Harlock Roadmap

A pure-Elixir TUI framework for Unix terminals. TEA-style model/update/view loop on top of OTP, with a thin termios NIF for direct /dev/tty control.

This roadmap is the working plan through v1.0 (stable API). It's a living document — revised as the design settles. v0.7.0 is current and published on Hex.

Status snapshot (v0.7.0, current)

What works:

• OTP supervision tree (Keeper → Writer → Reader → Runtime, rest_for_one, max_restarts: 0). Terminal is restored on any crash via Keeper.terminate/2. This is correct and load-bearing — don't regress it.
• TEA loop with init/1, update/2, view/1, optional subs/1. Dirty-flag rendering, no periodic polling.
• Cmd executor for supervised side effects: Cmd.from/1, Cmd.batch/1, Cmd.map/2. Results arrive back through update/2 as messages.
• Focus traversal (Tab / Shift-Tab), focus traps for modals with automatic stash/restore on open/close.
• Focus-aware key routing (v0.4): the runtime dispatches navigation keys straight to the focused viewport / tabs / text_input / textarea / menu / select / tree and delivers the result as a message, so apps no longer hand-wire apply_key helpers. box(focus_proxy:) lets a container mirror a child's focus for styling without joining traversal.
• Push-shaped subscriptions: Sub.telemetry and Sub.logger turn :telemetry events and log calls into update/2 messages, Sub.source subscribes to anything that sends to its subscriber (Phoenix.PubSub, registries, :global groups), plus sparkline for trends.
• table takes a fn offset, limit -> rows window function as well as an enumerable, so a query- or stream-backed table is asked only for what fits.
• Harlock.Bench measures render and diff cost over five scenarios, reporting percentiles. Layout solver covered by property tests as well as examples.
• Constraint layout solver: :length, :percentage, :fill, :min, :max. Deterministic round-off absorption; graceful truncation on over-constraint (logs warning, never crashes).
• Cell-grid renderer with frame diffing. ANSI output via Writer.
• Theming (v0.4): full token set (:header, :focus, :selection, :border, :primary, :accent, :muted, :error), three built-in themes (:default / :dark / :high_contrast), caps-aware colour downgrade (truecolor → 256 → 16 → mono), and a table style cascade. :default-theme output is pinned byte-for-byte against v0.3.0 by a golden-frame test.
• Primitives: text, vbox, hbox, spacer, box (4 border styles + title + padding), overlay (5 anchors + focus trap), table / list (row-id identity, single/multi selection, header), text_input, textarea (multi-line, opt-in word wrap), viewport (render-then-clip + scroll-into-view + cursor remap), progress, spinner, statusbar, keybar, tabs, menu, select (dropdown that flips rather than clipping near a margin), tree (flat projection, id-keyed expansion, lazily loaded children).
• Undo / redo via Harlock.UndoStack — bounded snapshots the app holds in its model, with coalescing that breaks on a newline, a cursor jump, or a delete after an insert.
• Wide-grapheme width (CJK, emoji, ZWJ sequences, flags).
• SIGWINCH resize via an ioctl(TIOCGWINSZ) NIF — terminal resize reflows.
• Input parser handles CSI/SS3, bracketed paste, XTerm focus reporting, modified arrows (CSI 1;5A), Home / End, and F-keys.
• :telemetry events for frame render, input dispatch, cmd, and reader.
• Headless IO.Test backend selectable via backend: :test for deterministic tests without a TTY, plus the Harlock.Test helper API.
• Examples: counter, sysmon, contacts, showcase, overview, notes, explorer, dashboard. Smoke tests driven by script(1) (handles BSD vs util-linux flag differences).
• Packaging and quality gates: Hex package metadata, published hexdocs, CI, Dialyzer, and Credo all wired in.

