diff --git a/docs/CURRENT-STATE.md b/docs/CURRENT-STATE.md
index 9a530406c..fad19c1f9 100644
--- a/docs/CURRENT-STATE.md
+++ b/docs/CURRENT-STATE.md
@@ -7459,6 +7459,28 @@ apply to `public/` assets or built-preview serving.
     holds the way it stripped detection's would take bracket promptness with it, silently.
   - Known, pre-existing, and deliberately out of scope here: the overlay's backing store does not
     re-derive on a DPR change mid-session (`worldOverlay` sizing — untouched by this work).
+- **A detection paint reuses the sweep before it when nothing it reads has changed
+  (`src/data/detectionPaintCache.js`).** The sweep in `_drawOverlay` (project every contact, fade
+  it, append its bracket, build callout candidates) depends only on the view and on the contacts.
+  Each paint still asks every layer for its contacts, then compares them with what the last sweep
+  was built from: the same objects in the same order, the same position coordinates, and the same
+  value in every field the sweep reads (`PAINTED_OBJECT_FIELDS`), under the same view (profile,
+  style, canvas size, keyhole, camera position, view-projection matrix, UI rectangles; see
+  `_paintViewOf`). On a match the kept bracket paths, counts and callout candidates are used again
+  and only the strokes and callouts are repainted. No layer has to announce a change, so none can
+  leave a stale picture behind. A sweep that READS a new object field or a new view value must add
+  it to those two lists; `detectionHost.test.mjs` compares 41 frames against a full sweep, canvas
+  call for canvas call.
+  - Never reused or kept: a solve paint (the sweep gathers the cohort), any paint while the Cyber
+    sonar sweep or a Global Context echo is live (brackets change with the clock), and any sweep
+    that reached a depth-tested contact (its answer comes from the scene).
+  - Where it pays: a held camera with contacts that move in steps. Live Flights re-positions the
+    fleet every 80 ms, so at 60 fps about seven paints in ten reuse a sweep. Once a frame takes
+    longer than 80 ms every paint sees moved aircraft and none is reused; after eight such sweeps
+    the contacts are only recorded on every eighth, so the comparison costs nothing there.
+    Satellites move on every frame and are never reused.
+  - `getDetectionDiagnostics().paintReuseCount` (and `data-paint-reuse-count`) counts reused
+    paints since `initDetection`; `placementBuildCount` is 0 on a reused paint.
 - The detection MODE BANNER (`DENSE VIS:… SRC:… DENS:…% ELASTIC …ms`) is
   developer telemetry and is HIDDEN by default. It paints only under
   `?detectDebug=1` (the `trafficDebug` convention), resolved once per
diff --git a/scripts/package-boundaries.json b/scripts/package-boundaries.json
index 918e3ea61..521afe83a 100644
--- a/scripts/package-boundaries.json
+++ b/scripts/package-boundaries.json
@@ -2690,6 +2690,7 @@
       "src/data/detectionDraw.js",
       "src/data/detectionPolicy.js",
       "src/data/detectionOcclusion.js",
+      "src/data/detectionPaintCache.js",
       "src/data/detectionPresentation.js",
       "src/data/detectionRenderDemand.js",
       "src/data/sceneOcclusionProbe.js",
@@ -4372,6 +4373,7 @@
       "src/data/detectionRenderDemand.js",
       "src/data/detectionPresentation.js",
       "src/data/detectionOcclusion.js",
+      "src/data/detectionPaintCache.js",
       "src/data/sceneOcclusionProbe.js",
       "src/data/modelEligibility.js",
       "src/app/layers/recentImagery.js",
diff --git a/src/data/detection.js b/src/data/detection.js
index 770b63fef..be6d355bb 100644
--- a/src/data/detection.js
+++ b/src/data/detection.js
@@ -51,6 +51,7 @@ import {
 } from './detectionPolicy.js';
 import { detectionBracketOpacity } from './detectionPresentation.js';
 import { SurfaceOcclusion } from './detectionOcclusion.js';
+import { createDetectionPaintCache } from './detectionPaintCache.js';
 import { SceneOcclusionProbe } from './sceneOcclusionProbe.js';
 import {
   createCyberSonarSampler,
@@ -243,6 +244,15 @@ let _hostLane = null;
 let _calloutLane = null;
 let _transitBracketPaths = new Map();
 let _transitBracketAlpha = 1;
+/**
+ * What the last full sweep of the objects built, kept for the paints that
+ * follow it with the same view and the same objects. See `_drawOverlay`.
+ */
+const _paintCache = createDetectionPaintCache();
+/** Scratch for `_paintViewOf`; the cache copies it when it keeps a paint. */
+const _paintView = [];
+/** Paints since `initDetection` that reused a kept sweep (diagnostics). */
+let _paintReuseCount = 0;
 /**
  * Callouts solved by the sensor lane this frame, replayed by the callout lane.
