Add resizable panes with drag handles, min widths, and persistence

src/app/App.module.css

@@ -40,28 +40,24 @@
   min-height: 0;
 }
 
+/*
+ * Side panes (library, preview) carry an explicit width (set inline from the
+ * PanesStore) and don't grow or shrink — the drag handles change that width.
+ * The editor between them flexes to fill the remainder, so a drag leaves the
+ * opposite side pane untouched (spec §01A). Panes are separated by the
+ * ResizeHandle, so no inter-pane borders here.
+ */
 .pane {
-  flex: 1 1 0;
+  flex: 0 0 auto;
   min-width: 0;
   overflow: auto;
-  border-right: var(--border-width) solid var(--border);
   background: var(--bg);
 }
 
-/* Library is a fixed-ish sidebar; editor + preview share the rest. */
-.panes > .pane:first-child {
-  flex: 0 0 280px;
-}
-
 /* Editor pane: Monaco manages its own scroll/layout, so no padding. */
 .paneEditor {
   flex: 1 1 0;
   min-width: 0;
   overflow: hidden;
-  border-right: var(--border-width) solid var(--border);
   background: var(--bg);
 }
-
-.pane:last-child {
-  border-right: none;
-}

src/app/App.tsx

@@ -1,19 +1,25 @@
 import { ConfirmDialog } from './components/ConfirmDialog';
 import { LivePreview } from './components/LivePreview';
+import { ResizeHandle } from './components/ResizeHandle';
 import { SnippetLibrary } from './components/SnippetLibrary';
 import { SpecEditor } from './components/SpecEditor';
 import { ThemeToggle } from './components/ThemeToggle';
+import { usePanesStore } from './stores/PanesStore';
 import styles from './App.module.css';
 
 /**
  * Application shell — the three-pane workspace from spec §01A
  * (library · editor · preview) under a fixed header.
  *
- * M1 fills the panes with the MVP authoring loop. Pane resizing/toggling,
- * modals, routing, and shortcuts arrive in later milestones (see
+ * 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).
  */
 export function App() {
+  const libraryWidth = usePanesStore((s) => s.libraryWidth);
+  const previewWidth = usePanesStore((s) => s.previewWidth);
+
   return (
     <div className={styles.app}>
       <header className={styles.header}>
@@ -24,13 +30,19 @@ export function App() {
       </header>
 
       <main className={styles.panes}>
-        <section className={styles.pane} aria-label="Snippet library">
+        <section
+          className={styles.pane}
+          style={{ width: libraryWidth }}
+          aria-label="Snippet library"
+        >
           <SnippetLibrary />
         </section>
+        <ResizeHandle side="library" label="Resize snippet library" />
         <section className={styles.paneEditor} aria-label="Spec editor">
           <SpecEditor />
         </section>
-        <section className={styles.pane} aria-label="Live preview">
+        <ResizeHandle side="preview" label="Resize live preview" />
+        <section className={styles.pane} style={{ width: previewWidth }} aria-label="Live preview">
           <LivePreview />
         </section>
       </main>

src/app/components/ResizeHandle.module.css

@@ -0,0 +1,35 @@
+/*
+ * 6px-wide hit target with a thin visible grip. Width is part of the layout
+ * budget (HANDLES_TOTAL in PanesStore); keep them in sync if this changes.
+ */
+.handle {
+  flex: 0 0 6px;
+  position: relative;
+  cursor: col-resize;
+  background: var(--border);
+  touch-action: none; /* let pointer drags own the gesture, not scroll */
+  transition: background var(--dur-fast) var(--ease);
+}
+
+.handle:hover,
+.handle:focus-visible {
+  background: var(--accent);
+  outline: none;
+}
+
+/* A short centered grip line, so the handle reads as a draggable divider. */
+.grip {
+  position: absolute;
+  top: 50%;
+  left: 50%;
+  width: 2px;
+  height: 24px;
+  transform: translate(-50%, -50%);
+  background: var(--border-strong);
+  border-radius: var(--radius);
+}
+
+.handle:hover .grip,
+.handle:focus-visible .grip {
+  background: var(--accent-contrast);
+}

