global);
+    const layer = findLayer(target.asset, target.layerId);
+    const at = this.cellUnder(target.item, global);
+    if (!layer || !inGrid(target.item.props, at.x, at.y)) return true;
+    if (tool === 'pick') this.pickCell(layer, at);
+    else this.fillFrom(target, layer, at);
+    return true;
+  }
+
+  /** Follows the pointer with the stroke under way; false when there is none. */
+  move(global: Vec2): boolean {
+    const stroke = this.stroke;
+    if (!stroke) return false;
+    const item = this.host.items().find((i) => i.id === stroke.objectId);
+    if (!item) return true;
+    const at = this.cellUnder(item, global);
+    if (stroke.last && at.x === stroke.last.x && at.y === stroke.last.y) return true;
+    const from = stroke.last ?? stroke.from;
+    stroke.last = at;
+    stroke.to = at;
+    // A pointer jumps between events, so the cells in between are filled in as well.
+    if (stroke.tool !== 'rect') this.paintCells(item, cellsAlong(from, at));
+    this.host.redraw();
+    return true;
+  }
+
+  /** Ends the stroke under way; false when there was none. */
+  release(): boolean {
+    if (!this.stroke) return false;
+    this.endStroke();
+    return true;
+  }
+
+  /** The cell under a pointer position, in the tilemap's own grid. */
+  private cellUnder(item: DrawItem, global: Vec2): Vec2 {
+    const inMap = applyPoint(invert(item.m), this.host.worldAt(global));
+    return cellAtPoint(item.props, inMap.x, inMap.y);
+  }
+
+  /** Takes the tile of a cell on the painted layer – the one painted on next (#133). */
+  private pickCell(layer: TilemapLayer, at: Vec2): void {
+    const state = this.host.state;
+    const tileId = cellAt(layer, at.x, at.y);
+    if (!tileId) state.setNotice('This cell is empty on the active layer.');
+    else if (!findAssetOf(state.project, tileId, 'tile'))
+      state.setNotice('This cell names a tile that no longer exists.');
+    else pickTile(state, tileId);
+  }
+
+  /** Fills the connected cells of one tile in a single command, so one click is one undo step. */
+  private fillFrom(target: PaintTarget, layer: TilemapLayer, at: Vec2): void {
+    const state = this.host.state;
+    const tileId = state.tilemapPaint.tileId;
+    if (!tileId) {
+      state.setNotice('Choose a tile to paint with.');
+      return;
+    }
+    const cells = floodFill(target.item.props, layer, at, tileId);
+    const command = paintTiles(
+      layer,
+      target.asset.id,
+      cells.map((cell) => ({ ...cell, tileId })),
+      'Fill tiles',
+    );
+    if (command) state.execute(command);
+  }
+
+  private startStroke(target: PaintTarget, global: Vec2): boolean {
+    const paint = this.host.state.tilemapPaint;
+    const tileId = paint.tool === 'erase' ? '' : paint.tileId;
+    if (paint.tool !== 'erase' && !tileId) {
+      this.host.state.setNotice('Choose a tile to paint with.');
+      return false;
+    }
+    const at = this.cellUnder(target.item, global);
+    this.stroke = {
+      objectId: target.item.id,
+      assetId: target.asset.id,
+      layerId: target.layerId,
+      tool: paint.tool,
+      tileId,
+      edits: [],
+      previous: new Map(),
+      from: at,
+      last: null,
+      to: at,
+    };
+    // A rectangle only shows where it would go until the button comes up; the other two draw at once.
