Add pane show/hide toggle strip with persisted visibility (M6, §01A)

A persistent left-rail toolbar (APG toolbar + roving tabindex) toggles the
library / editor / preview panes; a hidden pane frees its space and the rest
redistribute proportionally to their remembered widths. Visibility persists to
astrolabe:ux-prefs (separate key from widths, so a hidden pane keeps its width).
Adds the positional pane-* glyph sub-family to Icon and a Datasets shortcut
button in the strip.

docs/architecture/10-interaction-and-feedback.md

@@ -162,13 +162,41 @@ lives in [04 · Routing & Global Events](04-routing-and-events.md).
   is unit-tested, not trapped in the component); the live value needs the container width,
   observed via `ResizeObserver` so it tracks window resizes, not just drags.
 - `aria-controls` points at the pane it sizes (`pane-library` / `pane-preview`).
-- **Keyboard**: ←/→ nudge; **Home** → smallest pane size, **End** → largest. **Enter-to-collapse
-  is deferred to M6** with the pane-visibility / toggle strip — collapsing needs a
-  hidden-pane state that doesn't exist yet, so we don't fake it.
+- **Keyboard**: ←/→ nudge; **Home** → smallest pane size, **End** → largest. **Enter-to-collapse**
+  now has a home — the hidden-pane state landed in M6 with the toggle strip (below). The strip
+  owns show/hide; wiring the splitter's Enter to it is an optional convenience, not required.
 
 _(Consulted via `/council` → WAI-ARIA APG `windowsplitter`. This bullet is the contract;
 cite it, not the APG file.)_
 
+**Resolved — pane toggle strip.** The persistent show/hide strip (spec §01A) is a **WAI-ARIA
+APG `toolbar`** (`role="toolbar"`, `aria-orientation="vertical"`, an `aria-label` such as
+"Workspace panes") — **not** a row of independently-tabbable buttons. Grouping into a toolbar
+gives the cluster a **single tab stop** with a **roving tabindex**, which APG names as the way
+to reduce tab stops for a control group. Vertical keyboard model: **Up/Down** move among
+controls, **Home/End** jump to first/last, **Tab/Shift+Tab** move into/out and restore the
+last-focused control on re-entry.
+
+- The three pane controls are **toggle buttons** — `aria-pressed` with a **stable** accessible
+  name that does **not** change with state (`aria-pressed="true"` ⇔ pane visible; the name stays
+  "Library pane" / "Editor pane" / "Preview pane"; only the icon may swap). This matches the
+  `ThemeToggle` precedent and APG's toggle-button rule — _"it is critical the label on a toggle
+  does not change when its state changes."_ These are **independent booleans**, so toggle
+  buttons — never a radio/segmented group; reserve `role="switch"` for genuine single-setting
+  on/off.
+- The **Datasets** control is a plain **command button** (no `aria-pressed`) in the _same_
+  toolbar — APG permits mixed control types — set off from the toggles by a visual divider (and
+  optionally a nested `role="group"`), but kept in the roving sequence as its last element.
+- **Focus**: show/hide is only ever initiated **from the strip**, so the activating toggle
+  already holds focus when its pane disappears and **retains it** (the button stays, flips to
+  not-pressed) — no orphaned focus, no restoration logic. The strip is **never itself hidden**,
+  so even with **all panes hidden** it stays the always-reachable "emergency exit" (NN/g #3 user
+  control). The pane appearing/disappearing plus the `aria-pressed` flip is the status feedback
+  (NN/g #1 visibility of system status).
+
+_(Consulted via `/council` → WAI-ARIA APG `toolbar` + `button` (toggle); NN/g #1/#3. This
+bullet is the contract; cite it, not the APG files.)_
+
 **Resolved — segmented (single-select) controls.** A "pick one of N" control (fit modes,
 the Draft/Published view) is a **radio group**, never a row of `aria-pressed` toggles
 (those model N independent booleans). Use the shared `SegmentedControl`: `role="radiogroup"`