What's stubbed / missing — the honest list:

• Sub: :interval, :telemetry, :logger and the generic :source exist. file and port are v0.8 — the seam is built, so they are additions rather than design work. pubsub is not coming: source/3 covers it.
• No windowed aggregation for metrics (rates, percentiles over a trailing window) — v0.8. Counts and means are a few lines of Enum in the model.
• Mouse events: SGR parser only — runtime enabling is deferred.
• Kitty keyboard protocol: parser only — runtime push is deferred.
• No mouse-driven interaction; table has no auto-routing, so a windowed table scrolls by app-handled keys writing :offset, as viewport did pre-R2.
• Windows native is unsupported (WSL works); the termios NIF targets POSIX.

Guiding principles

1. OTP-first. The supervision tree is the architecture. New features that need processes get their own child, supervised correctly.
2. No NIFs in the rendering path. ANSI in, ANSI out. The termios NIF shipped in v0.2 is the one allowed exception, strictly scoped to /dev/tty control; new NIFs require the same level of justification.
3. Phoenix devs feel at home. update/view/subs mirrors LiveView's mental model on purpose, and subscribing to a Phoenix.PubSub topic is a one-liner — via Sub.source/3 rather than a named Sub.pubsub, because principle 7 says the dependency belongs in the app rather than here.
4. Headless-testable. Every new widget gets IO.Test-driven coverage. No "works on my terminal" features.
5. Terminal restoration is sacred. Any new IO path must survive crashes without leaving the terminal in raw mode / alt-screen. Test it.
6. Honest stubs over fake completeness. Stub modules clearly say so in their moduledoc. No silent half-features.
7. Seams, not adapters. Harlock provides the shape an integration plugs into and stops there. table takes a window function, not an Ecto.Queryable; Sub.telemetry takes event names, not an Oban dashboard. A terminal UI library should not acquire a dependency on a database, a web framework, or a job runner in order to display their output — the adapter is a few lines of app code, or its own package if it ever earns one. :telemetry is the sole first-party integration, and only because it is already a dependency and its seam is generic.

Versioning

• 0.x — API may break. We document breaking changes in CHANGELOG.
• 1.0 — locked public API for Harlock, Harlock.App, Harlock.Elements, Harlock.Cmd, Harlock.Sub, Harlock.Render.Style, Harlock.Layout. Internal modules — Harlock.App.Runtime, the rest of Harlock.Terminal.*, Harlock.Element.Renderer — stay @moduledoc false and remain free to change without notice.

v0.2-prep — tooling first (do before any v0.2 implementation)

CI gates the v0.2 implementation work; it does not land alongside it. Otherwise the Cmd executor, SIGWINCH path, and text_input land without type-checking or lint to catch regressions, and the first green build is also the first build that has to satisfy every new check at once.

• Add dev deps: :ex_doc, :dialyxir, :credo. Wire mix docs.
• .github/workflows/ci.yml: mix format --check-formatted, mix test, mix dialyzer, mix credo --strict on push + PR.
• .credo.exs tuned to a green baseline on v0.1 code.
• Dialyzer PLT cached in CI; baseline clean on v0.1 before opening v0.2 PRs.
• CHANGELOG.md seeded from v0.1.

v0.2 — "actually usable" (~2 weeks)

The minimum set that lets someone build a real internal tool. Ship together.

Cmd executor (the unblocker)

The runtime currently destructures {model, _cmd} and throws the cmd away. Wire a real executor.

• Add a Task.Supervisor child to the app supervisor, positioned between IO and Runtime so it's already up when Runtime's handle_continue dispatches the init-time cmd. (Earlier plan said "after Runtime"; that lost the init-cmd race against the supervisor's child-start loop.) Strategy: :one_for_one, restart :temporary on individual tasks.
• Implement Cmd.from/1: runs the 0-arity fn under Task.Supervisor, sends result back as {:harlock_event, result}.
• Implement Cmd.batch/1: spawns each child cmd concurrently, no ordering guarantees.
• Add Cmd.map/2 for tagging results: Cmd.map(cmd, fn r -> {:tag, r} end).
• Tasks linked to runtime; if runtime exits, tasks die. If a task crashes, log + emit {:harlock_event, {:cmd_error, reason}} (don't kill runtime).
• Runtime: on update/2 returning {model, cmd}, dispatch cmd, update model, render. On :quit with cmd, dispatch then quit.

