TUI: persist last-session UI state (active tab, selected instrument, sort, filters, depth preset, etc.)

Comienza localizando el modelo de estado de la TUI, la ruta de apagado y el helper existente mutate_tui_toml_at utilizado para los round trips de tui.toml. Implementa primero la Fase 1: persiste active_tab, selected_instrument, market.sort y market.category bajo [session], preservando las claves no relacionadas. Añade o amplía las pruebas alrededor del punto de integración TOML comprobable y verifica que los valores ausentes, malformados u obsoletos vuelvan a un valor seguro por defecto.

Summary

The TUI currently forgets all in-session UI state on quit. Every relaunch drops you on Market tab / default sort / BTC_USDT selected / depth 50 / category Spot, regardless of where you were when you quit. For power users who live in the TUI across many short sessions, this is friction.

Add a best-effort state-restore layer that remembers the last session's UI state and restores it on launch.

What to persist

Navigation

• Active tab (Market / Portfolio / Orders / History / Watchlist / Positions).
• Selected instrument on Market / Watchlist / Positions (falls back gracefully if the instrument is no longer available).

Market tab

• Category filter (Spot / Perpetual / Futures / ...).
• Sort field + direction (Volume / Change / Price / Name, asc/desc).
• Search query? — probably NOT; search feels ephemeral. Up for debate.
• View mode: Table / Detail / Chart / Compare? — probably NOT; rehydrating straight into a Compare view is jarring. Up for debate.
• Timeframe (1m / 5m / ... / 1D).
• Book depth preset (10 / 50 / 150) — from #26.
• Heatmap on/off.
• Volume unit (USD / Notional).
• Split-view state (table + chart).

Help overlay

• Last shortcuts page viewed? — probably NOT; #30 explicitly resets to page 1 every open, and that's the right UX. This one is low-value.

Orders / History / Portfolio

• Sort field + direction on any tab that has sortable columns.
• History pagination position? — probably NOT; data moves forward between sessions, so "resume on page 3" is stale.

Compare view

• List of pinned instruments (if view_mode is persisted).

Where to persist

~/.config/cdcx/tui.toml — same file #23 (watchlist) and settings already round-trip. One new top-level [session] table:

[session]
active_tab = \"market\"
selected_instrument = \"ETH_USDT\"
volume_unit = \"usd\"
split_view = false

[session.market]
category = \"PERPETUAL_SWAP\"
sort_field = \"volume\"
sort_asc = false
timeframe = \"4h\"
book_depth = 50
heatmap = false

Reuse the mutate_tui_toml_at helper from #28 so unrelated keys (theme, tick_rate, watchlist, custom themes) are preserved.

When to persist

Option A — on every change: write on each mutation. Simple, but O(n) disk writes per session.

Option B — on quit only: single write in the shutdown path. Lower-friction, but any crash loses the session.

Option C — debounced (every ~5s if dirty): best of both. Adds complexity.

Recommendation: Option B to start. Fast, simple, one integration point. If we notice state loss from crashes later, upgrade to C.

Scope / sequencing

Land in small increments, each an independently-useful PR:

1. Phase 1 (minimum useful): active_tab + selected_instrument + market.sort + market.category. Roughly half the annoyance, small diff.
2. Phase 2: timeframe, book_depth, heatmap, volume_unit, split_view.
3. Phase 3: sort state on Orders / Portfolio / Positions, compare-view instrument list.

Deliberately NOT in scope for the first cut: search query, help overlay page, history pagination, workflow modals, spotlight.

Non-goals

• Not a full settings-UI-exposed preference system. This is silent state capture; the user doesn't configure it.
• Not a per-profile state store (yet). One global [session] block. Add per-profile later if someone hits the edge case.
• Not a remote-sync system. Local file only.

Risk

• Stale instrument on restore: if selected_instrument was delisted, fall back to index 0 of the filtered list. Don't panic, don't toast.
• Schema evolution: treat [session] as a purely additive, tolerant block. Missing keys → sensible defaults. Unknown keys → ignore. Malformed values → fall through to default and don't error the user.
• File-size creep: negligible. Even with all phases, the [session] block stays under a few hundred bytes.

Related

• #23 — established the tui.toml round-trip pattern + save_watchlist_at testable seam.
• #26 — book_depth preset that would become persisted under [session.market].
• #30 — help overlay pagination (explicitly NOT persisted; called out here to avoid confusion).