src/app/App.tsx

@@ -2,6 +2,7 @@ import { useEffect, useRef } from 'react';
 import { ConfirmDialog } from './components/ConfirmDialog';
 import { LivePreview } from './components/LivePreview';
 import { ModalShell } from './components/ModalShell';
+import { PaneToggleStrip } from './components/PaneToggleStrip';
 import { ResizeHandle } from './components/ResizeHandle';
 import { SnippetLibrary } from './components/SnippetLibrary';
 import { SpecEditor } from './components/SpecEditor';
@@ -19,13 +20,23 @@ import styles from './App.module.css';
  * (library · editor · preview) under a fixed header.
  *
  * The center editor flexes; the library and preview carry remembered widths and
- * are resized via the drag handles between them (spec §01A). Pane show/hide
- * toggling, modals, routing, and shortcuts arrive in later milestones (see
- * docs/IMPLEMENTATION-PLAN.md).
+ * are resized via the drag handles between them (spec §01A). Each pane can be
+ * shown/hidden from the always-present toggle strip (the leftmost rail); a hidden
+ * pane frees its space and the rest redistribute — when the editor is hidden the
+ * two side panes flex proportionally to their remembered widths.
  */
 export function App() {
   const libraryWidth = usePanesStore((s) => s.libraryWidth);
   const previewWidth = usePanesStore((s) => s.previewWidth);
+  const libraryVisible = usePanesStore((s) => s.libraryVisible);
+  const editorVisible = usePanesStore((s) => s.editorVisible);
+  const previewVisible = usePanesStore((s) => s.previewVisible);
+
+  // Side-pane sizing: fixed remembered width while the editor (the flex filler) is
+  // present; when it's hidden, the side panes grow proportionally to those widths
+  // so they fill the freed space (spec §01A → "redistributing proportionally").
+  const sideStyle = (width: number): React.CSSProperties =>
+    editorVisible ? { width } : { flex: `${width} 1 0` };
   // Hidden file input driving Import — the header button proxies its click so the
   // browser file picker is the only chrome (spec §08 → no intermediate dialog).
   const fileInputRef = useRef<HTMLInputElement>(null);
@@ -112,27 +123,40 @@ export function App() {
 
       {/* tabIndex -1 makes the landmark a focus target for the skip link. */}
       <main id="main" className={styles.panes} tabIndex={-1}>
+        {/* Always-present rail: shows/hides panes and shortcuts to Datasets (§01A). */}
+        <PaneToggleStrip />
+        {libraryVisible && (
           <section
             id="pane-library"
             className={styles.pane}
-          style={{ width: libraryWidth }}
+            style={sideStyle(libraryWidth)}
             aria-label="Snippet library"
           >
             <SnippetLibrary />
           </section>
+        )}
+        {/* A resize handle only sits between two visible panes that flank the editor. */}
+        {libraryVisible && editorVisible && (
           <ResizeHandle side="library" label="Resize snippet library" />
-        <section className={styles.paneEditor} aria-label="Spec editor">
+        )}
+        {editorVisible && (
+          <section id="pane-editor" className={styles.paneEditor} aria-label="Spec editor">
             <SpecEditor />
           </section>
+        )}
+        {editorVisible && previewVisible && (
           <ResizeHandle side="preview" label="Resize live preview" />
+        )}
+        {previewVisible && (
           <section
             id="pane-preview"
             className={styles.pane}
-          style={{ width: previewWidth }}
+            style={sideStyle(previewWidth)}
             aria-label="Live preview"
           >
             <LivePreview />
           </section>
+        )}
       </main>
 