+    if (paint.tool !== 'rect') this.paintCells(target.item, [at]);
+    this.host.redraw();
+    return true;
+  }
+
+  private strokeLayer(stroke: PaintStroke): TilemapLayer | undefined {
+    const asset = findAssetOf(this.host.state.project, stroke.assetId, 'tilemap');
+    return asset && findLayer(asset, stroke.layerId);
+  }
+
+  /** Writes cells into the model as a preview, remembering what they held. */
+  private paintCells(item: DrawItem, cells: readonly Vec2[]): void {
+    const stroke = this.stroke;
+    const layer = stroke && this.strokeLayer(stroke);
+    if (!stroke || !layer) return;
+    let changed = false;
+    for (const at of cells) {
+      if (!inGrid(item.props, at.x, at.y)) continue;
+      const key = cellKey(at.x, at.y);
+      if (!stroke.previous.has(key)) stroke.previous.set(key, layer.cells[key] ?? '');
+      if ((layer.cells[key] ?? '') === stroke.tileId) continue;
+      stroke.edits.push({ x: at.x, y: at.y, tileId: stroke.tileId });
+      if (stroke.tileId) layer.cells[key] = stroke.tileId;
+      else delete layer.cells[key];
+      changed = true;
+    }
+    if (changed) this.host.state.preview();
+  }
+
+  /** Puts the preview back and lets the command do the edit, so one stroke is one undo step. */
+  private endStroke(): void {
+    const stroke = this.stroke;
+    this.stroke = null;
+    if (!stroke) return;
+    const layer = this.strokeLayer(stroke);
+    if (layer)
+      for (const [key, value] of stroke.previous) {
+        if (value) layer.cells[key] = value;
+        else delete layer.cells[key];
+      }
+    const item = this.host.items().find((i) => i.id === stroke.objectId);
+    const edits =
+      stroke.tool === 'rect' && item
+        ? cellsInRect(item.props, stroke.from, stroke.to).map((at) => ({
+            ...at,
+            tileId: stroke.tileId,
+          }))
+        : stroke.edits;
+    const command =
+      layer &&
+      paintTiles(layer, stroke.assetId, edits, stroke.tool === 'erase' ? 'Erase tiles' : 'Paint tiles');
+    if (command) this.host.state.execute(command);
+    else this.host.state.preview();
+    this.host.redraw();
+  }
+
+  /**
+   * While a painting tool is chosen, the selected tilemap shows its cells – without the grid one
+   * paints blind – and a rectangle drag shows the box it would fill.
+   */
+  draw(g: Graphics, items: readonly DrawItem[], view: View): void {
+    const state = this.host.state;
+    const target = state.tilemapPaint.tool === 'select' ? null : paintedTilemap(state);
+    if (!target) return;
+    const item = items.find((i) => i.id === target.objectId && i.type === 'tilemap');
+    if (!item) return;
+    const tile = tilemapTileSize(item.props);
+    const columns = tilemapColumns(item.props);
+    const rows = tilemapRows(item.props);
+    const corner = (x: number, y: number): Vec2 => {
+      const at = cellOrigin(item.props, x, y);
+      return worldToScreen(
+        view,
+        item.m.tx + at.x * item.m.a + at.y * item.m.c,
+        item.m.ty + at.x * item.m.b + at.y * item.m.d,
+      );
+    };
+    if (tile * view.scale >= MIN_GRID_STEP) {
+      for (let x = 0; x <= columns; x++) {
