diff --git a/explorer.qmd b/explorer.qmd
index 6448f77..cf13394 100644
--- a/explorer.qmd
+++ b/explorer.qmd
@@ -3484,19 +3484,48 @@ zoomWatcher = {
         }, 250);
     }
 
-    document.getElementById('heatmapToggle')?.addEventListener('change', () => {
+    document.getElementById('heatmapToggle')?.addEventListener('change', async () => {
         if (heatmapEnabled()) {
             refreshHeatmap();
+            // #233 phase 3: heatmap is mutually exclusive with the marker
+            // layers, so flip dot visibility and re-evaluate the facet apology
+            // note (which is meaningless once the heatmap shows filtered
+            // density directly). Both helpers read live state, so order vs the
+            // refresh/clear above does not matter.
+            applyLayerVisibility();
+            syncFacetNote();
         } else {
+            // #262 fix: heatmap-OFF must reload the altitude-appropriate marker
+            // layer, not just flip visibility. If the session lived entirely in
+            // heatmap mode the marker layer was never (re)loaded for the current
+            // view; toggling .show = true without data leaves an empty globe.
+            // Nulling currentRes defeats the "same-res skip" guard in the camera
+            // handler AND in the inline loadRes call below so the reload always
+            // fires even if the target resolution equals the stale stored value.
             clearHeatmap();
+            applyLayerVisibility();
+            syncFacetNote();
+            busyAcquire();
+            try {
+                currentRes = null;  // defeat same-res skip guard (#262)
+                const h = viewer.camera.positionCartographic.height;
+                if (searchIsActive() || h < ENTER_POINT_ALT) {
+                    // Search active: clusters cannot be text-filtered, so stay/
+                    // enter point mode.  Low altitude: enter point mode normally.
+                    await enterPointMode(false);
+                } else {
+                    // Cluster altitude: load the altitude-appropriate resolution.
+                    const target = h > 3000000 ? 4 : h > 300000 ? 6 : 8;
+                    const applied = await loadRes(target, { 4: h3_res4_url, 6: h3_res6_url, 8: h3_res8_url }[target]);
+                    // Chase: user may have zoomed inside ENTER_POINT_ALT while
+                    // the cluster load was in flight (mirrors the camera handler
+                    // pattern from issue #193).
+                    if (applied) await tryEnterPointModeIfNeeded();
+                }
+            } finally {
+                busyRelease();
+            }
         }
-        // #233 phase 3: heatmap is mutually exclusive with the marker
-        // layers, so flip dot visibility and re-evaluate the facet apology
-        // note (which is meaningless once the heatmap shows filtered
-        // density directly). Both helpers read live state, so order vs the
-        // refresh/clear above does not matter.
-        applyLayerVisibility();
-        syncFacetNote();
     });
     viewer._heatmapOverlay = { enabled: false, layer: null, lastRefreshAt: 0, lastPointCount: 0, lastKey: null };
     viewer._heatmapSkips = 0;  // tolerance-dedupe skip counter (test signal)
@@ -4230,6 +4259,7 @@ zoomWatcher = {
                         samplePointsLen: viewer.samplePoints.length,
                         samplePointsShown: viewer.samplePoints.show,
                         h3PointsShown: viewer.h3Points.show,
+                        h3PointsLen: viewer.h3Points.length,  // #262: needed to assert clusters loaded
                     };
                 } catch (e) { return { error: String(e) }; }
             };
diff --git a/tests/playwright/heatmap-off-restore.mjs b/tests/playwright/heatmap-off-restore.mjs
new file mode 100644
index 0000000..1bfd6bc
--- /dev/null
+++ b/tests/playwright/heatmap-off-restore.mjs
@@ -0,0 +1,332 @@
+// heatmap-off-restore.mjs — #262 mode-matrix regression harness
+//
+// Verifies that toggling heatmap OFF correctly (re)loads the altitude-
+// appropriate marker layer.  Mirrors a1-verify.mjs: plain `chromium`,
+// run with `node tests/playwright/heatmap-off-restore.mjs`.  NOT a
+// .spec.js so it stays out of the default Playwright suite.
+//
+// Pre-requisites (same as a1-verify.mjs):
+//   1. Build:  quarto render explorer.qmd
+//   2. Serve:  python3 dev_server.py --dir docs --port 8076
+//   3. Run:    node tests/playwright/heatmap-off-restore.mjs
+//
+// Set HEADLESS=1 for CI.  Set PORT=xxxx to override default 8076.