src/app/components/ResizeHandle.tsx

@@ -0,0 +1,99 @@
+/**
+ * Vertical drag handle between two panes (spec §01A).
+ *
+ * Sits between a side pane and the editor; dragging resizes the side pane while
+ * the editor absorbs the change, so the opposite side pane is unaffected. Width
+ * is clamped (pure `clampSideWidth`) so neither the dragged pane nor the editor
+ * falls below its minimum. Keyboard accessible: focus and use ←/→ to nudge.
+ *
+ * The handle reads the panes-row width from its own parent at interaction time,
+ * so it needs no layout props — it just controls the `side` it is told to.
+ */
+
+import { useRef } from 'react';
+import { clampSideWidth, usePanesStore, type PaneSide } from '../stores/PanesStore';
+import styles from './ResizeHandle.module.css';
+
+/** Keyboard nudge step (px) per arrow press. */
+const KEY_STEP = 16;
+
+interface ResizeHandleProps {
+  /** Which side pane this handle resizes. */
+  side: PaneSide;
+  /** Accessible label, e.g. "Resize snippet library". */
+  label: string;
+}
+
+export function ResizeHandle({ side, label }: ResizeHandleProps) {
+  const ref = useRef<HTMLDivElement>(null);
+
+  /** Full panes-row width — the handle's parent (`.panes`). */
+  const containerWidth = (): number => ref.current?.parentElement?.clientWidth ?? 0;
+
+  /** Apply a desired width for this side, clamped against the current layout. */
+  const applyWidth = (desired: number) => {
+    const { libraryWidth, previewWidth, setWidth } = usePanesStore.getState();
+    const other = side === 'library' ? previewWidth : libraryWidth;
+    setWidth(side, clampSideWidth(side, desired, containerWidth(), other));
+  };
+
+  const onPointerDown = (e: React.PointerEvent<HTMLDivElement>) => {
+    if (e.button !== 0) return; // primary button only
+    e.preventDefault();
+    const startX = e.clientX;
+    const startWidth =
+      side === 'library'
+        ? usePanesStore.getState().libraryWidth
+        : usePanesStore.getState().previewWidth;
+
+    const onMove = (ev: PointerEvent) => {
+      const delta = ev.clientX - startX;
+      // The left handle grows its pane as it moves right; the right handle (left
+      // of the preview) shrinks the preview as it moves right.
+      const desired = side === 'library' ? startWidth + delta : startWidth - delta;
+      applyWidth(desired);
+    };
+    const onUp = () => {
+      window.removeEventListener('pointermove', onMove);
+      window.removeEventListener('pointerup', onUp);
+      document.body.style.cursor = '';
+      document.body.style.userSelect = '';
+    };
+
+    window.addEventListener('pointermove', onMove);
+    window.addEventListener('pointerup', onUp);
+    // While dragging, force the resize cursor and suppress text selection.
+    document.body.style.cursor = 'col-resize';
+    document.body.style.userSelect = 'none';
+  };
+
+  const onKeyDown = (e: React.KeyboardEvent<HTMLDivElement>) => {
+    if (e.key !== 'ArrowLeft' && e.key !== 'ArrowRight') return;
+    e.preventDefault();
+    const dir = e.key === 'ArrowRight' ? 1 : -1;
+    const current =
+      side === 'library'
+        ? usePanesStore.getState().libraryWidth
+        : usePanesStore.getState().previewWidth;
+    const delta = side === 'library' ? dir * KEY_STEP : -dir * KEY_STEP;
+    applyWidth(current + delta);
+  };
+
+  return (
+    // TODO: a focusable window-splitter should also expose aria-valuenow/min/max
+    // (the side pane's current/min/max width) so assistive tech can announce the
+    // size as it changes. Wire it when the pane toggle strip lands in M6.
+    <div
+      ref={ref}
+      className={styles.handle}
+      role="separator"
+      aria-orientation="vertical"
+      aria-label={label}
+      tabIndex={0}
+      onPointerDown={onPointerDown}
+      onKeyDown={onKeyDown}
+    >
+      <span className={styles.grip} aria-hidden="true" />
+    </div>
+  );
+}

