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+/**
+ * Explorer URL state round-trip regressions (addresses #209).
+ *
+ * CI-safe spec set extracted from `url_roundtrip_investigation.js` (which
+ * remains as a manual diagnostic targeting the live deploy). These tests
+ * run against `localhost:5860` (Quarto preview) and cover the URL state
+ * contract slices fixed in PRs #203, #205, #210, #212.
+ *
+ * Coverage matrix:
+ *   - Point-mode deep-link with `mode=point` (Bug B fix from #203)
+ *   - Low-alt deep-link WITHOUT `mode=point` (#207 item 4)
+ *   - Sub-threshold pan settles via `moveEnd` (#205)
+ *   - Round-trip across contexts: copy URL, paste in fresh context,
+ *     same camera/mode (#203/#205/#207 combined)
+ *   - `h3` hashchange null-result clears `_globeState.selectedH3`
+ *     (#207 item 6)
+ *
+ * These specs use the Quarto preview against warm-cache artifacts.
+ * Cold-cache live-site behavior and the rest of issue #209's checklist
+ * (`pid`+filters, `search`/`search_scope`, facet filters, `view=table`,
+ * search-result flight) stay in `url_roundtrip_investigation.js` and a
+ * follow-up issue.
+ */
+
+const { test, expect } = require('@playwright/test');
+
+const EXPLORER_PATH = '/explorer.html';
+
+// Cyprus / Polis — confirmed dense region (~23k samples), used in #206/#210.
+const LAT = 34.9957;
+const LNG = 33.6798;
+const ALT_POINT = 8000;    // < ENTER_POINT_ALT = 120000 → point mode
+const ALT_POINT_DEEP = 62054;
+const ALT_CLUSTER = 500000;
+const ENTER_POINT_ALT = 120000;
+
+/** Wait until `viewer._globeState.mode` equals `expected`.
+ *  Cold-cache point-mode boot can take 60–90s per #190; allow 3 minutes. */
+async function waitForMode(page, expected, timeoutMs = 180000) {
+  await page.waitForFunction(
+    async (expectedMode) => {
+      try {
+        const v = await window._ojs?.ojsConnector?.mainModule?.value('viewer');
+        return v?._globeState?.mode === expectedMode;
+      } catch { return false; }
+    },
+    expected,
+    { timeout: timeoutMs }
+  );
+}
+
+/** Wait until `loadViewportSamples` has settled into the point-mode done message.
+ *  Matches on the trailing "Click one for details" phrase — it's the common
+ *  denominator across both done-state phaseMsg branches at explorer.qmd:1610-1612
+ *  (normal: `"<n> individual samples. Click one for details."` and cap-reached:
+ *  `"<n> samples in view (showing m — zoom in for more). Click one for details."`).
+ *  Codex review (PR #214 round 1) suggested switching to the count phrase
+ *  `\d[\d,]*\s+individual\s+samples`, but that pattern misses the cap-reached
+ *  branch which has no "individual" word, so we stay with the trailing phrase. */
+async function waitForPointModeSettled(page, timeoutMs = 120000) {
+  await page.waitForFunction(
+    () => {
+      const msg = document.getElementById('phaseMsg')?.textContent || '';
+      return msg.includes('Click one for details');
+    },
+    null,
+    { timeout: timeoutMs }
+  );
+}
+
+/** Wait for boot to complete enough that the `hashchange` listener (registered
+ *  late in the explorer cell at explorer.qmd:2210) is installed. `_globeState`
+ *  is initialized very early (line 871), so `waitForMode` alone is not enough
+ *  to guarantee the listener exists. `_suppressHashWrite` flips to false either
+ *  via zoomWatcher init or at the end of the boot deep-link section (line 2734);
+ *  by the time it's false, the hashchange listener is definitely registered. */
+async function waitForBootSettled(page, timeoutMs = 180000) {
+  await page.waitForFunction(
+    async () => {
+      try {
+        const v = await window._ojs?.ojsConnector?.mainModule?.value('viewer');
+        return v?._suppressHashWrite === false;
+      } catch { return false; }
+    },
+    null,
+    { timeout: timeoutMs }
+  );
+}
+
+/** Wait until the URL hash's lat/lng (and optionally alt) match the expected
+ *  values within `eps` / `altEps`. Replaces fixed sleeps with a precise settle
+ *  condition for moveEnd-driven hash writes (regression for #205). */
+async function waitForHashLatLng(page, expectedLat, expectedLng, opts = {}) {
+  const eps = opts.eps ?? 0.001;
+  const expectedAlt = opts.alt ?? null;
+  const altEps = opts.altEps ?? 100;
+  const timeoutMs = opts.timeoutMs ?? 10000;
+  await page.waitForFunction(
+    ({ lat, lng, eps, alt, altEps }) => {
+      const params = new URLSearchParams(location.hash.slice(1));
+      const ul = parseFloat(params.get('lat'));
+      const un = parseFloat(params.get('lng'));
+      if (!Number.isFinite(ul) || !Number.isFinite(un)) return false;
+      if (Math.abs(ul - lat) >= eps || Math.abs(un - lng) >= eps) return false;
+      if (alt != null) {
+        const ua = parseFloat(params.get('alt'));
+        if (!Number.isFinite(ua) || Math.abs(ua - alt) >= altEps) return false;
+      }
+      return true;
+    },
+    { lat: expectedLat, lng: expectedLng, eps, alt: expectedAlt, altEps },
+    { timeout: timeoutMs }
+  );
+}
+
+/** Snapshot camera + mode + selection + URL hash from the live page.