+
+import { chromium } from 'playwright';
+
+const PORT = process.env.PORT || '8076';
+const BASE = `http://localhost:${PORT}/explorer.html?data_base=/data&debug=a1`;
+const HEADLESS = process.env.HEADLESS === '1';
+
+// Cluster altitude — well above ENTER_POINT_ALT (120 km)
+const CLUSTER_ALT = 9_000_000;   // 9 000 km → res4
+// Point altitude — well below ENTER_POINT_ALT
+const POINT_ALT   = 8_000;       // 8 km
+
+const TIMEOUT_INIT   = 180_000;  // cold DuckDB-WASM boot
+const TIMEOUT_LOAD   =  90_000;  // cluster / point data load
+const TIMEOUT_TOGGLE =  60_000;  // heatmap toggle → data settle
+
+const browser = await chromium.launch({ headless: HEADLESS });
+
+// ─── helpers ─────────────────────────────────────────────────────────────────
+
+function makeUrl(lat, lng, alt, extra = '') {
+    return `${BASE}&${extra}#v=1&lat=${lat}&lng=${lng}&alt=${alt}`;
+}
+
+async function waitAppLive(page, url) {
+    await page.goto(url, { waitUntil: 'domcontentloaded' });
+    await page.waitForFunction(
+        () => typeof window.a1dbg === 'function' &&
+              typeof window.__a1globe === 'function' &&
+              !!document.querySelector('#heatmapToggle'),
+        null, { timeout: TIMEOUT_INIT });
+}
+
+async function globe(page) {
+    return page.evaluate(() => window.__a1globe?.());
+}
+
+async function waitFor(page, predFn, timeout, label) {
+    try {
+        await page.waitForFunction(predFn, null, { timeout });
+    } catch (_) {
+        const g = await globe(page);
+        console.error(`  TIMEOUT waiting for: ${label}`);
+        console.error(`  globe state: ${JSON.stringify(g)}`);
+        throw new Error(`Timeout: ${label}`);
+    }
+}
+
+async function heatmapOn(page) {
+    // Toggle heatmap on (if not already).
+    const on = await page.evaluate(() => document.getElementById('heatmapToggle')?.checked);
+    if (!on) {
+        await page.click('#heatmapToggle');
+    }
+    // Wait for the heatmap to be reflected in __a1globe (h3Points hidden).
+    await waitFor(page, () => window.__a1globe?.()?.h3PointsShown === false, TIMEOUT_TOGGLE, 'heatmap ON → h3Points hidden');
+}
+
+async function heatmapOff(page) {
+    // Toggle heatmap off (if not already).
+    const on = await page.evaluate(() => document.getElementById('heatmapToggle')?.checked);
+    if (on) {
+        await page.click('#heatmapToggle');
+    }
+}
+
+let passed = 0;
+let failed = 0;
+const failures = [];
+
+function assert(cond, msg, ctx) {
+    if (cond) {
+        console.log(`  ✓ ${msg}`);
+        passed++;
+    } else {
+        console.error(`  ✗ ${msg}  (${JSON.stringify(ctx)})`);
+        failed++;
+        failures.push(msg);
+    }
+}
+
+// ─── Test 1: boot with heatmap=1 at cluster altitude ─────────────────────────
+// Assert heatmap visible, markers hidden.
+{
+    console.log('\n[T1] Boot with heatmap=1 at cluster altitude → assert heatmap shown, markers hidden');
+    const page = await browser.newPage();
+    try {
+        const url = makeUrl(43.15, 11.40, CLUSTER_ALT, 'heatmap=1&sources=OPENCONTEXT');
+        await waitAppLive(page, url);
+
+        // Phase 1 should have loaded; give heatmap a moment to render.
+        await waitFor(page,
+            () => document.getElementById('heatmapToggle')?.checked === true,
+            TIMEOUT_TOGGLE, 'T1: heatmap toggle checked');
+
+        const g = await globe(page);
+        assert(g?.h3PointsShown === false, 'T1: h3Points hidden while heatmap on', g);
+        assert(g?.samplePointsShown === false, 'T1: samplePoints hidden while heatmap on', g);
+        assert(g?.mode === 'cluster', 'T1: mode is cluster at cluster altitude', g);
+        console.log(`  globe: ${JSON.stringify(g)}`);
+    } finally {
+        await page.close();
+    }
+}
+
+// ─── Test 2: THE BUG — boot heatmap=1 at cluster alt, toggle OFF → clusters must load ─
+{
+    console.log('\n[T2] Boot heatmap=1 at cluster altitude, toggle OFF → H3 clusters must load (#262 regression)');
+    const page = await browser.newPage();
+    try {
+        const url = makeUrl(43.15, 11.40, CLUSTER_ALT, 'heatmap=1&sources=OPENCONTEXT');
+        await waitAppLive(page, url);
+
+        // Wait for toggle to be checked (boot hydration).
