diff --git a/src/web/public/index.html b/src/web/public/index.html
index a5a4250d..27bbd942 100644
--- a/src/web/public/index.html
+++ b/src/web/public/index.html
@@ -567,6 +567,7 @@
           <div class="file-preview-actions">
             <button class="btn-icon-sm file-preview-edit-btn" id="filePreviewEditBtn" onclick="app.enterFilePreviewEdit()" title="Edit file" aria-label="Edit file" hidden><svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M17 3a2.85 2.83 0 1 1 4 4L7.5 20.5 2 22l1.5-5.5z"/></svg></button>
             <button class="btn-icon-sm" onclick="app.copyFilePreviewContent()" title="Copy content">&#x2398;</button>
+            <button class="btn-icon-sm" id="filePreviewDetachBtn" onclick="app.detachFilePreview()" title="Open in new tab" aria-label="Open in new tab" hidden><svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M18 13v6a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h6"/><polyline points="15 3 21 3 21 9"/><line x1="10" y1="14" x2="21" y2="3"/></svg></button>
             <button class="btn-icon-sm" onclick="app.closeFilePreview()" title="Close">&times;</button>
           </div>
         </div>
diff --git a/src/web/public/panels-ui.js b/src/web/public/panels-ui.js
index 43286b4f..52a4c8ea 100644
--- a/src/web/public/panels-ui.js
+++ b/src/web/public/panels-ui.js
@@ -3313,6 +3313,11 @@ Object.assign(CodemanApp.prototype, {
     // Stop whatever the previous preview was playing. Overwriting innerHTML
     // only DETACHES a <video>/<audio>; a detached media element keeps playing.
     this._stopFilePreviewMedia();
+    // Disarm detach until this load has a URL of its own: an early error return
+    // must not leave the button opening the PREVIOUS file in a new tab.
+    this.filePreviewDetachUrl = '';
+    const detachBtn = this.$('filePreviewDetachBtn');
+    if (detachBtn) detachBtn.hidden = true;
 
     // Show overlay with loading state
     overlay.classList.add('visible');
@@ -3340,6 +3345,19 @@ Object.assign(CodemanApp.prototype, {
       return;
     }
 
+    // Every branch below renders from one of these routes, so the detach button
+    // can always offer the same bytes in a browser tab: docx/pptx through the
+    // server-converted PDF preview, everything else through the raw route.
+    // (html/htm arrive as a download there by design — file-raw serves them
+    // attachment-only so widening READ never widens RUN.)
+    const officeDoc = ext === 'docx' || ext === 'pptx';
+    this.filePreviewDetachUrl = attachmentId
+      ? `/api/sessions/${sessionId}/attachments/${encodeURIComponent(attachmentId)}/${officeDoc ? 'preview' : 'raw'}`
+      : officeDoc
+        ? `/api/sessions/${sessionId}/file-preview?path=${encodeURIComponent(filePath)}`
+        : `/api/sessions/${sessionId}/file-raw?path=${encodeURIComponent(filePath)}`;
+    if (detachBtn) detachBtn.hidden = false;
+
     // Registered attachment: render straight from its by-id routes — images and
     // PDFs inline, Office docs via the server-converted PDF preview, text fetched
     // raw. (Workspace-path previews fall through to the file-content endpoint.)
@@ -3489,6 +3507,29 @@ Object.assign(CodemanApp.prototype, {
     // audible and keeps streaming from the server. Closing has to stop it.
     this._stopFilePreviewMedia();
     this.filePreviewContent = '';
+    this.filePreviewDetachUrl = '';
+    const detachBtn = this.$('filePreviewDetachBtn');
+    if (detachBtn) detachBtn.hidden = true;
+  },
+
+  /**
+   * Open the previewed file in a browser tab and close the overlay.
+   *
+   * window.open is called WITHOUT the 'noopener' feature string: with it the
+   * call returns null even on success, which would make a blocked pop-up
+   * indistinguishable from a working one. The opener link is severed by hand
+   * instead, and a null return then reliably means the browser blocked it, in
+   * which case the overlay stays up so the user has not lost the file.
