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>

tests/core/physics-engine.test.ts

@@ -0,0 +1,60 @@
+import { describe, expect, it } from 'vitest';
+import {
+  loadPhysics,
+  PIXELS_PER_METRE,
+  physicsStatus,
+  type Rapier,
+  toMetres,
+  toPixels,
+} from '../../src/runtime/physics/engine';
+
+const STEP = 1 / 60;
+const GRAVITY = 980;
+
+/** A 400 × 20 px floor whose top is at y = 290, and a 32 px crate dropped from y = 0. */
+function crateOverFloor(rapier: Rapier) {
+  const world = new rapier.World({ x: 0, y: toMetres(GRAVITY) });
+  world.timestep = STEP;
+  const floor = world.createRigidBody(rapier.RigidBodyDesc.fixed().setTranslation(0, toMetres(300)));
+  world.createCollider(rapier.ColliderDesc.cuboid(toMetres(200), toMetres(10)), floor);
+  const crate = world.createRigidBody(rapier.RigidBodyDesc.dynamic().setTranslation(0, 0));
+  world.createCollider(rapier.ColliderDesc.cuboid(toMetres(16), toMetres(16)), crate);
+  const run = (steps: number) => {
+    for (let i = 0; i < steps; i++) world.step();
+  };
+  const y = () => toPixels(crate.translation().y);
+  return { world, crate, run, y };
+}
+
+describe('physics engine', () => {
+  it('loads once and reports the Rapier version', async () => {
+    const first = loadPhysics();
+    expect(loadPhysics()).toBe(first);
+    const rapier = await first;
+    expect(rapier.version()).toBe('0.20.0');
+    expect(physicsStatus()).toMatchObject({ state: 'ready', version: '0.20.0' });
+  });
+
+  it('converts at 100 px to the metre', () => {
+    expect(PIXELS_PER_METRE).toBe(100);
+    expect(toMetres(980)).toBe(9.8);
+    expect(toPixels(toMetres(123.5))).toBeCloseTo(123.5, 10);
+  });
+
+  it('lets a crate fall freely in pixels per second squared', async () => {
+    const { run, y, crate } = crateOverFloor(await loadPhysics());
+    run(30);
+    // ½ g t² = 122.5 px after half a second; the integrator lands about a pixel past it.
+    expect(Math.abs(y() - 0.5 * GRAVITY * 0.25)).toBeLessThan(1.5);
+    expect(toPixels(crate.linvel().y)).toBeCloseTo(GRAVITY * 0.5, 3);
+  });
+
+  it('keeps a crate resting on the floor', async () => {
+    const { run, y, crate } = crateOverFloor(await loadPhysics());
+    run(180);
+    expect(Math.abs(y() - 274)).toBeLessThan(0.5);
+    expect(Math.abs(toPixels(crate.linvel().y))).toBeLessThan(0.5);
+    run(120);
+    expect(Math.abs(y() - 274)).toBeLessThan(0.5);
+  });
+});