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

@@ -0,0 +1,63 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { loadPanelLayout, savePanelLayout } from './ux-prefs';
+
+const KEY = 'astrolabe:ux-prefs';
+
+/** In-memory localStorage stub (Node's global one is non-functional). */
+function makeStorageStub() {
+  const map = new Map<string, string>();
+  return {
+    getItem: (k: string) => (map.has(k) ? map.get(k)! : null),
+    setItem: (k: string, v: string) => void map.set(k, String(v)),
+    removeItem: (k: string) => void map.delete(k),
+    clear: () => map.clear(),
+    key: (i: number) => [...map.keys()][i] ?? null,
+    get length() {
+      return map.size;
+    },
+  };
+}
+
+describe('ux-prefs · panelLayout', () => {
+  beforeEach(() => vi.stubGlobal('localStorage', makeStorageStub()));
+  afterEach(() => vi.unstubAllGlobals());
+
+  it('returns empty widths when nothing is stored', () => {
+    expect(loadPanelLayout()).toEqual({ libraryWidth: undefined, previewWidth: undefined });
+  });
+
+  it('round-trips stored widths', () => {
+    savePanelLayout({ libraryWidth: 300, previewWidth: 420 });
+    expect(loadPanelLayout()).toEqual({ libraryWidth: 300, previewWidth: 420 });
+  });
+
+  it('merges partial writes without dropping the other width', () => {
+    savePanelLayout({ libraryWidth: 300, previewWidth: 420 });
+    savePanelLayout({ libraryWidth: 250 });
+    expect(loadPanelLayout()).toEqual({ libraryWidth: 250, previewWidth: 420 });
+  });
+
+  it('rejects non-positive / non-finite junk', () => {
+    localStorage.setItem(
+      KEY,
+      JSON.stringify({ panelLayout: { libraryWidth: -5, previewWidth: 'wide' } }),
+    );
+    expect(loadPanelLayout()).toEqual({ libraryWidth: undefined, previewWidth: undefined });
+  });
+
+  it('tolerates malformed JSON', () => {
+    localStorage.setItem(KEY, '{ not json');
+    expect(loadPanelLayout()).toEqual({ libraryWidth: undefined, previewWidth: undefined });
+  });
+
+  it('preserves unrelated keys in the record (forward-compatible merge)', () => {
+    localStorage.setItem(KEY, JSON.stringify({ sort: { sortBy: 'name', sortOrder: 'asc' } }));
+    savePanelLayout({ libraryWidth: 260 });
+    const stored = JSON.parse(localStorage.getItem(KEY)!) as {
+      sort: { sortBy: string };
+      panelLayout: { libraryWidth: number };
+    };
+    expect(stored.sort.sortBy).toBe('name');
+    expect(stored.panelLayout.libraryWidth).toBe(260);
+  });
+});

src/app/infrastructure/ux-prefs.ts

@@ -0,0 +1,78 @@
+/**
+ * UX preferences persistence (localStorage) — docs/architecture/02 §5, spec §09D.
+ *
+ * These preferences persist **separately** from UserSettings so they can change
+ * frequently (a drag emits many width updates) without rewriting the settings
+ * 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.
+ *
+ * Per the architecture rule this is one of the only modules that may touch
+ * `localStorage`; everything else goes through these typed functions.
+ */
+
+const KEY = 'astrolabe:ux-prefs';
+
+/** Per-pane widths (px). Optional — a missing field falls back to its default. */
+export interface PanelLayout {
+  libraryWidth?: number;
+  previewWidth?: number;
+}
+
+interface StoredPrefs {
+  panelLayout?: PanelLayout;
+  [k: string]: unknown;
+}
+
+function available(): boolean {
+  try {
+    return typeof localStorage !== 'undefined' && typeof localStorage.getItem === 'function';
+  } catch {
+    return false;
+  }
+}
+
+function readRaw(): StoredPrefs {
+  if (!available()) return {};
+  try {
+    const raw = localStorage.getItem(KEY);
+    if (!raw) return {};
+    const parsed: unknown = JSON.parse(raw);
+    return parsed && typeof parsed === 'object' ? (parsed as StoredPrefs) : {};
+  } catch (err) {
+    console.warn('[ux-prefs] failed to read, using defaults', err);
+    return {};
+  }
+}
+
+function writeRaw(next: StoredPrefs): void {
+  if (!available()) return;
+  try {
+    localStorage.setItem(KEY, JSON.stringify(next));
+  } catch (err) {
+    console.warn('[ux-prefs] failed to write', err);
+  }
+}
+
+/** A finite positive number, or undefined — guards against junk in storage. */
+function posNumber(v: unknown): number | undefined {
+  return typeof v === 'number' && Number.isFinite(v) && v > 0 ? v : undefined;
+}
+
+/** The persisted panel layout, with only valid numeric widths surfaced. */
+export function loadPanelLayout(): PanelLayout {
+  const stored = readRaw().panelLayout ?? {};
+  return {
+    libraryWidth: posNumber(stored.libraryWidth),
+    previewWidth: posNumber(stored.previewWidth),
+  };
+}
+
+/** Persist the panel layout, preserving every other key already in the record. */
+export function savePanelLayout(layout: PanelLayout): void {
+  const current = readRaw();
+  writeRaw({ ...current, panelLayout: { ...current.panelLayout, ...layout } });
+}