+   */
+  detachFilePreview() {
+    if (!this.filePreviewDetachUrl) return;
+    const win = window.open(this.filePreviewDetachUrl, '_blank');
+    if (!win) {
+      this.showToast('Pop-up blocked: allow pop-ups for this site to detach previews', 'error');
+      return;
+    }
+    win.opener = null;
+    this.closeFilePreview();
   },
 
   /**
@@ -4127,6 +4168,10 @@ Object.assign(CodemanApp.prototype, {
       }).catch(() => {
         this.showToast('Failed to copy', 'error');
       });
+    } else {
+      // Media/PDF/binary previews have no text buffer. Saying so beats the
+      // dead-button silence this used to be.
+      this.showToast('Nothing to copy in this preview', 'info');
     }
   },
 
diff --git a/test/file-preview-detach.test.ts b/test/file-preview-detach.test.ts
new file mode 100644
index 00000000..9e31df9c
--- /dev/null
+++ b/test/file-preview-detach.test.ts
@@ -0,0 +1,157 @@
+/**
+ * @fileoverview File viewer detach button: pop the previewed file into a browser tab.
+ *
+ * The header used to end in [copy ⎘] [close ×], and for a PDF/media preview the
+ * copy button was completely dead: `filePreviewContent` stays empty for those
+ * branches, the `if (content)` guard swallowed the click, and the ⎘ glyph reads
+ * as a pop-out icon — so the visible symptom was "the detach button next to the
+ * X does nothing". There is now a real detach button (`filePreviewDetachBtn`)
+ * that opens the preview's own raw/preview route in a new tab, and the copy
+ * button toasts instead of silently doing nothing.
+ *
+ * What is pinned here:
+ *   1. opening a workspace PDF arms the detach URL (file-raw) and reveals the button,
+ *   2. an attachment docx/pptx detaches through the converted-PDF /preview route,
+ *      other attachments through /raw,
+ *   3. detach opens the URL, severs opener, and closes the overlay,
+ *   4. a blocked pop-up (window.open → null) keeps the overlay up and toasts,
+ *   5. closing the preview disarms the button (no stale URL for the next file),
+ *   6. copy with no text buffer toasts instead of the old dead-button silence.
+ *
+ * Loaded via `vm` against a stub app, same harness style as
+ * file-preview-media.test.ts (no jsdom).
+ */
+
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+import vm from 'node:vm';
+import { describe, expect, it, vi } from 'vitest';
+
+const PUBLIC = resolve(import.meta.dirname, '../src/web/public');
+const panelsJs = readFileSync(resolve(PUBLIC, 'panels-ui.js'), 'utf8');
+
+function loadApp() {
+  const CodemanApp = function CodemanApp(this: unknown) {} as unknown as new () => Record<string, unknown>;
+  const windowStub: Record<string, unknown> = { addEventListener: vi.fn(), open: vi.fn() };
+  const context = vm.createContext({
+    CodemanApp,
+    console: { ...console, warn: vi.fn(), error: vi.fn() },
+    localStorage: { getItem: () => null, setItem: () => {}, removeItem: () => {} },
+    escapeHtml: (s: string) => String(s),
+    document: { getElementById: () => null, addEventListener: vi.fn() },
+    window: windowStub,
+    setTimeout,
+    clearTimeout,
+    confirm: () => true,
+    fetch: () => {
+      throw new Error('fetch not stubbed');
+    },
+  });
+  vm.runInContext(panelsJs, context, { filename: 'panels-ui.js' });
+
+  const body = {
+    innerHTML: '',
+    querySelectorAll: () => [] as unknown[],
+    querySelector: () => null,
+  };
+  const overlay = {
+    classes: new Set<string>(['visible']),
+    classList: {
+      add: (c: string) => overlay.classes.add(c),