+ *  `selectedPid` and `selectedH3` are intentionally read from `viewer._globeState`
+ *  — that's the canonical URL-selection backing state today. If a future refactor
+ *  renames or relocates these fields, this spec is the place to update. */
+async function snapshot(page) {
+  return await page.evaluate(async () => {
+    const v = await window._ojs?.ojsConnector?.mainModule?.value('viewer');
+    const carto = v?.camera?.positionCartographic;
+    if (!carto) return null;
+    return {
+      url: location.href,
+      hash: location.hash,
+      lat: Cesium.Math.toDegrees(carto.latitude),
+      lng: Cesium.Math.toDegrees(carto.longitude),
+      alt: carto.height,
+      mode: v._globeState.mode,
+      selectedPid: v._globeState.selectedPid || null,
+      selectedH3: v._globeState.selectedH3 || null,
+    };
+  });
+}
+
+// ---- Tests ----------------------------------------------------------------
+
+test.describe('Explorer URL state round-trip (issue #209)', () => {
+  // Default per-test cap: point-mode boot in headless can take 60–90s per #190.
+  // The multi-context round-trip test overrides this to a longer cap below.
+  test.setTimeout(180000);
+
+
+  test('deep-link with mode=point enters point mode (Bug B from #203)', async ({ page }) => {
+    const url = `${EXPLORER_PATH}#v=1&lat=${LAT}&lng=${LNG}&alt=${ALT_POINT_DEEP}&mode=point`;
+    await page.goto(url);
+    await waitForMode(page, 'point');
+    await waitForPointModeSettled(page);
+    const s = await snapshot(page);
+    expect(s.mode).toBe('point');
+    expect(Math.abs(s.alt - ALT_POINT_DEEP)).toBeLessThan(100);
+  });
+
+  test('deep-link with low altitude AND no mode=point still enters point mode (#207 item 4)', async ({ page }) => {
+    // No `mode=point` in URL. Boot should enter point based on altitude alone.
+    const url = `${EXPLORER_PATH}#v=1&lat=${LAT}&lng=${LNG}&alt=${ALT_POINT}`;
+    await page.goto(url);
+    // Wait for the settled point-mode done message — more reliable than
+    // waitForMode alone, which can match a transient mode flip during boot
+    // (the dual-mode-state anomaly being fixed in #208).
+    await waitForPointModeSettled(page);
+    const s = await snapshot(page);
+    expect(s.mode).toBe('point');
+    expect(s.alt).toBeLessThan(ENTER_POINT_ALT);
+  });
+
+  test('sub-threshold pan updates URL hash via moveEnd (#205)', async ({ page }) => {
+    // Start at a settled point-mode view.
+    const url = `${EXPLORER_PATH}#v=1&lat=${LAT}&lng=${LNG}&alt=${ALT_POINT_DEEP}&mode=point`;
+    await page.goto(url);
+    await waitForMode(page, 'point');
+    await waitForPointModeSettled(page);
+
+    // Programmatically drive a SMALL pan (Δlat ≈ 0.02°). The pan is small
+    // enough that `camera.percentageChanged = 0.1` may not raise
+    // `camera.changed`, but `moveEnd` always fires once on flight complete.
+    const newLat = LAT + 0.02;
+    const newLng = LNG + 0.02;
+    await page.evaluate(async ({ lat, lng, alt }) => {
+      const v = await window._ojs.ojsConnector.mainModule.value('viewer');
+      v.scene.requestRenderMode = false;  // keep render loop alive in headless
+      v.camera.cancelFlight();
+      v.camera.flyTo({
+        destination: Cesium.Cartesian3.fromDegrees(lng, lat, alt),
+        duration: 1.0,
+      });
+    }, { lat: newLat, lng: newLng, alt: ALT_POINT_DEEP });
+
+    // Poll the URL hash until lat/lng reflect the new pan — replaces a
+    // fixed 2.5s sleep that was tight against flight-complete + moveEnd
+    // debounce timing. URL hash settling is what we actually care about.
+    await waitForHashLatLng(page, newLat, newLng);
+  });
+
+  test('URL round-trips across browser contexts (#203 + #205 combined)', async ({ browser }) => {
+    // Round-trip test opens two fresh contexts, each paying the cold-cache
+    // point-mode boot cost (60–90s per #190). Override describe-level 180s cap.
+    test.setTimeout(360000);
+
+    // Context A: navigate, settle, programmatically pan + zoom, capture URL.
