fix(perf): free a turned-off layer's entities that Cesium keeps (#1367)

Release stale Cesium command references after disabling a layer while preserving current-frame commands.

Contribution by @quanticsoul4772.

src/sceneCommandRelease.js

@@ -0,0 +1,74 @@
+/**
+ * Let a turned-off layer's memory go.
+ *
+ * Cesium (engine 22.3, `Scene/View.js`) reuses each view's per-pass command
+ * arrays: every frame resets `frustumCommands.indices[pass]` to 0 and
+ * overwrites `commands[pass][i]` from the start. `Scene.js` truncates the
+ * voxel, Gaussian-splat and translucent passes to their counts; the opaque
+ * and other passes keep the slots past the new count. When a layer is turned
+ * off the next frame has fewer commands, so that tail keeps the destroyed
+ * primitives' DrawCommands, and through `_owner` → primitive → entity →
+ * EntityCollection the whole data source stays reachable. The offscreen view
+ * `scene.sampleHeight` / `pickFromRay` render into keeps its last list the
+ * same way, until the next height sample.
+ *
+ * Measured (production build, heap after GC, Data Centers on → off): 116 MiB
+ * kept on main, 13 MiB with this module; heap snapshots traced the entities
+ * through `View.frustumCommandsList` and `Picking._pickOffscreenView`.
+ *
+ * After a layer finishes turning off, this renders one frame, trims the main
+ * view's arrays to that frame's counts, and empties the offscreen pick view's
+ * lists (it rebuilds them in full on every pick). It reads private Cesium
+ * fields: `src/sceneCommandRelease.test.mjs` fails if any of them is renamed
+ * or if Cesium changes which passes it truncates, and once Cesium clears the
+ * tail itself this module can go.
+ */
+
+/** Drop command references a view no longer draws. */
+function trimView(view, { keepCurrent }) {
+  for (const frustum of view?.frustumCommandsList || []) {
+    const { commands, indices } = frustum;
+    for (let pass = 0; pass < commands.length; pass += 1) {
+      const keep = keepCurrent ? indices[pass] : 0;
+      if (commands[pass].length > keep) commands[pass].length = keep;
+      if (!keepCurrent) indices[pass] = 0;
+    }
+  }
+  if (!keepCurrent)
+    for (const extent of view?._commandExtents || [])
+      extent.command = undefined;
+}
+
+/**
+ * Release stale command references in the scene's main view (keeping what
+ * the last frame drew) and in its offscreen pick view (all of it).
+ * @param {object} scene Cesium Scene, right after a render.
+ */
+export function trimStaleSceneCommands(scene) {
+  trimView(scene._defaultView, { keepCurrent: true });
+  trimView(scene._picking?._pickOffscreenView, { keepCurrent: false });
+}
+
+/**
+ * After every layer disable, render one frame and trim once it is drawn.
+ * @param {{scene: object, dataManager: object, requestRender: (reason: string) => void}} deps
+ * @returns {() => void} uninstall
+ */
+export function installLayerReleaseTrim({ scene, dataManager, requestRender }) {
+  let removeOneShot = null;
+  const unsubscribe = dataManager.subscribe((change) => {
+    if (change?.type !== 'visibility' || change.enabled !== false) return;
+    if (removeOneShot) return; // a trim is already waiting for its frame
+    removeOneShot = scene.postRender.addEventListener(() => {
+      removeOneShot();
+      removeOneShot = null;
+      trimStaleSceneCommands(scene);
+    });
+    requestRender('layer-release-trim');
+  });
+  return () => {
+    unsubscribe();
+    removeOneShot?.();
+    removeOneShot = null;
+  };
+}