  * The array and its rows are POOLED — `_calloutCount` is the live length — so a
@@ -379,6 +389,8 @@ export function destroyDetection() {
   _calloutLane?.unregister?.();
   _calloutLane = null;
   _transitBracketPaths.clear();
+  _paintCache.clear();
+  _paintReuseCount = 0;
   _calloutCount = 0;
   _viewer = null;
   _layers = [];
@@ -643,6 +655,7 @@ function _publishDiagnostics() {
   dataset.solveMs = String(_lastDiagnostics.solveMs ?? 0);
   dataset.frameTotalMs = String(_lastDiagnostics.frameTotalMs ?? 0);
   dataset.throttleSkipCount = String(_lastDiagnostics.throttleSkipCount ?? 0);
+  dataset.paintReuseCount = String(_lastDiagnostics.paintReuseCount ?? 0);
 }
 
 /**
@@ -718,6 +731,7 @@ function _applyModeState() {
     _syncSurfaceVisibility();
     _setLanesActive(false);
     _labelArbiter.clear();
+    _paintCache.clear();
     _surfaceOcclusion.clear();
     _surfaceSuppressedKeys.clear();
     _occlusionProbe.reset();
@@ -1259,15 +1273,53 @@ function _materializeCandidate(
   };
 }
 
+/**
+ * Every value of the view that the object sweep in `_drawOverlay` and the
+ * callout candidates it builds read, in a fixed order, for
+ * `detectionPaintCache`. A value the sweep starts to read belongs here.
+ */
+function _paintViewOf(frame, keyholeOutsideOpacity, cyberMapActive) {
+  const view = _paintView;
+  const { keyhole, cameraPosition, viewProjectionMatrix, uiRects } = frame;
+  let n = 0;
+  view[n++] = _mode;
+  view[n++] = _theme;
+  view[n++] = _charWidth;
+  view[n++] = frame.width;
+  view[n++] = frame.height;
+  view[n++] = keyholeOutsideOpacity;
+  view[n++] = cyberMapActive;
+  view[n++] = keyhole?.centerX;
+  view[n++] = keyhole?.centerY;
+  view[n++] = keyhole?.radius;
+  view[n++] = keyhole?.featherPx;
+  view[n++] = cameraPosition?.x;
+  view[n++] = cameraPosition?.y;
+  view[n++] = cameraPosition?.z;
+  for (let i = 0; i < 16; i++) view[n++] = viewProjectionMatrix[i];
+  const rectCount = Array.isArray(uiRects) ? uiRects.length : -1;
+  view[n++] = rectCount;
+  for (let i = 0; i < rectCount; i++) {
+    const rect = uiRects[i];
+    view[n++] = rect?.x;
+    view[n++] = rect?.y;
+    view[n++] = rect?.w;
+    view[n++] = rect?.h;
+  }
+  view.length = n;
+  return view;
+}
+
 /**
  * Paint detection against the shared host frame. Brackets remain broad and
  * per-frame; rich placements rebuild only for selected/fading identities and
  * for a bounded cohort on solve ticks.
  */
 function _drawOverlay(frame) {
-  _transitBracketPaths.clear();
   const { width, height } = frame;
   if (!_ctx || width <= 0 || height <= 0) {
+    _transitBracketPaths.clear();
+    _paintCache.forget();
     return {
       didSolve: false,
       solveMs: 0,
@@ -1309,6 +1361,8 @@ function _drawOverlay(frame) {
     // callout lane repaints the last solve on frames the sensor lane skips,
     // leaving rows here would strand the final callsigns on screen after the
     // last data layer is switched off.
+    _transitBracketPaths.clear();
+    _paintCache.clear();
     _calloutCount = 0;
     _ctx.font = FONT;
     _drawScanlines(width, height, now);
@@ -1361,6 +1415,9 @@ function _drawOverlay(frame) {
   // every host lane. Detection retains the manual scalar projection below.
   const occluder = frame.occluder;
   const camPos = frame.cameraPosition;
+  const camX = camPos.x;
+  const camY = camPos.y;
+  const camZ = camPos.z;
   // Answers to the occlusion tests started on earlier frames land here.
   _surfaceOcclusion.beginFrame(now);
   _occlusionProbe.beginFrame(_viewer?.scene, _surfaceOcclusion.now);
@@ -1390,7 +1447,7 @@ function _drawOverlay(frame) {
   if (!_charWidth) _charWidth = _ctx.measureText('0000000000').width / 10 || 6;
   const tiers = _theme.tiers || null;
   const colorFor = (key) => (tiers && tiers[key]) || _theme.line;
-  const bracketPaths = new Map();
+  let bracketPaths = new Map();
   // Global Context sonar echoes: a second bracket, banded by strength, around
   // each flight or vessel a pulse front has just crossed.
   // Resting pulses light nothing, so the per-contact echo work is skipped.
@@ -1415,14 +1472,41 @@ function _drawOverlay(frame) {
   };
 
   let visibleCount = 0;
-  const bracketOpacityCounts = { full: 0, partial: 0, hidden: 0 };
-  const aircraftBracketSectors = { left: 0, front: 0, right: 0 };
+  let bracketOpacityCounts = { full: 0, partial: 0, hidden: 0 };
+  let aircraftBracketSectors = { left: 0, front: 0, right: 0 };
   let protectedVisibleCount = 0;
-  const candidateMap = new Map();
+  let candidateMap = new Map();
   const cohortBuilders = shouldSolve ? new Map() : null;
   const demandByLayer = shouldSolve ? new Map() : null;
   let placementBuildCount = 0;
-  for (let i = 0; i < objects.length; i++) {
+
+  // The sweep below is the cost of this paint: every object projected, faded
+  // and given its bracket. What it builds depends only on the view and on the
+  // objects, so when both are exactly what the last sweep had, that sweep's
+  // paths, counts and callout candidates are used again and only the strokes
+  // and the callouts are repainted. Three things keep a paint from being
+  // reused or kept:
+  // - the Cyber sonar sweep and Global Context echoes change brackets with the
+  //   clock alone;
+  // - a solve needs the sweep to gather its cohort;
+  // - a depth-tested contact gets its answer from the scene, not from here.
+  // A kept solve paint holds the solve's own candidates too, so the callouts a
+  // solve has just chosen are placed from the same map either way.