       {/* The one feature modal (Datasets / Extract / …), rendered from the

src/app/components/Icon.tsx

@@ -28,6 +28,11 @@ export type IconName =
   | 'delete' // delete — Carbon TrashCan
   | 'add' // add / create-new — Carbon Add
   | 'settings' // per-pane settings disclosure (gear) — Carbon Settings
+  // Pane-toggle sub-family (spec §01A): a panel frame with one region filled, so the
+  // glyph shows *which* pane it controls by position (left / centre / right).
+  | 'pane-library' // toggle the library pane (left)
+  | 'pane-editor' // toggle the editor pane (centre)
+  | 'pane-preview' // toggle the preview pane (right)
   // Status sub-family (arch 09 §5.2) — Carbon's FILLED notification glyphs, coloured
   // by status (not text). A redundant non-colour severity channel (WCAG 1.4.1): the
   // triangle shape-codes warning apart from the round error/success/info.
@@ -70,6 +75,35 @@ const GLYPHS: Record<IconName, ReactNode> = {
       <path d="M27,3H5A2.0025,2.0025,0,0,0,3,5V27a2.0025,2.0025,0,0,0,2,2H27a2.0025,2.0025,0,0,0,2-2V5A2.0025,2.0025,0,0,0,27,3Zm0,2,0,4H5V5ZM5,27V11H27l0,16Z" />
     </>
   ),
+  // Panel frame (x4–28 / y6–26, 2px border) with one third filled. The filled bar's
+  // position maps to the pane: left = library, centre = editor, right = preview.
+  'pane-library': (
+    <>
+      <rect x="4" y="6" width="24" height="2" />
+      <rect x="4" y="24" width="24" height="2" />
+      <rect x="4" y="6" width="2" height="20" />
+      <rect x="26" y="6" width="2" height="20" />
+      <rect x="7" y="10" width="4" height="12" />
+    </>
+  ),
+  'pane-editor': (
+    <>
+      <rect x="4" y="6" width="24" height="2" />
+      <rect x="4" y="24" width="24" height="2" />
+      <rect x="4" y="6" width="2" height="20" />
+      <rect x="26" y="6" width="2" height="20" />
+      <rect x="14" y="10" width="4" height="12" />
+    </>
+  ),
+  'pane-preview': (
+    <>
+      <rect x="4" y="6" width="24" height="2" />
+      <rect x="4" y="24" width="24" height="2" />
+      <rect x="4" y="6" width="2" height="20" />
+      <rect x="26" y="6" width="2" height="20" />
+      <rect x="21" y="10" width="4" height="12" />
+    </>
+  ),
   moon: (
     <path d="M13.5025,5.4136A15.0755,15.0755,0,0,0,25.096,23.6082a11.1134,11.1134,0,0,1-7.9749,3.3893c-.1385,0-.2782.0051-.4178,0A11.0944,11.0944,0,0,1,13.5025,5.4136M14.98,3a1.0024,1.0024,0,0,0-.1746.0156A13.0959,13.0959,0,0,0,16.63,28.9973c.1641.006.3282,0,.4909,0a13.0724,13.0724,0,0,0,10.702-5.5556,1.0094,1.0094,0,0,0-.7833-1.5644A13.08,13.08,0,0,1,15.8892,4.38,1.0149,1.0149,0,0,0,14.98,3Z" />
   ),

src/app/components/PaneToggleStrip.module.css

@@ -0,0 +1,57 @@
+/*
+ * The pane toggle strip — a slim vertical rail at the left edge of the work area
+ * (spec §01A). Always present (never hidden), so it's the way back when panes are
+ * hidden. Visual look only; semantics live in PaneToggleStrip.tsx.
+ */
+.strip {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: var(--space-2);
+  flex: 0 0 auto;
+  padding: var(--space-3) var(--space-2);
+  border-right: var(--border-width) solid var(--border);
+  background: var(--layer-01);
+}
+
+.button {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 32px;
+  height: 32px;
+  padding: 0;
+  border: var(--border-width) solid transparent;
+  border-radius: var(--radius);
+  background: transparent;
+  /* Not-pressed (hidden pane) reads as muted; pressed (shown) lifts to full text. */
+  color: var(--text-secondary);
+  cursor: pointer;
+  transition:
+    background var(--dur-fast) var(--ease),
+    color var(--dur-fast) var(--ease);
+}
+
+.button:hover {
+  background: var(--layer-02);
+  color: var(--text);
+}
+
+.button:focus-visible {
+  outline: 2px solid var(--focus);
+  outline-offset: 2px;
+}
+
+/* Shown pane: the toggle is "on" — full-strength glyph on a filled chip. */
+.pressed {
+  color: var(--text);
+  background: var(--layer-02);
+  border-color: var(--border-strong);
+}
+
+.divider {
+  width: 20px;
+  height: var(--border-width);
+  margin: var(--space-1) 0;
+  background: var(--border);
+}