+        await waitFor(page,
+            () => document.getElementById('heatmapToggle')?.checked === true,
+            TIMEOUT_TOGGLE, 'T2: heatmap toggle checked at boot');
+
+        // Turn heatmap OFF.
+        await heatmapOff(page);
+
+        // Wait for clusters to load and be visible.
+        await waitFor(page,
+            () => {
+                const g = window.__a1globe?.();
+                return g?.mode === 'cluster' &&
+                       g?.h3PointsShown === true &&
+                       (g?.h3PointsLen ?? 0) > 0;
+            },
+            TIMEOUT_LOAD, 'T2: h3Points loaded and shown after heatmap OFF');
+
+        const g = await globe(page);
+        assert(g?.mode === 'cluster', 'T2: mode remains cluster', g);
+        assert(g?.h3PointsShown === true, 'T2: h3Points shown', g);
+        assert((g?.h3PointsLen ?? 0) > 0, `T2: h3Points non-empty (got ${g?.h3PointsLen})`, g);
+        assert(g?.samplePointsShown === false, 'T2: samplePoints still hidden', g);
+        console.log(`  globe: ${JSON.stringify(g)}`);
+    } finally {
+        await page.close();
+    }
+}
+
+// ─── Test 3: heatmap OFF at point altitude → sample points load ───────────────
+{
+    console.log('\n[T3] Heatmap ON at point altitude, toggle OFF → sample points load');
+    const page = await browser.newPage();
+    try {
+        // Boot at cluster alt so we get a stable cluster-mode base; heatmap on.
+        const url = makeUrl(35.0, 33.0, POINT_ALT, 'heatmap=1&sources=OPENCONTEXT');
+        await waitAppLive(page, url);
+
+        // At low altitude the app may boot directly into point mode.
+        // Wait for heatmap to be checked (boot hydration) AND for the app to settle.
+        await waitFor(page,
+            () => document.getElementById('heatmapToggle')?.checked === true,
+            TIMEOUT_TOGGLE, 'T3: heatmap toggle checked at boot');
+
+        // Turn heatmap OFF at low altitude.
+        await heatmapOff(page);
+
+        // Should enter point mode with sample points loaded.
+        await waitFor(page,
+            () => {
+                const g = window.__a1globe?.();
+                return g?.mode === 'point' &&
+                       g?.samplePointsShown === true &&
+                       (g?.samplePointsLen ?? 0) > 0;
+            },
+            TIMEOUT_LOAD, 'T3: sample points loaded after heatmap OFF at low alt');
+
+        const g = await globe(page);
+        assert(g?.mode === 'point', 'T3: mode is point at low altitude', g);
+        assert(g?.samplePointsShown === true, 'T3: samplePoints shown', g);
+        assert((g?.samplePointsLen ?? 0) > 0, `T3: samplePoints non-empty (got ${g?.samplePointsLen})`, g);
+        assert(g?.h3PointsShown === false, 'T3: h3Points hidden in point mode', g);
+        console.log(`  globe: ${JSON.stringify(g)}`);
+    } finally {
+        await page.close();
+    }
+}
+
+// ─── Test 4: heatmap OFF with search active → filtered point markers ──────────
+{
+    console.log('\n[T4] Heatmap ON with search active, toggle OFF → filtered point markers');
+    const page = await browser.newPage();
+    try {
+        // Boot at cluster altitude, no heatmap yet.
+        const url = makeUrl(43.15, 11.40, CLUSTER_ALT, 'sources=OPENCONTEXT');
+        await waitAppLive(page, url);
+
+        // Commit a search to force point mode.
+        const TERM = 'bucchero';
+        await page.fill('#sampleSearch', TERM);
+        await page.press('#sampleSearch', 'Enter');
+        // Use page.waitForFunction directly here so we can pass TERM as the arg.
+        await page.waitForFunction(
+            (t) => window.__searchFilter?.active === true &&
+                   window.__searchFilter?.term === t &&
+                   window.__searchFilter?.total > 0,
+            TERM, { timeout: TIMEOUT_LOAD });
+
+        // Wait for point mode with filtered sample points.
+        await waitFor(page,
+            () => {
+                const g = window.__a1globe?.();
+                return g?.mode === 'point' && (g?.samplePointsLen ?? 0) > 0;
+            },
+            TIMEOUT_LOAD, 'T4: point mode with samples after search');
+
+        // Now enable heatmap.
+        await heatmapOn(page);
+
+        // With heatmap on and search active, markers should be hidden.