+  const paintKeepable = !sonarActive && !echoField;
+  const paintView = paintKeepable
+    ? _paintViewOf(frame, keyholeOutsideOpacity, cyberMapActive)
+    : null;
+  const kept =
+    paintKeepable && !shouldSolve
+      ? _paintCache.reuse(paintView, objects)
+      : null;
+  let depthTestedSwept = false;
+  let identityLayerId;
+  let renderSources;
+  let selectedSources;
+  if (!kept) _transitBracketPaths.clear();
+  const sweepCount = kept ? 0 : objects.length;
+  for (let i = 0; i < sweepCount; i++) {
     const obj = objects[i];
     if (!obj.position) continue;
     // Skip objects occluded by the ellipsoid (behind the horizon)
@@ -1450,8 +1534,12 @@ function _drawOverlay(frame) {
     let halfW;
     let halfH;
     if (obj.type === 'AIR') {
+      // Cartesian3.distance, without its two argument checks per object.
+      const dx = camX - px;
+      const dy = camY - py;
+      const dz = camZ - pz;
       const bscale = nearFarScale(
-        Cesium.Cartesian3.distance(camPos, obj.position),
+        Math.sqrt(dx * dx + dy * dy + dz * dz),
         BILL_NEAR,
         BILL_NEAR_SCALE,
         BILL_FAR,
@@ -1477,6 +1565,7 @@ function _drawOverlay(frame) {
     // Something the scene drew in front of a depth-tested contact hides its
     // bracket and callout too, on every frame, not only on solves.
     if (obj.depthTested === true) {
+      depthTestedSwept = true;
       const testedLayerId = obj._layerId || 'unknown';
       const testedSourceId = obj.sourceId ?? obj.id ?? i;
       const hidden = _surfaceOcclusion.isHidden(
@@ -1557,14 +1646,29 @@ function _drawOverlay(frame) {
     if (!primary && !micro) continue;
     const layerId = obj._layerId || 'unknown';
     const sourceId = obj.sourceId ?? obj.id ?? i;
-    const liveForRender = renderIdentities.get(layerId)?.has(sourceId) || false;
+    // Objects arrive a layer at a time, so the layer's sets are looked up once
+    // per run of objects, not once per object.
+    if (layerId !== identityLayerId) {
+      identityLayerId = layerId;
+      renderSources = renderIdentities.get(layerId);
+      selectedSources = selectedIdentities?.get(layerId);
+    }
+    const liveForRender = renderSources?.has(sourceId) || false;
     if (!shouldSolve && !liveForRender) continue;
 
     obj._cohortSourceId = sourceId;
     obj._cohortPriority = _semanticPriority(obj);
     obj._cohortBand =
       admissionAlpha >= 0.999 ? 8 : Math.floor(admissionAlpha * 8);
-    obj._cohortHash = stableIdentityHash(layerId, sourceId);
+    // The hash is of the identity alone, so it is worked out once per identity.
+    if (
+      obj._cohortHashLayerId !== layerId ||
+      obj._cohortHashSourceId !== sourceId
+    ) {
+      obj._cohortHash = stableIdentityHash(layerId, sourceId);
+      obj._cohortHashLayerId = layerId;
+      obj._cohortHashSourceId = sourceId;
+    }
     obj._candidateScreenX = sx;
     obj._candidateScreenY = sy;
     obj._candidateHalfW = halfW;
@@ -1578,8 +1682,7 @@ function _drawOverlay(frame) {
       demandByLayer.set(layerId, (demandByLayer.get(layerId) || 0) + 1);
       if (!cohortBuilders.has(layerId))
         cohortBuilders.set(layerId, new BoundedCohort(256));
-      const incumbent =
-        selectedIdentities?.get(layerId)?.has(sourceId) || false;
+      const incumbent = selectedSources?.has(sourceId) || false;
       cohortBuilders.get(layerId).consider(obj, incumbent);
     }
 
@@ -1599,6 +1702,29 @@ function _drawOverlay(frame) {
   }
   // Start this frame's occlusion tests; nothing here waits for them.
   _occlusionProbe.endFrame();
+  if (kept) {
+    _paintReuseCount++;
+    ({
+      bracketPaths,
+      visibleCount,
+      bracketOpacityCounts,
+      aircraftBracketSectors,
+      protectedVisibleCount,
+      candidateMap,
+    } = kept);
+  } else if (paintKeepable && !depthTestedSwept) {
+    // `candidateMap` is kept by reference: a solve below adds to it.
+    _paintCache.keep(paintView, objects, {
+      bracketPaths,
+      visibleCount,
+      bracketOpacityCounts,
+      aircraftBracketSectors,
+      protectedVisibleCount,
+      candidateMap,
+    });
+  } else {
+    _paintCache.forget();
+  }
 
   const altitude = _viewer?.camera?.positionCartographic?.height ?? 1e9;
   const collectiveBudget = labelBudgetFor(altitude, _densityPct);
@@ -1782,6 +1908,7 @@ function _drawOverlay(frame) {
       // drawing them cost it.