src/app/components/PaneToggleStrip.test.tsx

@@ -0,0 +1,99 @@
+import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
+import { act } from 'react';
+import { createRoot, type Root } from 'react-dom/client';
+import { usePanesStore } from '../stores/PanesStore';
+import { PaneToggleStrip } from './PaneToggleStrip';
+
+vi.mock('../modals/ModalCoordinator', () => ({ openModal: vi.fn() }));
+import { openModal } from '../modals/ModalCoordinator';
+
+(globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
+
+let container: HTMLDivElement;
+let root: Root;
+
+const toggleButtons = () =>
+  Array.from(container.querySelectorAll<HTMLButtonElement>('button[aria-pressed]'));
+const datasetsButton = () =>
+  container.querySelector<HTMLButtonElement>('button[aria-label="Datasets"]')!;
+
+beforeEach(() => {
+  usePanesStore.getState().hydrate({}, {});
+  container = document.createElement('div');
+  document.body.appendChild(container);
+  root = createRoot(container);
+  act(() => root.render(<PaneToggleStrip />));
+});
+
+afterEach(() => {
+  act(() => root.unmount());
+  container.remove();
+  vi.clearAllMocks();
+});
+
+describe('PaneToggleStrip', () => {
+  test('renders an APG toolbar: three pane toggles + a Datasets command button', () => {
+    expect(container.querySelector('[role="toolbar"][aria-orientation="vertical"]')).not.toBeNull();
+    expect(toggleButtons()).toHaveLength(3);
+    // The Datasets control is a command button, not a toggle (no aria-pressed).
+    expect(datasetsButton().hasAttribute('aria-pressed')).toBe(false);
+  });
+
+  test('aria-pressed mirrors pane visibility and flips on click', () => {
+    const [library] = toggleButtons();
+    expect(library.getAttribute('aria-pressed')).toBe('true');
+
+    act(() => library.click());
+
+    expect(usePanesStore.getState().libraryVisible).toBe(false);
+    expect(library.getAttribute('aria-pressed')).toBe('false');
+  });
+
+  test('the accessible name is stable across the pressed/not-pressed flip (APG)', () => {
+    const [library] = toggleButtons();
+    expect(library.getAttribute('aria-label')).toBe('Library pane');
+    act(() => library.click());
+    expect(library.getAttribute('aria-label')).toBe('Library pane');
+  });
+
+  test('roving tabindex: exactly one control is in the tab order at a time', () => {
+    const all = [...toggleButtons(), datasetsButton()];
+    expect(all.filter((b) => b.tabIndex === 0)).toHaveLength(1);
+    expect(all[0].tabIndex).toBe(0); // first control by default
+  });
+
+  test('ArrowDown moves the roving tab stop to the next control', () => {
+    const all = [...toggleButtons(), datasetsButton()];
+    act(() => {
+      all[0].dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowDown', bubbles: true }));
+    });
+    expect(all[0].tabIndex).toBe(-1);
+    expect(all[1].tabIndex).toBe(0);
+  });
+
+  test('End jumps to the last control (Datasets); Home returns to the first', () => {
+    const all = [...toggleButtons(), datasetsButton()];
+    act(() => {
+      all[0].dispatchEvent(new KeyboardEvent('keydown', { key: 'End', bubbles: true }));
+    });
+    expect(datasetsButton().tabIndex).toBe(0);
+
+    act(() => {
+      datasetsButton().dispatchEvent(new KeyboardEvent('keydown', { key: 'Home', bubbles: true }));
+    });
+    expect(all[0].tabIndex).toBe(0);
+  });
+
+  test('ArrowUp from the first control wraps to the last', () => {
+    const all = [...toggleButtons(), datasetsButton()];
+    act(() => {
+      all[0].dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowUp', bubbles: true }));
+    });
+    expect(datasetsButton().tabIndex).toBe(0);
+  });
+
+  test('the Datasets button opens the Datasets manager', () => {
+    act(() => datasetsButton().click());
+    expect(openModal).toHaveBeenCalledWith('datasets');
+  });
+});