+      remove: (c: string) => overlay.classes.delete(c),
+      contains: (c: string) => overlay.classes.has(c),
+    },
+  };
+  const detachBtn = { hidden: true };
+  const elements: Record<string, unknown> = {
+    filePreviewBody: body,
+    filePreviewOverlay: overlay,
+    filePreviewTitle: { textContent: '' },
+    filePreviewFooter: { textContent: '' },
+    filePreviewDetachBtn: detachBtn,
+  };
+
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const app = new CodemanApp() as Record<string, any>;
+  app.$ = (id: string) => elements[id] ?? null;
+  app._resetFilePreviewEdit = () => {};
+  app._isExternalPreviewPath = () => false;
+  app.showToast = vi.fn();
+  app.filePreviewContent = '';
+  return { app, body, overlay, detachBtn, windowStub };
+}
+
+describe('file viewer detach button', () => {
+  it('arms the detach URL and reveals the button for a workspace PDF', async () => {
+    const { app, detachBtn, body } = loadApp();
+
+    await app.openFilePreview('/ws/report.pdf', 's1');
+
+    expect(app.filePreviewDetachUrl).toBe('/api/sessions/s1/file-raw?path=%2Fws%2Freport.pdf');
+    expect(detachBtn.hidden).toBe(false);
+    expect(body.innerHTML).toContain('<iframe');
+  });
+
+  it('routes attachment office docs through /preview and other attachments through /raw', async () => {
+    const { app } = loadApp();
+
+    await app.openFilePreview('/tmp/deck.pptx', 's1', 'att-1');
+    expect(app.filePreviewDetachUrl).toBe('/api/sessions/s1/attachments/att-1/preview');
+
+    await app.openFilePreview('/tmp/scan.pdf', 's1', 'att-2');
+    expect(app.filePreviewDetachUrl).toBe('/api/sessions/s1/attachments/att-2/raw');
+  });
+
+  it('opens the URL, severs opener, and closes the overlay on detach', () => {
+    const { app, overlay, windowStub } = loadApp();
+    const win: Record<string, unknown> = { opener: {} };
+    (windowStub.open as ReturnType<typeof vi.fn>).mockReturnValue(win);
+    app.filePreviewDetachUrl = '/api/sessions/s1/file-raw?path=doc.pdf';
+
+    app.detachFilePreview();
+
+    expect(windowStub.open).toHaveBeenCalledWith('/api/sessions/s1/file-raw?path=doc.pdf', '_blank');
+    expect(win.opener).toBeNull();
+    expect(overlay.classList.contains('visible')).toBe(false);
+  });
+
+  it('keeps the overlay and toasts when the pop-up is blocked', () => {
+    const { app, overlay, windowStub } = loadApp();
+    (windowStub.open as ReturnType<typeof vi.fn>).mockReturnValue(null);
+    app.filePreviewDetachUrl = '/api/sessions/s1/file-raw?path=doc.pdf';
+
+    app.detachFilePreview();
+
+    expect(overlay.classList.contains('visible')).toBe(true);
+    expect(app.showToast).toHaveBeenCalledWith(expect.stringContaining('Pop-up blocked'), 'error');
+  });
+
+  it('does nothing when no preview is armed', () => {
+    const { app, windowStub } = loadApp();
+    app.filePreviewDetachUrl = '';
+
+    app.detachFilePreview();
+
+    expect(windowStub.open).not.toHaveBeenCalled();
+  });
+
+  it('disarms the button when the preview closes', () => {
+    const { app, detachBtn } = loadApp();
+    app.filePreviewDetachUrl = '/api/sessions/s1/file-raw?path=doc.pdf';
+    detachBtn.hidden = false;
+
+    app.closeFilePreview();
+
+    expect(app.filePreviewDetachUrl).toBe('');
+    expect(detachBtn.hidden).toBe(true);
+  });
+
+  it('copy with no text buffer toasts instead of staying silent', () => {
+    const { app } = loadApp();
+    app.filePreviewContent = '';
+
+    app.copyFilePreviewContent();
+
+    expect(app.showToast).toHaveBeenCalledWith('Nothing to copy in this preview', 'info');
+  });
+});