       surfaceProbeTests: _occlusionProbe.lastFrameTests,
       surfaceProbeMs: _occlusionProbe.lastFrameMs,
+      paintReuseCount: _paintReuseCount,
     };
   } else {
     const selectedCount = _labelArbiter.selectedKeys.size;
@@ -1795,6 +1922,7 @@ function _drawOverlay(frame) {
     _lastDiagnostics.aircraftBracketSectors = aircraftBracketSectors;
     _lastDiagnostics.bracketPresentationOpacity = bracketPresentationOpacity;
     _lastDiagnostics.protectedVisibleCount = protectedVisibleCount;
+    _lastDiagnostics.paintReuseCount = _paintReuseCount;
   }
 
   _drawScanlines(width, height, now);
diff --git a/src/data/detectionHost.test.mjs b/src/data/detectionHost.test.mjs
index c60255993..1a69580ea 100644
--- a/src/data/detectionHost.test.mjs
+++ b/src/data/detectionHost.test.mjs
@@ -1661,3 +1661,234 @@ test('a WebGL 1 scene reads depth synchronously and reports it once', () => {
     env.cleanup();
   }
 });
+
+// --- Reusing a paint -------------------------------------------------------
+
+/**
+ * Contacts as the fleet layers return them: the same objects on every paint,
+ * changed in place. `fresh` returns equal copies instead, which no paint can
+ * reuse, so the two can be compared.
+ */
+function steadyContacts({ fresh = false } = {}) {
+  const objects = [
+    { position: new Cesium.Cartesian3(-0.4, 0.2, 6_356_752), sourceId: 'air-1', id: 'AIRONE', metric: 'FL350', type: 'AIR' },
+    { position: new Cesium.Cartesian3(0.4, -0.2, 6_356_752), sourceId: 'sat-1', id: 'SATONE', metric: '412KM', type: 'SAT' },
+    { position: new Cesium.Cartesian3(0.1, 0.5, 6_356_752), sourceId: 'sea-1', id: 'SEAONE', metric: '12 kn', type: 'SEA' },
+    { position: new Cesium.Cartesian3(-0.2, -0.5, 6_356_752), sourceId: 'mil-1', id: 'MILONE', metric: 'FL410', type: 'AIR', tier: 'military' },
+  ];
+  let calls = 0;
+  return {
+    objects,
+    get calls() { return calls; },
+    layer: {
+      id: 'flights',
+      getDetectableObjects() {
+        calls += 1;
+        return fresh
+          ? objects.map((object) => ({ ...object, position: Cesium.Cartesian3.clone(object.position) }))
+          : objects;
+      },
+    },
+  };
+}
+
+/** Everything painted since the trace was last emptied, with each path as its points. */
+function takePainted(env) {
+  const painted = env.paintTrace.map((call) =>
+    call.map((value) => (value instanceof MockPath2D ? value.points.map((point) => [...point]) : value)));
+  env.paintTrace.length = 0;
+  return painted;
+}
+
+const bracketPointCount = (env) => env.detectionCtx.calls
+  .filter(([name]) => name === 'stroke')
+  .reduce((sum, [, path]) => sum + (path?.points?.length ?? 0), 0);
+
+test('a paint with the same view and the same contacts reuses the last sweep, and paints exactly what a full sweep paints', () => {
+  // The same frames twice: once with contacts a paint can reuse, once with
+  // equal copies it cannot. Every canvas call must match, frame for frame.
+  const frames = (fresh) => {
+    const env = installEnvironment();
+    const contacts = steadyContacts({ fresh });
+    const painted = [];
+    try {
+      initWorldOverlay(env.viewer);
+      initDetection(env.viewer, [contacts.layer], () => {});
+      setMode('DENSE');
+      settleFrame(env);
+      painted.push(takePainted(env));
+      for (let frame = 1; frame <= 40; frame++) {
+        // Through the run: a contact moves, one is relabelled, one changes
+        // tier, and the camera turns.
+        if (frame === 9) contacts.objects[0].position.x += 0.05;
+        if (frame === 17) contacts.objects[1].metric = '398KM';
+        if (frame === 23) contacts.objects[2].tier = 'military';
+        if (frame === 31) env.viewer.camera.viewMatrix[12] = 0.03;
+        nextFrame(env);
+        painted.push(takePainted(env));
+      }
+      return { painted, reused: getDetectionDiagnostics().paintReuseCount };
+    } finally {
+      env.cleanup();
+    }
+  };
+  const reusing = frames(false);
+  const sweeping = frames(true);
+  assert.equal(sweeping.reused, 0, 'copies are swept on every paint');
+  // 40 frames 16 ms apart: a solve about every eighth, four changes, and the
+  // rest reuse the sweep before them.