src/app/components/PaneToggleStrip.tsx

@@ -0,0 +1,149 @@
+/**
+ * PaneToggleStrip — the persistent show/hide strip (spec §01A).
+ *
+ * Per the interaction contract (docs/architecture/10 → "Resolved — pane toggle
+ * strip", consulted via /council → WAI-ARIA APG `toolbar` + `button`):
+ *
+ * - It is an APG **toolbar** (`role="toolbar"`, vertical) — a **single tab stop**
+ *   with a **roving tabindex**, so the whole strip is one stop in the tab order
+ *   and Up/Down (Left/Right duplicate) move among its controls; Home/End jump to
+ *   the ends. Re-entry restores the last-focused control (the `focusIndex` state).
+ * - The three pane controls are **toggle buttons** (`aria-pressed`) with a
+ *   **stable** accessible name ("Library pane" …) that never changes with state;
+ *   `aria-pressed=true` ⇔ pane visible. The positional glyph shows which pane.
+ * - The **Datasets** control is a plain command button (no `aria-pressed`) — APG
+ *   permits mixed control types in one toolbar — set off by a divider, kept last
+ *   in the roving order.
+ * - Focus is never orphaned: hiding is only ever initiated *from here*, so the
+ *   activating toggle keeps focus (it stays mounted, flips to not-pressed). The
+ *   strip is never itself hidden, so it stays reachable even with all panes hidden.
+ */
+
+import { useRef, useState } from 'react';
+import { openModal } from '../modals/ModalCoordinator';
+import { usePanesStore, type PaneName } from '../stores/PanesStore';
+import { Icon, type IconName } from './Icon';
+import styles from './PaneToggleStrip.module.css';
+
+interface PaneItem {
+  pane: PaneName;
+  icon: IconName;
+  /** Stable accessible name (APG: never changes with the pressed state). */
+  label: string;
+  /** The id of the pane section this toggle controls. */
+  controls: string;
+}
+
+const PANES: readonly PaneItem[] = [
+  { pane: 'library', icon: 'pane-library', label: 'Library pane', controls: 'pane-library' },
+  { pane: 'editor', icon: 'pane-editor', label: 'Editor pane', controls: 'pane-editor' },
+  { pane: 'preview', icon: 'pane-preview', label: 'Preview pane', controls: 'pane-preview' },
+];
+
+export function PaneToggleStrip() {
+  const libraryVisible = usePanesStore((s) => s.libraryVisible);
+  const editorVisible = usePanesStore((s) => s.editorVisible);
+  const previewVisible = usePanesStore((s) => s.previewVisible);
+  const togglePane = usePanesStore((s) => s.togglePane);
+  const visible: Record<PaneName, boolean> = {
+    library: libraryVisible,
+    editor: editorVisible,
+    preview: previewVisible,
+  };
+
+  // Roving tabindex over the toggles + the trailing Datasets action (the divider
+  // is not a control). One tab stop; the focused index is the only `tabIndex=0`.
+  const count = PANES.length + 1;
+  const datasetsIndex = PANES.length;
+  const refs = useRef<Array<HTMLButtonElement | null>>([]);
+  const [focusIndex, setFocusIndex] = useState(0);
+
+  const focusAt = (i: number) => {
+    const idx = ((i % count) + count) % count;
+    setFocusIndex(idx);
+    refs.current[idx]?.focus();
+  };
+
+  const onKeyDown = (e: React.KeyboardEvent, index: number) => {
+    switch (e.key) {
+      case 'ArrowDown':
+      case 'ArrowRight':
+        focusAt(index + 1);
+        break;
+      case 'ArrowUp':
+      case 'ArrowLeft':
+        focusAt(index - 1);
+        break;
+      case 'Home':
+        focusAt(0);
+        break;
+      case 'End':
+        focusAt(count - 1);
+        break;
+      default:
+        return; // not ours — let it bubble
+    }
+    e.preventDefault();
+  };
+
+  return (
+    <div
+      role="toolbar"
+      aria-orientation="vertical"
+      aria-label="Workspace panes"
+      className={styles.strip}
+    >
+      {PANES.map((item, i) => (
+        <button
+          key={item.pane}
+          ref={(el) => {
+            refs.current[i] = el;
+          }}
+          type="button"
+          aria-pressed={visible[item.pane]}
+          aria-label={item.label}
+          // TODO: when a pane is hidden, App unmounts its <section>, so this
+          // aria-controls IDREF dangles until the pane is shown again. Harmless
+          // (AT ignores unresolved IDREFs) but technically invalid — consider
+          // keeping the section mounted-but-hidden, or dropping aria-controls.
+          aria-controls={item.controls}
+          // Description (not the name): hints what activating does. The name stays
+          // stable (aria-label) per APG's toggle-button rule.
+          title={`${visible[item.pane] ? 'Hide' : 'Show'} ${item.label.toLowerCase()}`}
+          tabIndex={focusIndex === i ? 0 : -1}
+          className={[styles.button, visible[item.pane] && styles.pressed]
+            .filter(Boolean)
+            .join(' ')}
+          onClick={() => {
+            togglePane(item.pane);
+            setFocusIndex(i);
+          }}
+          onKeyDown={(e) => onKeyDown(e, i)}
+        >
+          <Icon name={item.icon} />
+        </button>
+      ))}
+
+      <div className={styles.divider} aria-hidden="true" />
+
+      <button
+        ref={(el) => {
+          refs.current[datasetsIndex] = el;
+        }}
+        type="button"
+        aria-label="Datasets"
+        aria-keyshortcuts="Meta+K Control+K"
+        title="Datasets (⌘/Ctrl+K)"
+        tabIndex={focusIndex === datasetsIndex ? 0 : -1}
+        className={styles.button}
+        onClick={() => {
+          openModal('datasets');
+          setFocusIndex(datasetsIndex);
+        }}
+        onKeyDown={(e) => onKeyDown(e, datasetsIndex)}
+      >
+        <Icon name="dataset" />
+      </button>
+    </div>
+  );
+}