src/app/orchestration/panes.ts

@@ -0,0 +1,35 @@
+/**
+ * Pane-layout orchestration — bridges the (browser-free) PanesStore to the
+ * ux-prefs adapter. Same store↔adapter pattern as theme/preferences.
+ *
+ * `initPanes` hydrates persisted widths into the store before render. `wirePanes`
+ * persists changes, **debounced**, because a drag emits a width update on every
+ * pointer move — writing localStorage on each would thrash. The debounce settles
+ * the write to the final resting widths (the same approach as snippet auto-save).
+ */
+
+import { loadPanelLayout, savePanelLayout } 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. */
+export function initPanes(): void {
+  usePanesStore.getState().hydrate(loadPanelLayout());
+}
+
+/** Persist width changes (debounced). Returns a teardown that detaches the subscriber. */
+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;
+    }
+    clearTimeout(timer);
+    timer = setTimeout(() => {
+      const { libraryWidth, previewWidth } = usePanesStore.getState();
+      savePanelLayout({ libraryWidth, previewWidth });
+    }, PANES_PERSIST_DEBOUNCE_MS);
+  });
+}

src/app/stores/PanesStore.test.ts

@@ -0,0 +1,55 @@
+import { beforeEach, describe, expect, test } from 'vitest';
+import { clampSideWidth, HANDLES_TOTAL, PANE_DEFAULT, PANE_MIN, usePanesStore } from './PanesStore';
+
+const store = () => usePanesStore.getState();
+
+describe('clampSideWidth', () => {
+  // A roomy container where nothing is constrained.
+  const W = 1400;
+
+  test('passes a comfortable width through unchanged', () => {
+    expect(clampSideWidth('library', 300, W, PANE_DEFAULT.preview)).toBe(300);
+    expect(clampSideWidth('preview', 400, W, PANE_DEFAULT.library)).toBe(400);
+  });
+
+  test('never goes below the pane minimum', () => {
+    expect(clampSideWidth('library', 50, W, PANE_DEFAULT.preview)).toBe(PANE_MIN.library);
+    expect(clampSideWidth('preview', 10, W, PANE_DEFAULT.library)).toBe(PANE_MIN.preview);
+  });
+
+  test('caps the width so the editor keeps at least its minimum', () => {
+    const other = PANE_DEFAULT.preview;
+    const max = W - other - HANDLES_TOTAL - PANE_MIN.editor;
+    // Asking for far more than the editor can spare clamps to that maximum.
+    expect(clampSideWidth('library', W, W, other)).toBe(max);
+    // The editor sits exactly at its minimum at that point.
+    expect(W - max - other - HANDLES_TOTAL).toBe(PANE_MIN.editor);
+  });
+
+  test('a container too narrow for all minimums still never drops below the min', () => {
+    // 500px total can't fit library(180)+preview(240)+editor(320)+handles.
+    expect(clampSideWidth('library', 300, 500, PANE_MIN.preview)).toBe(PANE_MIN.library);
+  });
+});
+
+describe('usePanesStore', () => {
+  beforeEach(() =>
+    store().hydrate({ libraryWidth: PANE_DEFAULT.library, previewWidth: PANE_DEFAULT.preview }),
+  );
+
+  test('setWidth updates the targeted side only', () => {
+    store().setWidth('library', 320);
+    expect(store().libraryWidth).toBe(320);
+    expect(store().previewWidth).toBe(PANE_DEFAULT.preview);
+
+    store().setWidth('preview', 420);
+    expect(store().previewWidth).toBe(420);
+    expect(store().libraryWidth).toBe(320);
+  });
+
+  test('hydrate fills missing values from defaults', () => {
+    store().hydrate({ libraryWidth: 250 });
+    expect(store().libraryWidth).toBe(250);
+    expect(store().previewWidth).toBe(PANE_DEFAULT.preview);
+  });
+});