+  assert.ok(reusing.reused >= 28, `${reusing.reused} of 40 paints reused a sweep`);
+  assert.equal(reusing.painted.length, sweeping.painted.length);
+  reusing.painted.forEach((frame, index) => {
+    assert.ok(frame.some(([target, name]) => target === 'detection' && name === 'stroke'), `frame ${index} strokes brackets`);
+    assert.deepEqual(frame, sweeping.painted[index], `frame ${index}`);
+  });
+  assert.ok(
+    reusing.painted.some((frame) => frame.some(([, name, text]) => name === 'fillText' && text === 'AIRONE')),
+    'callouts were painted during the run',
+  );
+});
+
+test('a reused paint builds no bracket paths and still strokes the kept ones', () => {
+  const env = installEnvironment();
+  const contacts = steadyContacts();
+  const builtPaths = [];
+  class CountingPath2D extends MockPath2D {
+    constructor() { super(); builtPaths.push(this); }
+  }
+  try {
+    globalThis.Path2D = CountingPath2D;
+    initWorldOverlay(env.viewer);
+    initDetection(env.viewer, [contacts.layer], () => {});
+    setMode('DENSE');
+    settleFrame(env);
+    assert.equal(getDetectionDiagnostics().paintReuseCount, 0);
+    const built = builtPaths.length;
+    const points = builtPaths.reduce((sum, path) => sum + path.points.length, 0);
+    assert.ok(points >= 4 * 12, 'four brackets of twelve points were built');
+
+    env.detectionCtx.calls.length = 0;
+    nextFrame(env);
+    assert.equal(getDetectionDiagnostics().paintReuseCount, 1);
+    assert.equal(builtPaths.length, built, 'no path was built');
+    assert.equal(builtPaths.reduce((sum, path) => sum + path.points.length, 0), points, 'no point was added to a kept path');
+    assert.equal(bracketPointCount(env), 4 * 12, 'the kept brackets are stroked');
+    assert.equal(getDetectionDiagnostics().visibleCount, 4);
+    assert.equal(getDetectionDiagnostics().placementBuildCount, 0, 'no placement was built on the reused paint');
+  } finally {
+    env.cleanup();
+  }
+});
+
+test('any change to a contact or to the view is swept on the very next paint', () => {
+  const changes = {
+    'a contact moves': (contacts) => { contacts.objects[0].position.y -= 0.01; },
+    'a contact gets a new position object': (contacts) => {
+      contacts.objects[0].position = new Cesium.Cartesian3(0.3, 0.3, 6_356_752);
+    },
+    'a contact is renamed': (contacts) => { contacts.objects[1].id = 'SATTWO'; },
+    'a contact reading changes': (contacts) => { contacts.objects[0].metric = 'FL360'; },
+    'a contact becomes the tracked one': (contacts) => { contacts.objects[0].skipLabel = true; },
+    'a contact changes tier': (contacts) => { contacts.objects[3].tier = undefined; },
+    'a contact changes type': (contacts) => { contacts.objects[2].type = 'VEH'; },
+    'a contact joins': (contacts) => {
+      contacts.objects.push({ position: new Cesium.Cartesian3(0.6, 0.6, 6_356_752), sourceId: 'air-2', id: 'AIRTWO', type: 'AIR' });
+    },
+    'a contact leaves': (contacts) => { contacts.objects.pop(); },
+    'a contact is replaced by an equal one': (contacts) => { contacts.objects[1] = { ...contacts.objects[1] }; },
+    'two contacts swap places in the list': (contacts) => { contacts.objects.reverse(); },
+    'the camera turns': (contacts, env) => { env.viewer.camera.viewMatrix[13] = 0.02; },
+    'the camera moves': (contacts, env) => {
+      env.viewer.camera.positionWC = new Cesium.Cartesian3(0, 100, 10_000_000);
+    },
+    'the style changes': () => { setDetectionStyle('thermal'); },
+    'the profile changes': () => { setMode('SPARSE'); },
+  };
+  for (const [name, change] of Object.entries(changes)) {
+    const env = installEnvironment();
+    const contacts = steadyContacts();
+    try {
+      initWorldOverlay(env.viewer);
+      initDetection(env.viewer, [contacts.layer], () => {});
+      setDetectionStyle('normal');
+      setMode('DENSE');
+      settleFrame(env);
+      nextFrame(env);
+      assert.equal(getDetectionDiagnostics().paintReuseCount, 1, `${name}: steady paints reuse`);
+
+      change(contacts, env);
+      nextFrame(env);
+      assert.equal(getDetectionDiagnostics().paintReuseCount, 1, `${name}: swept again`);
+      assert.equal(getDetectionDiagnostics().observationCount, contacts.objects.length, name);
+    } finally {
+      env.cleanup();
+    }
+  }
+});
+
+test('a depth-tested contact, the sonar sweep and echoes are swept on every paint', () => {
+  const run = ({ depthTested = false, sonar = false, echo = false } = {}) => {
+    const env = installEnvironment();
+    const contacts = steadyContacts();
+    if (depthTested) {
+      contacts.objects[0].type = 'VEH';
+      contacts.objects[0].depthTested = true;
+    }
+    if (sonar) {
+      env.document.documentElement = {
+        dataset: { uiTheme: 'cyber', cyberSonarOpacity: '35' },
+        style: { setProperty() {}, removeProperty() {} },
+      };
+      const hud = env.document.createElement('div');
+      hud.id = 'intel-hud';
+      hud.classList = { contains: (className) => className === 'active' };
+      env.document.body.appendChild(hud);
+    }
+    try {
+      if (echo)
+        setAwarenessEchoField({
+          center: Cesium.Cartesian3.fromDegrees(0, 89, 0),
+          radiusM: 250_000,
+          pulse: { count: 1000, periodMs: 1000, startFraction: 0 },
+          echo: { layerIds: ['flights'], fadeMs: 60_000, color: 'rgb(1, 2, 3)', spreadPx: 8, lineWidth: 3.25 },
+        });
+      initWorldOverlay(env.viewer);
+      initDetection(env.viewer, [contacts.layer], () => {});
+      setMode('DENSE');
+      settleFrame(env);
+      const sweeps = contacts.calls;
+      for (let i = 0; i < 30; i++) nextFrame(env);
+      assert.ok(contacts.calls > sweeps, 'frames painted');
+      return getDetectionDiagnostics().paintReuseCount;
+    } finally {
+      clearAwarenessEchoField();
+      env.cleanup();
+    }
+  };
+  assert.ok(run() > 15, 'the same contacts with none of these are reused');
+  assert.equal(run({ depthTested: true }), 0, 'depth-tested');
+  assert.equal(run({ sonar: true }), 0, 'sonar');
+  assert.equal(run({ echo: true }), 0, 'echo');
+});
+
+test('turning detection off lets go of the kept sweep', () => {
+  const env = installEnvironment();
+  const contacts = steadyContacts();
+  try {
+    initWorldOverlay(env.viewer);
+    initDetection(env.viewer, [contacts.layer], () => {});
+    setMode('DENSE');
+    settleFrame(env);
+    nextFrame(env);
+    assert.equal(getDetectionDiagnostics().paintReuseCount, 1);
+    setMode('OFF');
+    setMode('DENSE');
+    // The first paint back is a solve, and the one after it reuses that.
