[3DGS] Request a Navara re-render on SparkJS async sort/LoD updates

[Otokami-Orokabu]

Overview

ThreeView renders on demand by default (animation: false). SparkJS, however, updates sort / LoD asynchronously in workers in a camera-dependent way. After the camera comes to rest, SparkJS may finish computing a new sort order but Navara isn't notified, so the rendered splat can remain in a stale state.

Current behavior

SplatMeshDesc.createMesh() calls mesh.initialized.then(() => this.requestUpdate()) only once.

Expected behavior

Request a Navara frame whenever SparkJS's sort / LoD state changes. Avoid forcing continuous rendering (animation: true); use a SparkJS event hook (see Approach below).

Approach

SparkJS exposes a public onDirty option in SparkRendererOptions, fired internally whenever sort / LoD state changes:

if (!this.dirty) {
  this.dirty = true;
  this.onDirty?.();
}

Wire this callback to MeshDesc.requestUpdate() so Navara schedules a frame whenever SparkJS finishes an async update.

The shared SparkRenderer (one per transparent scene, refCount-managed) needs to fan out a single onDirty to every SplatMeshDesc listener.

This avoids forcing continuous rendering (animation: true).

Related upstream issue

onDirty itself may not fire under certain paged LoD + camera-static conditions (the upstream dirty chain can break when the LoD worker is busy and the camera is static, so background paged fetches don't trigger a re-render).

The Navara-side onDirty → requestUpdate bridge is therefore necessary but not always sufficient: when the upstream chain breaks, no event reaches Navara to relay.

Scope

• This issue (Navara side): subscribe to onDirty and request a frame. Sufficient for sort-order refinement after camera stop and for non-paged LoD splats.
• Upstream (sparkjsdev/spark#298): additional setDirty() calls at paged-fetch completion etc. Pending.