src/app/infrastructure/ux-prefs.test.ts

@@ -1,5 +1,10 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
-import { loadPanelLayout, savePanelLayout } from './ux-prefs';
+import {
+  loadPanelLayout,
+  loadPaneVisibility,
+  savePanelLayout,
+  savePaneVisibility,
+} from './ux-prefs';
 
 const KEY = 'astrolabe:ux-prefs';
 
@@ -61,3 +66,35 @@ describe('ux-prefs · panelLayout', () => {
     expect(stored.panelLayout.libraryWidth).toBe(260);
   });
 });
+
+describe('ux-prefs · paneVisibility', () => {
+  beforeEach(() => vi.stubGlobal('localStorage', makeStorageStub()));
+
+  it('round-trips persisted visibility; absent flags stay undefined', () => {
+    savePaneVisibility({ library: false, editor: true });
+    expect(loadPaneVisibility()).toEqual({ library: false, editor: true, preview: undefined });
+  });
+
+  it('surfaces only booleans — junk in storage falls back to undefined', () => {
+    localStorage.setItem(
+      KEY,
+      JSON.stringify({ paneVisibility: { library: 'nope', preview: false } }),
+    );
+    expect(loadPaneVisibility()).toEqual({
+      library: undefined,
+      editor: undefined,
+      preview: false,
+    });
+  });
+
+  it('preserves panelLayout when writing visibility (separate keys)', () => {
+    savePanelLayout({ libraryWidth: 300 });
+    savePaneVisibility({ editor: false });
+    const stored = JSON.parse(localStorage.getItem(KEY)!) as {
+      panelLayout: { libraryWidth: number };
+      paneVisibility: { editor: boolean };
+    };
+    expect(stored.panelLayout.libraryWidth).toBe(300);
+    expect(stored.paneVisibility.editor).toBe(false);
+  });
+});

src/app/infrastructure/ux-prefs.ts

@@ -6,9 +6,10 @@
  * record. Its own key, `astrolabe:ux-prefs`, holds the snippet sort preference
  * (lands with M5/M6) and the panel layout (per-pane widths + visibility).
  *
- * This slice persists the panel **widths** only; visibility joins it when the
- * toggle strip lands. Read-with-fallback + write-through merge, the same
- * contract as the settings adapter, so adding fields later upgrades cleanly.
+ * This slice persists the panel **widths** and per-pane **visibility** (spec §01A).
+ * Read-with-fallback + write-through merge, the same contract as the settings
+ * adapter, so adding fields later upgrades cleanly. Widths and visibility are kept
+ * as separate keys: a hidden pane retains its remembered width.
  *
  * Per the architecture rule this is one of the only modules that may touch
  * `localStorage`; everything else goes through these typed functions.
@@ -22,8 +23,16 @@ export interface PanelLayout {
   previewWidth?: number;
 }
 