+    nextFrame(env);
+    assert.equal(getDetectionDiagnostics().paintReuseCount, 1);
+    nextFrame(env);
+    assert.equal(getDetectionDiagnostics().paintReuseCount, 2);
+  } finally {
+    env.cleanup();
+  }
+});
diff --git a/src/data/detectionPaintCache.js b/src/data/detectionPaintCache.js
new file mode 100644
index 000000000..4fac97ec5
--- /dev/null
+++ b/src/data/detectionPaintCache.js
@@ -0,0 +1,206 @@
+/**
+ * @module detectionPaintCache
+ * @description Lets the detection overlay keep what one paint built (bracket
+ * paths, counts, callout candidates) and use it again on the next paint when
+ * that paint would build exactly the same thing.
+ *
+ * Nothing announces a change to this cache. Each paint hands over the values
+ * the sweep reads from the view and the objects the layers returned, and the
+ * cache compares them, value by value, with what the kept paint was built
+ * from. A layer therefore cannot leave a stale picture behind by forgetting to
+ * say it changed: an object that moved, appeared, left or was relabelled is
+ * seen here, and the sweep runs again.
+ *
+ * Pure: no Cesium, no DOM.
+ */
+
+/**
+ * The object fields the detection sweep and its callout candidates read.
+ * Position is compared separately, by coordinate. `sameFields` and
+ * `recordFields` below name each of these outright, in this order, because a
+ * field read by a name held in a variable costs several times as much, and
+ * this runs for every object on every paint.
+ */
+export const PAINTED_OBJECT_FIELDS = Object.freeze([
+  '_layerId',
+  'type',
+  'tier',
+  'skipLabel',
+  'id',
+  'metric',
+  'sourceId',
+  'bracketHalfWidth',
+  'bracketHalfHeight',
+  'depthTested',
+  'cardSourceId',
+  'cardEntryId',
+]);
+
+/**
+ * After this many sweeps in a row with no reuse between them, the objects are
+ * only recorded on every RECORD_STRIDE-th sweep. A scene whose objects move on
+ * every paint can never reuse one, so recording each of them would be work
+ * with no return; a scene that settles is picked up again within RECORD_STRIDE
+ * paints.
+ */
+export const SWEEPS_BEFORE_BACKOFF = 8;
+export const RECORD_STRIDE = 8;
+
+const FIELD_COUNT = PAINTED_OBJECT_FIELDS.length;
+
+function sameFields(object, fields, f) {
+  return (
+    object._layerId === fields[f] &&
+    object.type === fields[f + 1] &&
+    object.tier === fields[f + 2] &&
+    object.skipLabel === fields[f + 3] &&
+    object.id === fields[f + 4] &&
+    object.metric === fields[f + 5] &&
+    object.sourceId === fields[f + 6] &&
+    object.bracketHalfWidth === fields[f + 7] &&
+    object.bracketHalfHeight === fields[f + 8] &&
+    object.depthTested === fields[f + 9] &&
+    object.cardSourceId === fields[f + 10] &&
+    object.cardEntryId === fields[f + 11]
+  );
+}
+
+/** Most fields are what they were at the last sweep; only the others are written. */
+function recordFields(object, fields, f) {
+  let value;
+  if ((value = object._layerId) !== fields[f]) fields[f] = value;
+  if ((value = object.type) !== fields[f + 1]) fields[f + 1] = value;
+  if ((value = object.tier) !== fields[f + 2]) fields[f + 2] = value;
+  if ((value = object.skipLabel) !== fields[f + 3]) fields[f + 3] = value;
+  if ((value = object.id) !== fields[f + 4]) fields[f + 4] = value;
+  if ((value = object.metric) !== fields[f + 5]) fields[f + 5] = value;
+  if ((value = object.sourceId) !== fields[f + 6]) fields[f + 6] = value;
+  if ((value = object.bracketHalfWidth) !== fields[f + 7])
+    fields[f + 7] = value;
+  if ((value = object.bracketHalfHeight) !== fields[f + 8])
+    fields[f + 8] = value;
+  if ((value = object.depthTested) !== fields[f + 9]) fields[f + 9] = value;
+  if ((value = object.cardSourceId) !== fields[f + 10]) fields[f + 10] = value;
+  if ((value = object.cardEntryId) !== fields[f + 11]) fields[f + 11] = value;
+}
+
+/**
+ * @returns {{
+ *   reuse: (view: Array, objects: Array<object>) => (object|null),
+ *   keep: (view: Array, objects: Array<object>, built: object) => boolean,
+ *   forget: () => void,
+ *   clear: () => void,
+ * }}
+ */
+export function createDetectionPaintCache() {
+  let built = null;
+  let viewKept = [];
+  let count = 0;
+  let references = [];
+  let fields = [];
+  let coordinates = new Float64Array(0);
+  let positioned = new Uint8Array(0);
+  let sweepsSinceReuse = 0;
+
+  function sameView(view) {
+    if (view.length !== viewKept.length) return false;
+    for (let i = 0; i < view.length; i++)
+      if (view[i] !== viewKept[i]) return false;
+    return true;
+  }
+
+  function sameObjects(objects) {
+    if (objects.length !== count) return false;
+    for (let i = 0, c = 0, f = 0; i < count; i++, c += 3, f += FIELD_COUNT) {
+      const object = objects[i];
+      if (object !== references[i]) return false;
+      const position = object.position;
+      if (position) {
+        // A coordinate that is not a number never equals the kept one, so such
+        // an object is swept again on every paint.
