fix some quirks in World.step when attempting to account for variable framerate

src/world/World.ts

@@ -388,15 +388,13 @@ export class World extends EventTarget {
    *     world.step(1/60);
    *
    * @see http://bulletphysics.org/mediawiki-1.5.8/index.php/Stepping_The_World
+   * @see https://gafferongames.com/post/fix_your_timestep/
    */
-  step(dt: number, timeSinceLastCalled = 0, maxSubSteps = 10): void {
-    if (timeSinceLastCalled === 0) {
+  step(dt: number, timeSinceLastCalled?: number, maxSubSteps: number = 10): void {
+    if (timeSinceLastCalled === undefined) {
       // Fixed, simple stepping
 
       this.internalStep(dt)
-
-      // Increment time
-      this.time += dt
     } else {
       this.accumulator += timeSinceLastCalled
       let substeps = 0
@@ -407,14 +405,16 @@ export class World extends EventTarget {
         substeps++
       }
 
-      const t = (this.accumulator % dt) / dt
+      // prevent the accumulator to build up delay to catch up. The logic being: if the step did not catch up at this frame, it is unlikely to catch up in the next one. Even if it does, it will result in a speed up simulation which is arguably worse that staying behind the real time.
+      this.accumulator = Math.max(this.accumulator, dt)
+
+      const t = this.accumulator / dt
       for (let j = 0; j !== this.bodies.length; j++) {
         const b = this.bodies[j]
         b.previousPosition.lerp(b.position, t, b.interpolatedPosition)
         b.previousQuaternion.slerp(b.quaternion, t, b.interpolatedQuaternion)
         b.previousQuaternion.normalize()
       }
-      this.time += timeSinceLastCalled
     }
   }