diff --git a/docker/frontend/scripts/library-check.mjs b/docker/frontend/scripts/library-check.mjs
index 200f3f3..4e4255a 100644
--- a/docker/frontend/scripts/library-check.mjs
+++ b/docker/frontend/scripts/library-check.mjs
@@ -893,13 +893,14 @@ step('and taking it reads the folder again', (await page.$$eval(thumbs, (els) =>
 // the roll again, every folder is listed a second time, and the counter starts
 // from zero while the frames it already holds are read again to be skipped.
 //
-// The position belongs to the tab now, so the reading carries on instead of
+// The position belongs to the origin now, so the reading carries on instead of
 // starting over. `check.hold` stops the reading on the frame it is reading, which
 // is where a reload lands — and the reading it left behind is still there when the
 // key goes, so the same reading finishes the roll. Everything is taken out of the
 // catalogue first, so the reading has frames on both sides of the reload.
 await page.evaluate(async () => {
   sessionStorage.clear();
+  localStorage.removeItem('recipescam.library.walk.CheckRoll');
   localStorage.setItem('check.hold', '1');
   const db = await new Promise((resolve, reject) => {
     const req = indexedDB.open('recipescam-library');
@@ -913,10 +914,10 @@ await page.evaluate(async () => {
     tx.onerror = () => reject(tx.error);
   });
 });
-// The position the tab is holding, the line the toolbar reads out, and the wait
+// The position the origin is holding, the line the toolbar reads out, and the wait
 // for it: the line is how far the reading has come, and it is the one thing on
 // screen that says whether a reading started over.
-const position = () => page.evaluate(() => JSON.parse(sessionStorage.getItem('recipescam.library.walk.CheckRoll') ?? 'null'));
+const position = () => page.evaluate(() => JSON.parse(localStorage.getItem('recipescam.library.walk.CheckRoll') ?? 'null'));
 const readout = () => page.$eval('[data-key="lib-progress"]', (e) => e.textContent ?? '').catch(() => '');
 const counting = () =>
   page.waitForFunction(
diff --git a/docker/frontend/scripts/walk-share-check.mjs b/docker/frontend/scripts/walk-share-check.mjs
new file mode 100644
index 0000000..ddbc2bb
--- /dev/null
+++ b/docker/frontend/scripts/walk-share-check.mjs
@@ -0,0 +1,218 @@
+// Where a reading had got to is written down for the whole origin, not for the
+// tab. The tab that reads a roll for minutes is often not the one that finishes
+// it: the app is closed and the installed one opens, or the tab is thrown away
+// and the reader comes back. The position used to live in sessionStorage, which
+// is the tab's own, so the second context found nothing, walked the roll from the
+// top and listed every folder again — the frame counter starting over while the
+// frames it already held were read again only to be skipped.
+//
+// Two pages of one browser context are two contexts of one origin: the first
+// picks the stand-in roll and is closed mid-walk, the second opens and takes the
+// reading up. What it costs is what says whether it did — a reading that took the
+// position up lists nothing, since the folders behind it came back with it, where
+// one that started at the top lists the whole roll a second time.
+//
+//   npm run build && node scripts/walk-share-check.mjs
+//   PLAYWRIGHT_CORE=<path to playwright-core/index.mjs> node scripts/walk-share-check.mjs
+//
+// Playwright is not a dependency of this package, so a run without it says SKIP
+// and exits 0. APP_PORT says where the preview goes, CHROME says which browser.
+import { spawn } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
+
+const FRONTEND = fileURLToPath(new URL('..', import.meta.url));
+const PORT = Number(process.env.APP_PORT ?? 4197);
+const playwright = await import(process.env.PLAYWRIGHT_CORE ?? 'playwright-core').catch(() => null);
+if (!playwright) {
+  console.log('SKIP playwright-core is not installed — pass PLAYWRIGHT_CORE=<path to its index.mjs>');
+  process.exit(0);
+}
+
+// `--host 127.0.0.1`: without it vite's preview binds the name `localhost`, which
+// on a dual-stack box can be ::1 alone and every 127.0.0.1 fetch below fails.
+const app = spawn('npx', ['vite', 'preview', '--port', String(PORT), '--strictPort', '--host', '127.0.0.1'], {
+  cwd: FRONTEND,
+  stdio: 'ignore',
+});
+const base = `http://127.0.0.1:${PORT}`;
+let up = false;
+for (let i = 0; i < 60 && !up; i++) {
+  up = await fetch(`${base}/library`).then((r) => r.ok).catch(() => false);
+  if (!up) await new Promise((r) => setTimeout(r, 500));
+}
+if (!up) {
+  app.kill('SIGTERM');
+  console.log(`FAIL no preview on ${base} — run \`npm run build\` first (vite preview serves dist/)`);
+  process.exit(1);
+}
+
+// Dated folders of five frames each: a roll large enough that the reading is
+// still going when the first page is taken away, and small enough to finish.