+        const a = corner(x, 0);
+        const b = corner(x, rows);
+        g.moveTo(a.x, a.y).lineTo(b.x, b.y);
+      }
+      for (let y = 0; y <= rows; y++) {
+        const a = corner(0, y);
+        const b = corner(columns, y);
+        g.moveTo(a.x, a.y).lineTo(b.x, b.y);
+      }
+      g.stroke({ width: 1, color: Theme.colors.gizmo, alpha: 0.25 });
+    }
+    const stroke = this.stroke;
+    if (!stroke || stroke.tool !== 'rect') return;
+    const x0 = Math.min(stroke.from.x, stroke.to.x);
+    const y0 = Math.min(stroke.from.y, stroke.to.y);
+    const x1 = Math.max(stroke.from.x, stroke.to.x) + 1;
+    const y1 = Math.max(stroke.from.y, stroke.to.y) + 1;
+    g.poly([corner(x0, y0), corner(x1, y0), corner(x1, y1), corner(x0, y1)], true)
+      .fill({ color: Theme.colors.accent, alpha: 0.2 })
+      .stroke({ width: 2, color: Theme.colors.accent });
+  }
+}
diff --git a/src/editor/viewport/tilemapSolids.ts b/src/editor/viewport/tilemapSolids.ts
new file mode 100644
index 00000000..11838830
--- /dev/null
+++ b/src/editor/viewport/tilemapSolids.ts
@@ -0,0 +1,162 @@
+import { type Container, Graphics } from 'pixi.js';
+import { scaleOf } from '../../core/model/matrix';
+import type { BoundaryLine } from '../../core/model/polygon';
+import type { Vec2 } from '../../core/model/Properties';
+import { cellOrigin, tilemapTileSize } from '../../core/model/tilemap';
+import { type CellPolygon, type CellRect, solidBoundary } from '../../core/model/tilemapCollision';
+import { Theme } from '../../ui';
+import type { DrawItem } from '../../viewport/items';
+import { CONE_LENGTH, markBlockingSide, placeNode } from '../../viewport/symbols';
+
+/** Where a resolved tilemap blocks: what its outline in the scene view is worked out from. */
+type TilemapSolids = Pick<NonNullable<DrawItem['tilemap']>, 'solids' | 'outlines' | 'oneWay'>;
+
+function sameSolids(a: TilemapSolids, b: TilemapSolids): boolean {
+  const sameRects = (p: readonly CellRect[], q: readonly CellRect[]) =>
+    p.length === q.length &&
+    p.every((r, i) => r.x === q[i]!.x && r.y === q[i]!.y && r.w === q[i]!.w && r.h === q[i]!.h);
+  const samePolygons = (p: readonly CellPolygon[], q: readonly CellPolygon[]) =>
+    p.length === q.length &&
+    p.every(
+      (polygon, i) =>
+        polygon.outline.length === q[i]!.outline.length &&
+        polygon.outline.every((c, j) => c.x === q[i]!.outline[j]!.x && c.y === q[i]!.outline[j]!.y),
+    );
+  return (
+    sameRects(a.solids, b.solids) &&
+    samePolygons(a.outlines, b.outlines) &&
+    sameRects(a.oneWay.solids, b.oneWay.solids) &&
+    samePolygons(a.oneWay.outlines, b.oneWay.outlines)
+  );
+}
+
+/**
+ * Where each placed tilemap blocks, outlined like a collider (#132). Editor-only for the same
+ * reason as a particle source's icon: the game view never syncs decorations.
+ */
+export class TilemapSolidOutlines {
+  private readonly nodes = new Map<string, Graphics>();
+  /**
+   * The outer edges of each tilemap's solids and what they were worked out from (#173). A map is
+   * resolved afresh on every draw, so they are kept by what blocks rather than by the resolved map.