+/** Per-pane visibility. Optional — a missing field falls back to "shown". */
+export interface PaneVisibilityPref {
+  library?: boolean;
+  editor?: boolean;
+  preview?: boolean;
+}
+
 interface StoredPrefs {
   panelLayout?: PanelLayout;
+  paneVisibility?: PaneVisibilityPref;
   [k: string]: unknown;
 }
 
@@ -76,3 +85,24 @@ export function savePanelLayout(layout: PanelLayout): void {
   const current = readRaw();
   writeRaw({ ...current, panelLayout: { ...current.panelLayout, ...layout } });
 }
+
+/** A boolean, or undefined — guards against junk in storage. */
+function bool(v: unknown): boolean | undefined {
+  return typeof v === 'boolean' ? v : undefined;
+}
+
+/** The persisted per-pane visibility, with only valid booleans surfaced. */
+export function loadPaneVisibility(): PaneVisibilityPref {
+  const stored = readRaw().paneVisibility ?? {};
+  return {
+    library: bool(stored.library),
+    editor: bool(stored.editor),
+    preview: bool(stored.preview),
+  };
+}
+
+/** Persist per-pane visibility, preserving every other key already in the record. */
+export function savePaneVisibility(visibility: PaneVisibilityPref): void {
+  const current = readRaw();
+  writeRaw({ ...current, paneVisibility: { ...current.paneVisibility, ...visibility } });
+}

src/app/orchestration/panes.ts

@@ -8,28 +8,47 @@
  * the write to the final resting widths (the same approach as snippet auto-save).
  */
 
-import { loadPanelLayout, savePanelLayout } from '../infrastructure/ux-prefs';
+import {
+  loadPanelLayout,
+  loadPaneVisibility,
+  savePanelLayout,
+  savePaneVisibility,
+} from '../infrastructure/ux-prefs';
 import { usePanesStore } from '../stores/PanesStore';
 
 /** Delay before a settled resize is persisted. */
 export const PANES_PERSIST_DEBOUNCE_MS = 300;
 
-/** Hydrate persisted pane widths into the store. Call before render. */
+/** Hydrate persisted pane widths + visibility into the store. Call before render. */
 export function initPanes(): void {
-  usePanesStore.getState().hydrate(loadPanelLayout());
+  usePanesStore.getState().hydrate(loadPanelLayout(), loadPaneVisibility());
 }
 
-/** Persist width changes (debounced). Returns a teardown that detaches the subscriber. */
+/**
+ * Persist layout changes. Returns a teardown that detaches the subscriber.
+ * Widths are **debounced** (a drag emits an update per pointer move); visibility
+ * is a discrete click, so it's written **immediately**.
+ */
 export function wirePanes(): () => void {
   let timer: ReturnType<typeof setTimeout> | undefined;
   return usePanesStore.subscribe((state, prev) => {
-    if (state.libraryWidth === prev.libraryWidth && state.previewWidth === prev.previewWidth) {
-      return;
-    }
+    if (state.libraryWidth !== prev.libraryWidth || state.previewWidth !== prev.previewWidth) {
       clearTimeout(timer);
       timer = setTimeout(() => {
         const { libraryWidth, previewWidth } = usePanesStore.getState();
         savePanelLayout({ libraryWidth, previewWidth });
       }, PANES_PERSIST_DEBOUNCE_MS);
+    }
+    if (
+      state.libraryVisible !== prev.libraryVisible ||
+      state.editorVisible !== prev.editorVisible ||
+      state.previewVisible !== prev.previewVisible
+    ) {
+      savePaneVisibility({
+        library: state.libraryVisible,
+        editor: state.editorVisible,
+        preview: state.previewVisible,
+      });
+    }
   });
 }