src/app/stores/PanesStore.ts

@@ -0,0 +1,74 @@
+/**
+ * Pane layout state — the resizable three-pane shell (spec §01A, persisted per
+ * §09D "Panel layout").
+ *
+ * Model: the two **side** panes (library, preview) carry explicit remembered
+ * widths; the **center** editor flexes to fill the remainder. This is what makes
+ * a drag "leave the rest of the layout unaffected" (§01A): dragging the left
+ * handle trades width between library and editor, the right handle between
+ * preview and editor — the opposite side pane never moves.
+ *
+ * Per-pane visibility / the toggle strip (the other half of §01A) is not here
+ * yet; this slice is resize + persistence, pulled forward from M6 because it is
+ * coupled to the Live Preview's container sizing.
+ *
+ * Pure clamp helpers live alongside the store so the resize math is unit-tested
+ * without a DOM. Persistence is a startup subscriber (orchestration/panes), not
+ * done here — the store stays browser-free.
+ */
+
+import { create } from 'zustand';
+
+/** Which side pane a drag handle controls. */
+export type PaneSide = 'library' | 'preview';
+
+/** Minimum widths (px) enforced while resizing, so no pane collapses (§01A). */
+export const PANE_MIN = { library: 180, preview: 240, editor: 320 } as const;
+
+/** Initial side-pane widths (px) on first run, before any persisted layout. */
+export const PANE_DEFAULT = { library: 280, preview: 360 } as const;
+
+/** Combined width (px) the resize handles occupy between the panes. */
+export const HANDLES_TOTAL = 12;
+
+/**
+ * Clamp a desired side-pane width so it stays at least its own minimum and
+ * leaves the editor at least its minimum. Pure — the single place the resize
+ * constraint lives. `containerWidth` is the full panes-row width; `otherWidth`
+ * is the opposite side pane's current width.
+ */
+export function clampSideWidth(
+  side: PaneSide,
+  desired: number,
+  containerWidth: number,
+  otherWidth: number,
+): number {
+  const min = PANE_MIN[side];
+  // The widest this pane can be while the editor still meets its minimum.
+  const max = containerWidth - otherWidth - HANDLES_TOTAL - PANE_MIN.editor;
+  // If the container is too narrow for everyone, the min wins (never below it).
+  return Math.max(min, Math.min(desired, Math.max(min, max)));
+}
+
+export interface PanesState {
+  libraryWidth: number;
+  previewWidth: number;
+  /** 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;
+}
+
+export const usePanesStore = create<PanesState>((set) => ({
+  libraryWidth: PANE_DEFAULT.library,
+  previewWidth: PANE_DEFAULT.preview,
+
+  setWidth: (side, width) =>
+    set(side === 'library' ? { libraryWidth: width } : { previewWidth: width }),
+
+  hydrate: (layout) =>
+    set({
+      libraryWidth: layout.libraryWidth ?? PANE_DEFAULT.library,
+      previewWidth: layout.previewWidth ?? PANE_DEFAULT.preview,
+    }),
+}));

src/main.tsx

@@ -1,5 +1,6 @@
 import { createRoot } from 'react-dom/client';
 import { App } from './app/App';
+import { initPanes, wirePanes } from './app/orchestration/panes';
 import { initPreviewFitMode, wirePreviewFitMode } from './app/orchestration/preferences';
 import { initApp } from './app/orchestration/startup';
 import { initTheme, wireTheme } from './app/orchestration/theme';
@@ -11,10 +12,12 @@ import '../styles/base.css';
 initTheme();
 wireTheme();
 
-// Hydrate + persist the preview fit mode the same way (pulled ahead of the M5
-// Settings modal); hydrating before render keeps the store authoritative.
+// Hydrate + persist the small UI preferences (preview fit mode, pane widths) the
+// same way. Pane widths hydrate before render so the layout opens as left.
 initPreviewFitMode();
 wirePreviewFitMode();
+initPanes();
+wirePanes();
 
 // Load the library from IndexedDB (seeding a sample on first run) and wire
 // persistence. Fire-and-forget: the UI renders immediately and fills in when