+const DIRS = {};
+for (let i = 0; i < 40; i++) {
+  DIRS[`d${String(i).padStart(2, '0')}`] = { files: [1, 2, 3, 4, 5].map((n) => `F${i}_${n}.JPG`), t: 1_700_000_000_000 };
+}
+
+const browser = await playwright.chromium.launch({
+  // Playwright's own Chromium when it has one; CHROME points at any other.
+  executablePath: process.env.CHROME || undefined,
+  args: ['--no-sandbox', '--enable-unsafe-swiftshader', '--use-gl=angle', '--use-angle=swiftshader'],
+});
+const context = await browser.newContext();
+
+// The picker stands in for the person: a folder of handles over frames made in
+// the page (a beat apiece, so the reading can be caught in the middle of the
+// roll), and the same handles put back on the rows IndexedDB hands out — a cloned
+// handle is a plain object with no `getFile` on it.
+await context.addInitScript(
+  ({ dirs }) => {
+    let bytes = null;
+    const jpeg = () =>
+      (bytes ??= new Promise((resolve) => {
+        const c = document.createElement('canvas');
+        c.width = 24;
+        c.height = 16;
+        const g = c.getContext('2d');
+        g.fillStyle = '#b3412a';
+        g.fillRect(0, 0, 24, 16);
+        c.toBlob(async (b) => resolve(new Uint8Array(await b.arrayBuffer())), 'image/jpeg', 0.8);
+      }));
+    const fileHandle = (name, t) => {
+      const handle = { kind: 'file', name, __fake: true };
+      Object.defineProperty(handle, 'getFile', {
+        value: async () => {
+          await new Promise((r) => setTimeout(r, 120));
+          return new File([await jpeg()], name, { type: 'image/jpeg', lastModified: t });
+        },
+      });
+      return handle;
+    };
+    const dirHandle = (label, node) => {
+      const handle = { kind: 'directory', name: label, __fake: true };
+      Object.defineProperties(handle, {
+        values: {
+          value: () => {
+            // Every listing a reading asks for, counted for the whole origin: a
+            // reading that starts over lists the same folder a second time, and
+            // that second listing is the whole of what the position saves.
+            localStorage.setItem('walkcheck.lists', String(Number(localStorage.getItem('walkcheck.lists') ?? 0) + 1));
+            return [
+              ...(node.files ?? []).map((f) => fileHandle(f, node.t)),
+              ...Object.entries(node.dirs ?? {}).map(([name, kid]) => dirHandle(name, kid)),
+            ][Symbol.iterator]();
+          },
+        },
+        queryPermission: { value: async () => 'granted' },
+        requestPermission: { value: async () => 'granted' },
+        getDirectoryHandle: {
+          value: async (name) => {
+            const kid = node.dirs?.[name];
+            if (!kid) throw new Error(`no folder named ${name}`);
+            return dirHandle(name, kid);
+          },
+        },
+        getFileHandle: {
+          value: async (name) => {
+            if (!(node.files ?? []).includes(name)) throw new Error(`no frame named ${name}`);
+            return fileHandle(name, node.t);
+          },
+        },
+      });
+      return handle;
+    };
+    const root = () => dirHandle('BigRoll', { files: [], dirs });
+    window.showDirectoryPicker = async () => root();
+    const revive = (row) => {
+      if (row?.handle?.__fake && !Object.getOwnPropertyDescriptor(row.handle, 'getFile')) {
+        row.handle = row.handle.kind === 'directory' ? root() : fileHandle(row.handle.name, 1_700_000_000_000);
+      }
+      return row;
+    };
+    for (const method of ['get', 'getAll']) {
+      const original = IDBObjectStore.prototype[method];
+      IDBObjectStore.prototype[method] = function (...args) {
+        const request = original.apply(this, args);
+        const descriptor = Object.getOwnPropertyDescriptor(IDBRequest.prototype, 'result');
+        Object.defineProperty(request, 'result', {
+          configurable: true,
+          get() {
+            const value = descriptor.get.call(this);
+            return Array.isArray(value) ? value.map(revive) : revive(value);
+          },
+        });
+        return request;
+      };
+    }
+  },
+  { dirs: DIRS }
+);
+
+const progress = (p) => p.$eval('[data-key="lib-progress"]', (e) => e.textContent ?? '').catch(() => '');
+const at = (text) => {
+  const m = /(\d+)\/(\d+)/.exec(text);
+  return m ? { n: Number(m[1]), t: Number(m[2]) } : null;
+};
+const listed = (p) => p.evaluate(() => Number(localStorage.getItem('walkcheck.lists') ?? 0));
+const held = (p) =>
+  p.evaluate(() => {
+    const raw = localStorage.getItem('recipescam.library.walk.BigRoll');
+    if (!raw) return null;
+    const s = JSON.parse(raw);
+    return { done: s.progress?.done, walked: s.walked?.length, frames: s.frames?.length };
+  });
+
+// --- the first context: it starts the reading and is taken away -------------
+const first = await context.newPage();
+await first.goto(`${base}/library`);
+await first.evaluate(() => {
+  localStorage.removeItem('walkcheck.lists');
+  localStorage.removeItem('recipescam.library.walk.BigRoll');
+  sessionStorage.clear();
+});
+await first.click('[data-key="lib-add"]');
+let stopped = null;
+for (let i = 0; i < 600; i++) {
+  const now = at(await progress(first));
+  if (now && now.n >= 40 && now.t >= 200) {
+    stopped = now;
+    break;
+  }
+  await first.waitForTimeout(50);
+}
+const wrote = await held(first);
+const listsA = await listed(first);
+await first.close();
+if (!stopped || !wrote) {
+  await browser.close();
+  app.kill('SIGTERM');
+  console.log(`FAIL the first reading never got going — ${stopped ? `${stopped.n}/${stopped.t}` : 'no counter'}, position ${JSON.stringify(wrote)}`);
+  process.exit(1);
+}
+
+// --- the next context: it opens the app and reads where it starts -----------
+const second = await context.newPage();
+await second.goto(`${base}/library`);
+let opened = null;
+for (let i = 0; i < 1200; i++) {
+  const now = at(await progress(second));
+  if (now) {
+    opened = now;
+    break;
+  }
+  await second.waitForTimeout(25);
+}
+// Let it work for a moment, then look at what it was: a reading that took the
+// position up has nothing to walk — the queue and the folders behind it came
+// back with it — where one that started at the top lists the whole roll again.