+        if (
+          positioned[i] !== 1 ||
+          position.x !== coordinates[c] ||
+          position.y !== coordinates[c + 1] ||
+          position.z !== coordinates[c + 2]
+        )
+          return false;
+      } else if (positioned[i] !== 0) {
+        return false;
+      }
+      if (!sameFields(object, fields, f)) return false;
+    }
+    return true;
+  }
+
+  return {
+    /**
+     * What the kept paint built, when `view` and `objects` are what it was
+     * built from; otherwise null.
+     * @param {Array} view - Every value of the view the sweep reads, in a fixed order.
+     * @param {Array<object>} objects - The objects the layers returned this paint.
+     */
+    reuse(view, objects) {
+      if (!built || !sameView(view) || !sameObjects(objects)) return null;
+      sweepsSinceReuse = 0;
+      return built;
+    },
+
+    /**
+     * Keep `result` as the result of sweeping `objects` under `view`.
+     * @returns {boolean} False when recording was skipped (see RECORD_STRIDE).
+     */
+    keep(view, objects, result) {
+      sweepsSinceReuse++;
+      if (
+        sweepsSinceReuse > SWEEPS_BEFORE_BACKOFF &&
+        (sweepsSinceReuse - SWEEPS_BEFORE_BACKOFF) % RECORD_STRIDE !== 0
+      ) {
+        built = null;
+        return false;
+      }
+      viewKept = view.slice();
+      count = objects.length;
+      if (positioned.length < count) {
+        // Room to spare, so a feed that grows by a few contacts a poll does
+        // not allocate on each of them.
+        const room = Math.ceil(count * 1.25);
+        coordinates = new Float64Array(room * 3);
+        positioned = new Uint8Array(room);
+        references = new Array(room).fill(undefined);
+        fields = new Array(room * FIELD_COUNT).fill(undefined);
+      }
+      for (let i = 0, c = 0, f = 0; i < count; i++, c += 3, f += FIELD_COUNT) {
+        const object = objects[i];
+        if (references[i] !== object) references[i] = object;
+        const position = object.position;
+        positioned[i] = position ? 1 : 0;
+        if (position) {
+          coordinates[c] = position.x;
+          coordinates[c + 1] = position.y;
+          coordinates[c + 2] = position.z;
+        }
+        recordFields(object, fields, f);
+      }
+      built = result;
+      return true;
+    },
+
+    /** Reuse nothing until the next `keep`. */
+    forget() {
+      built = null;
+    },
+
+    /** Forget the kept paint and let go of its objects. */
+    clear() {
+      built = null;
+      viewKept = [];
+      count = 0;
+      references = [];
+      fields = [];
+      coordinates = new Float64Array(0);
+      positioned = new Uint8Array(0);
+      sweepsSinceReuse = 0;
+    },
+  };
+}
diff --git a/src/data/detectionPaintCache.test.mjs b/src/data/detectionPaintCache.test.mjs
new file mode 100644
index 000000000..3b68dc3c1
--- /dev/null
+++ b/src/data/detectionPaintCache.test.mjs
@@ -0,0 +1,197 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  createDetectionPaintCache,
+  PAINTED_OBJECT_FIELDS,
+  RECORD_STRIDE,
+  SWEEPS_BEFORE_BACKOFF,
+} from './detectionPaintCache.js';
+
+const contacts = () => [
+  {
+    position: { x: 1, y: 2, z: 3 },
+    _layerId: 'flights',
+    sourceId: 'a',
+    id: 'A',
+    metric: 'FL300',
+    type: 'AIR',
+  },
+  {
+    position: { x: 4, y: 5, z: 6 },
+    _layerId: 'vessels',
+    sourceId: 'b',
+    id: 'B',
+    type: 'SEA',
+  },
+  { _layerId: 'cctv', sourceId: 'c', id: 'C', type: 'CAM' },
+];
+const view = () => ['DENSE', 800, 600, 0.25, 1, 0, 0, 1];
+
+function kept() {
+  const cache = createDetectionPaintCache();
+  const objects = contacts();
+  const built = { paths: 'kept' };
+  assert.equal(cache.keep(view(), objects, built), true);
+  return { cache, objects, built };
+}
+
+test('the kept sweep is handed back for the same view and the same objects', () => {
+  const { cache, objects, built } = kept();
+  assert.equal(cache.reuse(view(), objects), built);
+  assert.equal(cache.reuse(view(), objects), built, 'and again');
+  // The same objects in a new list are still the same objects.