+    const ctxA = await browser.newContext();
+    let ctxB;
+    try {
+      const pageA = await ctxA.newPage();
+      await pageA.goto(`${EXPLORER_PATH}#v=1&lat=${LAT}&lng=${LNG}&alt=${ALT_POINT_DEEP}&mode=point`);
+      await waitForMode(pageA, 'point');
+      await waitForPointModeSettled(pageA);
+
+      const newLat = LAT + 0.01;
+      const newLng = LNG - 0.01;
+      const newAlt = 9500;  // still below ENTER_POINT_ALT
+      await pageA.evaluate(async ({ lat, lng, alt }) => {
+        const v = await window._ojs.ojsConnector.mainModule.value('viewer');
+        v.scene.requestRenderMode = false;
+        v.camera.cancelFlight();
+        v.camera.flyTo({
+          destination: Cesium.Cartesian3.fromDegrees(lng, lat, alt),
+          duration: 1.0,
+        });
+      }, { lat: newLat, lng: newLng, alt: newAlt });
+
+      // Wait for the URL hash to reflect the new camera (lat + lng + alt) —
+      // tighter assertion than lat/lng alone, since `buildHash` writes all
+      // three together.
+      await waitForHashLatLng(pageA, newLat, newLng, { alt: newAlt });
+
+      const snapA = await snapshot(pageA);
+      expect(snapA.mode).toBe('point');
+
+      // Context B: open the captured URL, verify camera/mode round-trip.
+      ctxB = await browser.newContext();
+      const pageB = await ctxB.newPage();
+      await pageB.goto(snapA.url);
+      await waitForPointModeSettled(pageB);
+      const snapB = await snapshot(pageB);
+
+      expect(Math.abs(snapA.lat - snapB.lat)).toBeLessThan(0.001);
+      expect(Math.abs(snapA.lng - snapB.lng)).toBeLessThan(0.001);
+      expect(Math.abs(snapA.alt - snapB.alt)).toBeLessThan(50);
+      expect(snapB.mode).toBe('point');
+    } finally {
+      await ctxA.close();
+      if (ctxB) await ctxB.close();
+    }
+  });
+
+  test('h3 hashchange with unknown cell clears selectedH3 (#207 item 6)', async ({ page }) => {
+    // Regression we're testing: the hashchange handler's null-result branch
+    // at explorer.qmd:2278-2289 must clear `_globeState.selectedH3` when
+    // `fetchClusterByH3` returns null.
+    //
+    // Codex round-2 review (PR #214) caught a subtle weakness: booting with
+    // `&h3=<invalid>` runs the BOOT deep-link path's own null-result branch
+    // (explorer.qmd:2728), which sets `selectedH3 = null` BEFORE the test
+    // even drives a hashchange. So a post-hashchange `selectedH3 === null`
+    // assertion is true regardless of whether the hashchange handler's own
+    // null-clear branch (line 2285) ran.
+    //
+    // Fix: gate the assertion on a handler-only side effect. The hashchange
+    // handler's `camera.flyTo` (lines 2220-2227) rotates `camera.heading` to
+    // the URL's `heading` value — a side effect that only the hashchange
+    // handler produces. Wait for that rotation BEFORE checking selectedH3;
+    // by then the handler has executed past line 2272 (which writes a new
+    // non-null selectedH3) AND reached line 2285 (the null-clear branch).
+    const invalidH3 = '0deadbeefffffff';  // 15 chars but not a real h3 cell
+    await page.goto(`${EXPLORER_PATH}#v=1&lat=${LAT}&lng=${LNG}&alt=${ALT_CLUSTER}&h3=${invalidH3}`);
+    await waitForMode(page, 'cluster');
+    // `_globeState.mode` is initialized at explorer.qmd:871, well before the
+    // hashchange listener is registered at line 2210; wait for boot settle.
+    await waitForBootSettled(page);
+
+    // Drive a hashchange to a DIFFERENT invalid h3, plus an explicit heading
+    // change to detect that the handler actually ran (boot's heading is 0).
+    await page.evaluate((newH3) => {
+      const params = new URLSearchParams(location.hash.slice(1));
+      params.set('h3', newH3);
+      params.set('heading', '5.0');  // distinctive value — only handler's flyTo writes this
+      location.hash = '#' + params.toString();
+    }, '0baadbeeffffffff');
+
+    // Wait for the hashchange handler's `flyTo` to rotate camera heading
+    // toward 5°. 5° = ~0.0873 radians; wait until heading is within a few
+    // degrees of that target. This proves the handler executed past line
+    // 2225 (camera.flyTo with the new heading from the URL).
+    await page.waitForFunction(async () => {
+      const v = await window._ojs.ojsConnector.mainModule.value('viewer');
+      const headingDeg = Cesium.Math.toDegrees(v.camera.heading) % 360;
+      return Math.abs(headingDeg - 5.0) < 1.0;
+    }, null, { timeout: 15000 });
+
+    // Now the handler has run. Line 2272 wrote `selectedH3` to a non-null
+    // value from the URL; line 2273 awaited `fetchClusterByH3` (which returns
+    // null at line 2134 for the malformed 16-char h3); line 2285 cleared it.
+    // If line 2285 is removed, selectedH3 stays as the URL's invalid h3, NOT
+    // null, and this assertion catches it.
+    const s = await snapshot(page);
+    expect(s.selectedH3).toBeNull();
+  });
+});