+   */
+  private readonly boundaries = new Map<
+    string,
+    { from: TilemapSolids; solid: BoundaryLine[]; oneWay: BoundaryLine[] }
+  >();
+
+  constructor(private readonly layer: Container) {}
+
+  dispose(id: string): void {
+    this.nodes.get(id)?.destroy();
+    this.nodes.delete(id);
+    this.boundaries.delete(id);
+  }
+
+  /** The blocking rectangles and outlined cells of a placed tilemap, and the outer edges drawn round them, in cells (test hook). */
+  info(item: DrawItem | undefined): {
+    visible: boolean;
+    rects: CellRect[];
+    outlines: Vec2[][];
+    oneWay: CellRect[];
+    oneWayOutlines: Vec2[][];
+    boundary: BoundaryLine[];
+  } | null {
+    const node = item && this.nodes.get(item.id);
+    if (!node || !item.tilemap) return null;
+    return {
+      visible: node.visible,
+      rects: item.tilemap.solids.map((rect) => ({ ...rect })),
+      outlines: item.tilemap.outlines.map((polygon) => polygon.outline.map((p) => ({ ...p }))),
+      oneWay: item.tilemap.oneWay.solids.map((rect) => ({ ...rect })),
+      oneWayOutlines: item.tilemap.oneWay.outlines.map((polygon) => polygon.outline.map((p) => ({ ...p }))),
+      boundary: this.boundaryOf(item).solid.map((line) => ({
+        points: line.points.map((p) => ({ ...p })),
+        closed: line.closed,
+      })),
+    };
+  }
+
+  private boundaryOf(item: DrawItem): { solid: BoundaryLine[]; oneWay: BoundaryLine[] } {
+    const from: TilemapSolids = item.tilemap!;
+    let boundary = this.boundaries.get(item.id);
+    if (!boundary || !sameSolids(boundary.from, from)) {
+      boundary = {
+        from,
+        solid: solidBoundary(from.solids, from.outlines),
+        oneWay: solidBoundary(from.oneWay.solids, from.oneWay.outlines),
+      };
+      this.boundaries.set(item.id, boundary);
+    }
+    return boundary;
+  }
+
+  /**
+   * The solid cells of a tilemap, drawn the way a collider is: tinted and outlined, above the
+   * content so a painted wall cannot hide them.
+   */
+  sync(item: DrawItem, show: boolean, scale: number): void {
+    let node = this.nodes.get(item.id);
+    const rects = item.tilemap?.solids ?? [];
+    const outlines = item.tilemap?.outlines ?? [];
+    const oneWay = item.tilemap?.oneWay ?? { solids: [], outlines: [] };
+    if (!show || rects.length + outlines.length + oneWay.solids.length + oneWay.outlines.length === 0) {
+      if (node) node.visible = false;
+      return;
+    }
+    if (!node) {
+      node = new Graphics();
+      node.eventMode = 'none';
+      this.nodes.set(item.id, node);
+      this.layer.addChild(node);
+    }
+    node.visible = true;
+    placeNode(node, item.m);
+    const tile = tilemapTileSize(item.props);
+    const width = 2 / (scale * scaleOf(item.m));
+    node.clear();
+    const color = Theme.colors.collider;
+    const boundary = this.boundaryOf(item);
+    // Filled piece by piece, but stroked only along the outer edges, so no seam shows (#173).
+    const fill = (rects: readonly CellRect[], outlines: readonly CellPolygon[], alpha: number) => {
+      if (rects.length + outlines.length === 0) return;
+      for (const rect of rects) {
+        const corner = cellOrigin(item.props, rect.x, rect.y);
+        node.rect(corner.x, corner.y, rect.w * tile, rect.h * tile);
+      }
+      for (const polygon of outlines)
+        node.poly(
+          polygon.outline.map((p) => cellOrigin(item.props, p.x, p.y)),
+          true,
+        );
+      node.fill({ color, alpha });
+    };
+    const stroke = (lines: readonly BoundaryLine[], lineWidth: number) => {
+      if (lines.length === 0) return;
+      for (const line of lines)
+        node.poly(
+          line.points.map((p) => cellOrigin(item.props, p.x, p.y)),
+          line.closed,
+        );
+      node.stroke({ width: lineWidth, color });
+    };
+    fill(rects, outlines, 0.12);
+    stroke(boundary.solid, width);
+    fill(oneWay.solids, oneWay.outlines, 0.06);
+    stroke(boundary.oneWay, width / 2);
+    // A platform's top is what blocks, so that is what stands out, with the cone it lands within.
+    const length = CONE_LENGTH / (scale * scaleOf(item.m));
+    for (const rect of oneWay.solids) {
+      const a = cellOrigin(item.props, rect.x, rect.y);
+      const b = cellOrigin(item.props, rect.x + rect.w, rect.y + rect.h);
+      markBlockingSide(
+        node,
+        { left: a.x, top: a.y, right: b.x, bottom: b.y },
+        { x: 0, y: -1 },
+        width * 2,
+        color,
+        { angle: rect.angle, length },
+      );
+    }
+  }
+}