perf(vessels): let the scene idle with AIS vessels enabled

Vessels move only when a poll lands, yet enabling the layer held the
continuous render loop for its lifetime, so a parked vessels-only scene
rendered every frame. The layer now:

- requests a frame for every forced visibility pass (new data, selection);
- leaves one trailing frame when the camera moved inside the 800 ms
  visibility cadence, so the view the camera stopped on is evaluated. It is
  keyed to camera motion alone: the pass's own label publication requests a
  frame, and scheduling on that looped at ~1.2 Hz;
- holds 'ais-vessels' only while focus emphasis is animating.

The 80 ms focus pass, which runs while anything is tracked, also skips hidden
sprites whose focus state is at rest (new spriteFocusAtRest) and whose alpha
is already 1: advancing them changes nothing, and with a 12k worldwide feed
they are most of the fleet.

Measured: a parked vessels-only scene (headless SwiftShader) went from
continuous mode and 28 frames in 5 s to idle and 0; horizon visibility had no
mismatches after the camera moved and settled. The focus pass over 12,000
vessels (11,353 hidden) went from ~4.0 ms to ~1.2 ms.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/vessels/rendering.js

@@ -74,7 +74,13 @@ export function createRendering({
     focusPassIsNeeded,
     advanceSpriteFocus,
     focusAlphaNeedsWrite,
+    spriteFocusAtRest,
   } = services.focus;
+  const {
+    holdContinuousRender,
+    releaseContinuousRender,
+    governorRequestRender,
+  } = services.render;
 
   function renderRowLimit() {
     const configured = Number(options.maxRows);
@@ -195,6 +201,14 @@ export function createRendering({
     return icon;
   }
 
+  // Vessels move only when a poll lands, so the layer does not hold the
+  // continuous render loop. Its passes run in preRender, which idle mode fires
+  // only for rendered frames: a forced pass (new data, selection) requests its
+  // frame, a frame that moved the camera but fell inside the 800 ms cadence
+  // leaves a trailing frame so the view the camera stopped on is always
+  // evaluated, and the continuous hold is kept only while focus emphasis is
+  // animating. The trailing frame is keyed to camera motion alone: the pass's
+  // own label publication requests a frame, and scheduling on that would loop.
   function installRuntime(viewer) {
     if (state.preRenderRemover || !viewer) return;
     state.preRenderRemover = viewer.scene.preRender.addEventListener(() =>
@@ -202,8 +216,27 @@ export function createRendering({
     );
   }
 
+  function scheduleTrailingVisibilityPass(now) {
+    if (state.trailingVisibilityTimer) return;
+    const waitMs = Math.max(
+      0,
+      VISIBILITY_UPDATE_MS - (now - state.lastVisibilityUpdate),
+    );
+    state.trailingVisibilityTimer = setTimeout(() => {
+      state.trailingVisibilityTimer = null;
+      if (state.feed.enabled) governorRequestRender('ais-vessels-visibility');
+    }, waitMs);
+  }
+
+  function cancelTrailingVisibilityPass() {
+    if (!state.trailingVisibilityTimer) return;
+    clearTimeout(state.trailingVisibilityTimer);
+    state.trailingVisibilityTimer = null;
+  }
+
   function updateVisibility(force = false) {
     if (!state.feed.enabled) return;
+    if (force) governorRequestRender('ais-vessels-update');
     const now = focusNowMs(performance.now());
     const focusTarget = getFocusTarget();
     const regularPass =
@@ -211,13 +244,22 @@ export function createRendering({
     const focusPass =
       focusPassIsNeeded(focusTarget, state.activeFocusCount) &&
       (force || now - state.lastFocusUpdate >= FOCUS_UPDATE_MS);
+    if (
+      !regularPass &&
+      state.viewer?.camera &&
+      cameraPoseSignature(state.viewer.camera) !== vesselState._lastCamPoseSig
+    )
+      scheduleTrailingVisibilityPass(now);
     if (!regularPass && !focusPass) return;
     if (regularPass) state.lastVisibilityUpdate = now;
     if (focusPass) state.lastFocusUpdate = now;
     if (!state.records.all.length) {
       // No records — flush any lingering card entries (vanished-feed case).
       if (regularPass) updateClusteredLabels([]);
-      if (focusPass) state.activeFocusCount = 0;
+      if (focusPass) {
+        state.activeFocusCount = 0;
+        releaseContinuousRender('ais-vessels');
+      }
       return;
     }
 
@@ -271,6 +313,9 @@ export function createRendering({
           Cesium.Cartesian3.distance(camera.positionWC, position),
       });
       state.activeFocusCount = result.activeCount;
+      if (result.activeCount > 0 || result.transitioning)
+        holdContinuousRender('ais-vessels');
+      else releaseContinuousRender('ais-vessels');
     }
   }
 
@@ -301,6 +346,16 @@ export function createRendering({
       const bb = visual?.billboard;
       const position = bb?.position || visual?.position;
       if (!bb || !position) continue;
+      // Most of a worldwide feed sits behind the horizon. A hidden sprite with
+      // no pending focus state and full alpha would advance to exactly what it
+      // already is, so skip it instead of paying its distance and state work
+      // every 80 ms while anything is tracked.
+      if (
+        bb.show === false &&
+        spriteFocusAtRest(bb) &&
+        !focusAlphaNeedsWrite(bb.color?.alpha, 1, params)
+      )
+        continue;
       const focus = advanceSpriteFocus(bb, {
         // Hidden/far-side sprites still finish any pending release so the active
         // count remains truthful and they cannot reappear with stale dim alpha.
@@ -506,6 +561,7 @@ export function createRendering({
     shipIcon,
     installRuntime,
     updateVisibility,
+    cancelTrailingVisibilityPass,
     applyVesselFocusDeemphasis,
     makeOccluder,
     isVisible,