feat(#487): left-navigation layout core and preferences (phase 1)

CHANGELOG.md

@@ -9,6 +9,40 @@ auto-generated per-PR notes; this file is the curated, human-readable history.
 
 ## [Unreleased]
 
+### Added
+- **The desktop left navigation's layout core and preferences** (#487, phase 1 of
+  4). A new pure `src/core/left-nav-layout.ts` owns every layout decision the
+  foldable left navigation needs: the named constants and thresholds, the
+  explicit `'wide' | 'rail'` mode, the hysteresis reducer that maps a proposed
+  width onto the next mode, the keyboard separator resolver, rail activation, the
+  separator's ARIA range, and the mobile projection that makes a phone ignore
+  rail mode without discarding the preference. Two new browser preferences back
+  it (`asb:leftNavMode`, `asb:leftNavDrawerPx`); the wide sidebar's width
+  deliberately stays on the existing `asb:sidebarPx` rather than gaining a second
+  owner, and the focused section is session state that does not reopen after a
+  reload. Hysteresis comes from the threshold *pair* — folding needs a proposal
+  below 140px and restoring one above 260px — so no single pointer pixel can
+  oscillate the mode, and the keyboard path routes through the same reducer as
+  the pointer path rather than reimplementing it. Every function takes the
+  navigation's proposed *total* width, which is what keeps a drag continuous
+  across a mode change: handing the reducer a mode-relative width instead made a
+  monotone rightward drag snap the navigation 108px backwards on the frame a
+  drawer converted to the sidebar. Arrow keys resize within a band and perform
+  the band edge's semantic transition, because an arrow's *relative* step cannot
+  cross a dead zone wider than itself — leaving that relative stranded a bare
+  rail's `ArrowRight` and a floored sidebar's `ArrowLeft` permanently, while
+  `aria-valuemin: 48` was advertised throughout. Opening a section is a separate
+  operation from the click toggle, since #428's bounded drag-hover re-asserts
+  intent repeatedly and a toggle would flap the drawer shut. Every reducer
+  normalizes its input, so the "a focused drawer exists only in rail mode"
+  invariant is an unconditional postcondition rather than a precondition the
+  caller has to honour. **No user-visible change yet**: the rail, the docked
+  focused drawer and the resize separator arrive in phase 3.
+- The sidebar's `'col'` drag axis now clamps through the same
+  `LEFT_PANEL_MIN_PX`/`LEFT_PANEL_MAX_PX` constants as the load path, instead of
+  repeating `180`/`420` as literals (#487). Behaviour is unchanged; it removes
+  the second owner of a range whose whole point is having one.
+
 ### Changed
 - **`VariableBarApp`'s shared activation port is now caller-neutral** (#478).
   `state.filterActive`/`params.saveFilterActive` — named after Workbench
@@ -25,6 +59,20 @@ auto-generated per-PR notes; this file is the curated, human-readable history.
   adapter refactor — no user-visible behavior changes.
 
 ### Fixed
+- **A corrupt `asb:sidebarPx` decodes to the default width instead of `NaN`**
+  (#487). The width decoded through a bare `clamp(parseInt(stored), 180, 420)`,
+  and `clamp` is not NaN-safe (`Math.max(180, NaN)` is `NaN`), so a non-numeric
+  stored value would reach the DOM as `width: NaNpx` — which the browser drops,
+  collapsing the sidebar with nothing to explain why, and the bad value would
+  persist across reloads. It now falls back to the documented 248px default, and
+  decoding requires the *whole* stored string to be a finite number — `parseInt`
+  accepted a numeric prefix, so a truncated write or a hand-edited `"200px"`
+  decoded to a plausible-looking width while the contract promised the default.
+  Hardening rather than a reproducible user-visible bug: no code path in the app
+  writes a malformed value (a real drag always carries a finite `clientX`), so
+  reaching it takes a hand-edited or foreign-origin `localStorage` entry. The
+  same NaN hole in `editorPct`/`sideSplitPct`/`cellDrawerPx`/`docPanePx` is
+  tracked separately in #570.
 - **The Dashboard tree no longer reveals two rows' pencil/trash actions at
   once, and its `· N` count now sits inline after the label** (#568). The
   hover/focus reveal rule (`.dash-tree-row:focus-within .dash-tree-act`)

src/application/app-preferences.ts

@@ -31,7 +31,11 @@ export type PreferenceKey =
   | 'sidePanel' | 'resultRowLimit'
   // #313 — the documentation pane's own persisted resize width, a sibling of
   // cellDrawerPx (never shared with it — see splitters.ts's 'docPane' axis).
-  | 'docPanePx';
+  | 'docPanePx'
+  // #487 — the desktop left navigation's semantic mode and its focused drawer's
+  // width. The WIDE sidebar width stays `sidebarPx` above: `core/left-nav-layout.ts`
+  // reuses that preference rather than introducing a second owner of one width.
+  | 'leftNavMode' | 'leftNavDrawerPx';
 
 /** The one state field this service reads/writes (`toggleTheme` only) — a
  *  plain settable property, not a signal (matches `AppState.theme`). */