+        const gHeat = await globe(page);
+        assert(gHeat?.samplePointsShown === false, 'T4: samplePoints hidden while heatmap+search on', gHeat);
+
+        // Toggle heatmap off — search is still active, must stay in point mode with filtered points.
+        await heatmapOff(page);
+
+        await waitFor(page,
+            () => {
+                const g = window.__a1globe?.();
+                return g?.mode === 'point' &&
+                       g?.samplePointsShown === true &&
+                       (g?.samplePointsLen ?? 0) > 0;
+            },
+            TIMEOUT_LOAD, 'T4: filtered point markers after heatmap OFF + active search');
+
+        const g = await globe(page);
+        assert(g?.mode === 'point', 'T4: still in point mode (search forces it)', g);
+        assert(g?.samplePointsShown === true, 'T4: samplePoints shown', g);
+        assert((g?.samplePointsLen ?? 0) > 0, `T4: samplePoints non-empty (got ${g?.samplePointsLen})`, g);
+        console.log(`  globe: ${JSON.stringify(g)}`);
+    } finally {
+        await page.close();
+    }
+}
+
+// ─── Test 5: normal cluster→point transition (no heatmap) still works ─────────
+{
+    console.log('\n[T5] Normal (no-heatmap) cluster boot → heatmap ON → OFF → clusters still show');
+    const page = await browser.newPage();
+    try {
+        const url = makeUrl(43.15, 11.40, CLUSTER_ALT, 'sources=OPENCONTEXT');
+        await waitAppLive(page, url);
+
+        // Wait for phase1 cluster data to load.
+        await waitFor(page,
+            () => {
+                const g = window.__a1globe?.();
+                return g?.mode === 'cluster' &&
+                       g?.h3PointsShown === true &&
+                       (g?.h3PointsLen ?? 0) > 0;
+            },
+            TIMEOUT_LOAD, 'T5: initial cluster load');
+
+        const gBefore = await globe(page);
+        assert(gBefore?.h3PointsShown === true, 'T5: clusters shown before heatmap', gBefore);
+
+        // Heatmap on → hides clusters.
+        await heatmapOn(page);
+        const gHeat = await globe(page);
+        assert(gHeat?.h3PointsShown === false, 'T5: clusters hidden while heatmap on', gHeat);
+
+        // Heatmap off → clusters must come back.
+        await heatmapOff(page);
+        await waitFor(page,
+            () => {
+                const g = window.__a1globe?.();
+                return g?.h3PointsShown === true && (g?.h3PointsLen ?? 0) > 0;
+            },
+            TIMEOUT_LOAD, 'T5: clusters restored after heatmap OFF');
+
+        const gAfter = await globe(page);
+        assert(gAfter?.h3PointsShown === true, 'T5: clusters shown after heatmap off', gAfter);
+        assert((gAfter?.h3PointsLen ?? 0) > 0, `T5: h3PointsLen > 0 (got ${gAfter?.h3PointsLen})`, gAfter);
+        console.log(`  globe: ${JSON.stringify(gAfter)}`);
+    } finally {
+        await page.close();
+    }
+}
+
+// ─── Test 6: heatmap ON still suppresses markers ─────────────────────────────
+{
+    console.log('\n[T6] Enabling heatmap suppresses h3Points (regression: heatmap ON must still hide markers)');
+    const page = await browser.newPage();
+    try {
+        const url = makeUrl(43.15, 11.40, CLUSTER_ALT, 'sources=OPENCONTEXT');
+        await waitAppLive(page, url);
+
+        // Wait for clusters to show.
+        await waitFor(page,
+            () => (window.__a1globe?.()?.h3PointsLen ?? 0) > 0,
+            TIMEOUT_LOAD, 'T6: initial cluster load');
+
+        await heatmapOn(page);
+
+        const g = await globe(page);
+        assert(g?.h3PointsShown === false, 'T6: h3Points hidden after heatmap ON', g);
+        assert(g?.samplePointsShown === false, 'T6: samplePoints hidden after heatmap ON', g);
+        console.log(`  globe: ${JSON.stringify(g)}`);
+    } finally {
+        await page.close();
+    }
+}
+
+// ─── summary ──────────────────────────────────────────────────────────────────
+
+await browser.close();
+
+console.log(`\n${'─'.repeat(60)}`);
+console.log(`Results: ${passed} passed, ${failed} failed`);
+if (failures.length) {
+    console.error('Failures:');
+    failures.forEach(f => console.error(`  - ${f}`));
+    process.exit(1);
+} else {
+    console.log('All assertions passed.');
+}