+await second.waitForTimeout(2500);
+const listsB = await listed(second);
+const session = await second.evaluate(() => sessionStorage.getItem('recipescam.library.walk.BigRoll') !== null);
+
+await browser.close();
+app.kill('SIGTERM');
+
+const ok = opened !== null && listsB === listsA && !session;
+console.log(`${ok ? 'ok  ' : 'FAIL'} walk share — a reading another context left behind carries on`);
+console.log(`     first context stopped at ${stopped.n}/${stopped.t}, ${listsA} folders listed, wrote ${JSON.stringify(wrote)}`);
+console.log(`     next context opened at ${opened ? `${opened.n}/${opened.t}` : 'nothing — it never started'}, ${listsB} folders listed, position under the tab's own key=${session}`);
+process.exit(ok ? 0 : 1);
diff --git a/docker/frontend/src/engine/library.ts b/docker/frontend/src/engine/library.ts
index 74b3276..46c94a5 100644
--- a/docker/frontend/src/engine/library.ts
+++ b/docker/frontend/src/engine/library.ts
@@ -361,10 +361,13 @@ export interface ScanProgress {
 // the catalogue only holds the frames that were read — so the reading would
 // start again at the top of the roll and walk every folder a second time.
 //
-// Session storage holds exactly that position, and holds it because it is the
-// tab's: the tab that reloads carries on from the frame it stopped at, and a new
-// tab starts a reading of its own from the top. The key goes when the reading
-// finishes.
+// Local storage holds exactly that position, and holds it for the whole origin,
+// which is the point: the tab that reloads carries on from the frame it stopped
+// at, and so does the next context to open the app — the installed one beside the
+// browser, where session storage, being the tab's own, handed it nothing and the
+// walk started at the top of the roll a second time, every folder listed again
+// and every frame it already held read again only to be skipped. The key goes
+// when the reading finishes.
 const WALK_KEY = (folder: string) => `recipescam.library.walk.${folder}`;
 
 interface WalkSaved {
@@ -383,8 +386,14 @@ interface WalkSaved {
 }
 
 function loadWalk(folder: string): WalkSaved | null {
+  // One reading at a time: a peer that has announced this folder is walking it
+  // right now, and what it has written down is a step of a walk still in
+  // progress. Taken up here, two readings would write the same roll down at
+  // once, each undoing the other's queue. In the tab's own storage this could not
+  // come up; now that the position is the origin's, it can.
+  if (peer?.folder === folder) return null;
   try {
-    const raw = sessionStorage.getItem(WALK_KEY(folder));
+    const raw = localStorage.getItem(WALK_KEY(folder));
     if (!raw) return null;
     const saved = JSON.parse(raw) as WalkSaved;
     return Array.isArray(saved?.walked) &&
@@ -410,7 +419,7 @@ function saveWalk(
   progress: ScanProgress
 ): void {
   try {
-    sessionStorage.setItem(WALK_KEY(folder), JSON.stringify({ walked, pending, frames, restored, progress } satisfies WalkSaved));
+    localStorage.setItem(WALK_KEY(folder), JSON.stringify({ walked, pending, frames, restored, progress } satisfies WalkSaved));
   } catch {
     // Nothing to write down: the reading starts from the top, as it did before.
   }
@@ -418,7 +427,7 @@ function saveWalk(
 
 function clearWalk(folder: string): void {
   try {
-    sessionStorage.removeItem(WALK_KEY(folder));
+    localStorage.removeItem(WALK_KEY(folder));
   } catch {
     // Nothing to clear.
   }