Rapier 2D (@dimforge/rapier2d-compat 0.20) comes in as the physics engine: the editor loads it once in the background right after opening a project from its own chunk, so the main bundle grows by 0.19 KB gzip and the first frame does not wait; loadPhysics, physicsStatus and the 100 px = 1 m conversion live in src/runtime/physics/engine.ts, __gm.physics() reports the state, unit tests pin free fall and a box resting on a box, and e2e module 41 pins that Rapier is ready from its own chunk without console noise (#145)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/editor/App.ts

@@ -2,6 +2,7 @@ import type { Application } from 'pixi.js';
 import { HttpFileStore } from '../core/store/HttpFileStore';
 import { HttpProjectStore } from '../core/store/HttpProjectStore';
 import type { StoredTexture, UploadedTexture } from '../core/store/TextureStore';
+import { loadPhysics } from '../runtime/physics/engine';
 import { requestRender, type VStack } from '../ui';
 import { fontLibrary } from '../viewport/FontLibrary';
 import { soundLibrary } from '../viewport/SoundLibrary';
@@ -42,5 +43,6 @@ export async function startEditor(app: Application, root: VStack): Promise<void>
   fontLibrary.onChange(() => requestRender());
   installDebugHook(app, state, editor);
   await editor.boot();
+  loadPhysics().catch((error: unknown) => console.error('[physics] Rapier could not be loaded:', error));
   await startPlugins(API_ENDPOINT, state, editor);
 }

src/editor/debugHook.ts

@@ -6,6 +6,7 @@ import { findAsset, findAssetOf, findObjectHolder } from '../core/model/Project'
 import type { FrameRect } from '../core/model/texture';
 import { findLayer } from '../core/model/tilemap';
 import { effectiveScale, gameClock } from '../core/time';
+import { physicsStatus } from '../runtime/physics/engine';
 import {
   activeDrag,
   afterFrames,
@@ -641,6 +642,7 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
     sceneSelect: () => ({ ...centreOf(editor.sceneButton), enabled: editor.sceneButton.enabled }),
     /** Behaviors running without Play (`Run in editor`), and how long they have been at it. */
     editorRun: () => editor.editorRunInfo(),
+    physics: () => physicsStatus(),
     /** Rows the hierarchy shows – collapsed subtrees are not in here. */
     hierarchyOrder: () => hierarchy.rowOrder(),
     hierarchyChevron: (id: string) => hierarchy.chevronPosition(id),

src/runtime/physics/engine.ts

@@ -0,0 +1,40 @@
+import type RAPIER from '@dimforge/rapier2d-compat';
+
+export type Rapier = typeof RAPIER;
+
+/** Everything outside the engine is in pixels; Rapier is tuned for metres. */
+export const PIXELS_PER_METRE = 100;
+
+export const toMetres = (pixels: number): number => pixels / PIXELS_PER_METRE;
+export const toPixels = (metres: number): number => metres * PIXELS_PER_METRE;
+
+export type PhysicsStatus =
+  | { state: 'idle' | 'loading' }
+  | { state: 'ready'; ms: number; version: string }
+  | { state: 'failed'; error: string };
+
+let status: PhysicsStatus = { state: 'idle' };
+let loading: Promise<Rapier> | null = null;
+
+/**
+ * Rapier is its own chunk with the WebAssembly inside. Every caller shares the one load, and a
+ * failed load stays failed rather than being tried again on each call.
+ */
+export function loadPhysics(): Promise<Rapier> {
+  if (loading) return loading;
+  const start = performance.now();
+  status = { state: 'loading' };
+  loading = import('@dimforge/rapier2d-compat').then(async ({ default: rapier }) => {
+    await rapier.init();
+    status = { state: 'ready', ms: Math.round(performance.now() - start), version: rapier.version() };
+    return rapier;
+  });
+  loading.catch((error: unknown) => {
+    status = { state: 'failed', error: error instanceof Error ? error.message : String(error) };
+  });
+  return loading;
+}
+
+export function physicsStatus(): PhysicsStatus {
+  return status;
+}