src/app/stores/PanesStore.test.ts

@@ -93,3 +93,48 @@ describe('usePanesStore', () => {
     expect(store().previewWidth).toBe(PANE_DEFAULT.preview);
   });
 });
+
+describe('pane visibility (§01A)', () => {
+  test('all panes are visible by default', () => {
+    store().hydrate({}, undefined);
+    expect(store().libraryVisible).toBe(true);
+    expect(store().editorVisible).toBe(true);
+    expect(store().previewVisible).toBe(true);
+  });
+
+  test('togglePane flips only the targeted pane', () => {
+    store().hydrate({}, {});
+    store().togglePane('editor');
+    expect(store().editorVisible).toBe(false);
+    expect(store().libraryVisible).toBe(true);
+    expect(store().previewVisible).toBe(true);
+
+    store().togglePane('editor');
+    expect(store().editorVisible).toBe(true);
+  });
+
+  test('hiding all panes is permitted', () => {
+    store().hydrate({}, {});
+    store().togglePane('library');
+    store().togglePane('editor');
+    store().togglePane('preview');
+    expect(store().libraryVisible).toBe(false);
+    expect(store().editorVisible).toBe(false);
+    expect(store().previewVisible).toBe(false);
+  });
+
+  test('hydrate restores explicit visibility; missing flags default to shown', () => {
+    store().hydrate({}, { library: false });
+    expect(store().libraryVisible).toBe(false);
+    expect(store().editorVisible).toBe(true);
+    expect(store().previewVisible).toBe(true);
+  });
+
+  test('a hidden pane keeps its remembered width (re-shows at it)', () => {
+    store().hydrate({ libraryWidth: 250 }, {});
+    store().togglePane('library'); // hide
+    expect(store().libraryWidth).toBe(250);
+    store().togglePane('library'); // re-show
+    expect(store().libraryWidth).toBe(250);
+  });
+});

src/app/stores/PanesStore.ts

@@ -22,6 +22,9 @@ import { create } from 'zustand';
 /** Which side pane a drag handle controls. */
 export type PaneSide = 'library' | 'preview';
 
+/** Every pane that can be shown/hidden via the toggle strip (spec §01A). */
+export type PaneName = 'library' | 'editor' | 'preview';
+
 /** Minimum widths (px) enforced while resizing, so no pane collapses (§01A). */
 export const PANE_MIN = { library: 180, preview: 240, editor: 320 } as const;
 
@@ -79,25 +82,56 @@ export function sideWidthValue(
   return Math.round(Math.min(100, Math.max(0, pct)));
 }
 
+/** Per-pane visibility (spec §01A). Widths are kept *independently* of visibility,
+ * so a hidden pane keeps its remembered width and re-shows at it (not a default). */
+export interface PaneVisibility {
+  library?: boolean;
+  editor?: boolean;
+  preview?: boolean;
+}
+
 export interface PanesState {
   libraryWidth: number;
   previewWidth: number;
+  /** Whether each pane is currently shown. All visible by default (§01A). */
+  libraryVisible: boolean;
+  editorVisible: boolean;
+  previewVisible: boolean;
   /** Set a side pane's width (already clamped by the caller). */
   setWidth: (side: PaneSide, width: number) => void;
-  /** Restore persisted widths on startup; missing values keep their defaults. */
-  hydrate: (layout: { libraryWidth?: number; previewWidth?: number }) => void;
+  /** Show/hide one pane. Hiding all panes is permitted (§01A) — the strip stays. */
+  togglePane: (pane: PaneName) => void;
+  /** Restore persisted widths + visibility on startup; missing values keep defaults. */
+  hydrate: (
+    layout: { libraryWidth?: number; previewWidth?: number },
+    visibility?: PaneVisibility,
+  ) => void;
 }
 
+const VISIBLE_KEY = {
+  library: 'libraryVisible',
+  editor: 'editorVisible',
+  preview: 'previewVisible',
+} as const;
+
 export const usePanesStore = create<PanesState>((set) => ({
   libraryWidth: PANE_DEFAULT.library,
   previewWidth: PANE_DEFAULT.preview,
+  libraryVisible: true,
+  editorVisible: true,
+  previewVisible: true,
 
   setWidth: (side, width) =>
     set(side === 'library' ? { libraryWidth: width } : { previewWidth: width }),
 
-  hydrate: (layout) =>
+  togglePane: (pane) => set((s) => ({ [VISIBLE_KEY[pane]]: !s[VISIBLE_KEY[pane]] })),
+
+  hydrate: (layout, visibility) =>
     set({
       libraryWidth: layout.libraryWidth ?? PANE_DEFAULT.library,
       previewWidth: layout.previewWidth ?? PANE_DEFAULT.preview,
+      libraryVisible: visibility?.library ?? true,
+      editorVisible: visibility?.editor ?? true,
+      previewVisible: visibility?.preview ?? true,
     }),
 }));