+  assert.equal(cache.reuse(view(), [...objects]), built);
+});
+
+test('nothing is handed back before a sweep is kept, or after it is forgotten', () => {
+  const cache = createDetectionPaintCache();
+  const objects = contacts();
+  assert.equal(cache.reuse(view(), objects), null);
+  cache.keep(view(), objects, {});
+  cache.forget();
+  assert.equal(cache.reuse(view(), objects), null);
+  cache.keep(view(), objects, {});
+  cache.clear();
+  assert.equal(cache.reuse(view(), objects), null);
+});
+
+test('any value of the view that differs is a miss', () => {
+  for (let i = 0; i < view().length; i++) {
+    const { cache, objects } = kept();
+    const changed = view();
+    changed[i] = typeof changed[i] === 'number' ? changed[i] + 1e-9 : 'SPARSE';
+    assert.equal(cache.reuse(changed, objects), null, `view value ${i}`);
+  }
+  const { cache, objects } = kept();
+  assert.equal(cache.reuse([...view(), 0], objects), null, 'one value more');
+  assert.equal(
+    cache.reuse(view().slice(0, -1), objects),
+    null,
+    'one value fewer',
+  );
+});
+
+test('a view value that is not a number never matches itself', () => {
+  const cache = createDetectionPaintCache();
+  const objects = contacts();
+  cache.keep([NaN], objects, {});
+  assert.equal(cache.reuse([NaN], objects), null);
+});
+
+test('the view is copied when kept, so the caller may write the next one over it', () => {
+  const cache = createDetectionPaintCache();
+  const objects = contacts();
+  const scratch = view();
+  const built = {};
+  cache.keep(scratch, objects, built);
+  scratch[1] = 1024;
+  assert.equal(cache.reuse(scratch, objects), null);
+  scratch[1] = 800;
+  assert.equal(cache.reuse(scratch, objects), built);
+});
+
+test('a coordinate changed in place is a miss, however small', () => {
+  for (const axis of ['x', 'y', 'z']) {
+    const { cache, objects } = kept();
+    objects[1].position[axis] += 1e-9;
+    assert.equal(cache.reuse(view(), objects), null, axis);
+  }
+});
+
+test('a position object swapped for an equal one is still the same paint', () => {
+  const { cache, objects, built } = kept();
+  objects[0].position = { x: 1, y: 2, z: 3 };
+  assert.equal(cache.reuse(view(), objects), built);
+});
+
+test('a position that appears, goes away or is not a number is a miss', () => {
+  let { cache, objects } = kept();
+  objects[2].position = { x: 0, y: 0, z: 0 };
+  assert.equal(cache.reuse(view(), objects), null, 'appears');
+
+  ({ cache, objects } = kept());
+  objects[0].position = null;
+  assert.equal(cache.reuse(view(), objects), null, 'goes away');
+
+  cache = createDetectionPaintCache();
+  objects = contacts();
+  objects[0].position.x = NaN;
+  cache.keep(view(), objects, {});
+  assert.equal(cache.reuse(view(), objects), null, 'not a number');
+});
+
+test('every field the sweep reads is compared', () => {
+  for (const field of PAINTED_OBJECT_FIELDS) {
+    const { cache, objects, built } = kept();
+    const before = objects[0][field];
+    objects[0][field] = before === 'changed' ? 'again' : 'changed';
+    assert.equal(cache.reuse(view(), objects), null, field);
+    objects[0][field] = before;
+    assert.equal(cache.reuse(view(), objects), built, `${field} put back`);
+  }
+});
+
+test('a field the sweep does not read may change freely', () => {
+  const { cache, objects, built } = kept();
+  objects[0]._candidateScreenX = 412;
+  objects[0]._cohortHash = 7;
+  assert.equal(cache.reuse(view(), objects), built);
+});
+
+test('a different object, count or order is a miss', () => {
+  let { cache, objects } = kept();
+  assert.equal(cache.reuse(view(), contacts()), null, 'equal copies');
+
+  ({ cache, objects } = kept());
+  assert.equal(cache.reuse(view(), objects.slice(0, 2)), null, 'one fewer');
+  assert.equal(cache.reuse(view(), [...objects, objects[0]]), null, 'one more');
+
+  ({ cache, objects } = kept());
+  assert.equal(
+    cache.reuse(view(), [objects[1], objects[0], objects[2]]),
+    null,
+    'order',
+  );
+});
+
+test('keeping again replaces what was kept, for more or fewer objects', () => {
+  const { cache, objects, built } = kept();
+  const more = [...objects, ...contacts(), ...contacts()];
+  const builtMore = {};
+  cache.keep(view(), more, builtMore);
+  assert.equal(cache.reuse(view(), objects), null);
+  assert.equal(cache.reuse(view(), more), builtMore);
+  cache.keep(view(), objects, built);
+  assert.equal(cache.reuse(view(), more), null);
+  assert.equal(cache.reuse(view(), objects), built);
+});
+
+test('objects that change on every paint are recorded only now and then', () => {
+  const cache = createDetectionPaintCache();
+  const objects = contacts();
+  const recorded = [];
+  const sweep = () => {
+    objects[0].position.x += 1;
+    assert.equal(cache.reuse(view(), objects), null);
+    recorded.push(cache.keep(view(), objects, {}));
+  };
+  for (let i = 0; i < SWEEPS_BEFORE_BACKOFF; i++) sweep();
+  assert.ok(recorded.every(Boolean), 'the first sweeps are all recorded');
+
+  recorded.length = 0;
+  for (let i = 0; i < RECORD_STRIDE * 5; i++) sweep();
+  assert.equal(recorded.filter(Boolean).length, 5, 'then one in every stride');
+
+  // A skipped sweep leaves nothing behind to reuse by mistake.
+  const built = {};
+  let wasRecorded = cache.keep(view(), objects, built);
+  assert.equal(wasRecorded, false);
+  assert.equal(cache.reuse(view(), objects), null);
+
+  // Once the objects hold still, the next recorded sweep is reused, and
+  // every sweep after a reuse is recorded again.
+  while (!wasRecorded) wasRecorded = cache.keep(view(), objects, built);
+  assert.equal(cache.reuse(view(), objects), built);
+  for (let i = 0; i < SWEEPS_BEFORE_BACKOFF; i++)
+    assert.equal(cache.keep(view(), objects, built), true);
+});