Tests: cmd that sleeps then returns; cmd that crashes; batch of three; quit-with-cmd ordering.

SIGWINCH + resize event

Terminal resize must reflow. Without this we can't ship.

• Use :os.set_signal(:sigwinch, :handle) (OTP 22+) in Keeper (the TTY-owning process). Signal arrives as {:signal, :sigwinch} in Keeper's mailbox.
• Keeper queries new size via ioctl(TIOCGWINSZ) through the termios NIF (Harlock.Terminal.Termios.winsize/1). The original plan considered tput cols/tput lines to avoid native code, but :os.cmd-based shell-outs lose the controlling tty (every shell spawned by ERTS is setsid()-detached), so a NIF was needed for termios anyway — TIOCGWINSZ goes through the same one.
• Keeper sends {:harlock_resize, rows, cols} to Runtime.
• Runtime handles {:harlock_resize, _, _}: update state.rows / state.cols, discard prev_frame (full redraw — diff against differently-sized buffer is meaningless), mark dirty, render.
• Initial size at Runtime startup also comes from Keeper.size/1 (synchronous GenServer.call, no race because Keeper starts first in the supervision tree). Test backend supplies explicit rows/cols via opts.

Tests: simulate resize event into IO.Test runtime, assert reflow (test/harlock/resize_test.exs).

Wide-grapheme width (prerequisite for text_input)

String.length ≠ display columns. Pulled into v0.2 from v0.3 because text_input cursor math depends on it — shipping text_input first would either land with broken CJK/emoji handling or force a width retrofit later. Verified usage at renderer.ex:188, 194, 200, 308, 315–325 and called out in code at renderer.ex:323-324 and cell.ex:7-8.

• New module Harlock.Width: width(grapheme) returns 0/1/2 based on Unicode East Asian Width + emoji presentation. Static table — pull from Unicode 15.1 EastAsianWidth.txt at build time via mix harlock.gen_width.
• Replace String.length with Harlock.Width.string_width/1 in: clip/2, align_text/3, draw_title/6, table column rendering, and the new text_input cursor math.
• Zero-width joiner / variation selectors: keep with the preceding grapheme, don't advance the cursor.
• Cell stays one codepoint per cell; wide graphemes occupy cell + cell' where cell' is a sentinel "continuation" cell. Frame diffing and clip math must skip continuations.

Tests: "héllo" (combining), "東京" (wide), "🇮🇹" (regional indicator pair), "👨‍👩‍👧" (ZWJ sequence).

Minimal theme tokens (prerequisite for v0.3 widgets)

Replace the hard-coded styles in the renderer with theme lookups now, so v0.3 widgets (progress, tabs, statusbar, keybar) aren't built against hard-coded values that need a sweep in v0.4. Full theming ergonomics still land in v0.4 — this is the minimum surface to avoid rework.

• Harlock.Theme struct with the four tokens the current renderer needs: header, focus, selection, border. Full token set (primary, accent, muted, error) deferred to v0.4.
• Harlock.Theme.get(token) available in callbacks via process dict (same pattern as Focus).
• App passes theme: %Theme{...} to Harlock.run/3; omitted = built-in default that matches today's hard-coded values exactly (no v0.1 → v0.2 visual diff for existing apps).
• Replace hard-coded styles at renderer.ex:165 (%Style{bold: true} header), :211-212 (%Style{reverse: true} / bold: true focused row), :254 (focus default), :214, 217 (bg: :cyan selection) with theme lookups.

Tests: app with custom theme overrides each token; app with no theme renders byte-identical to v0.1 baseline (golden frame).

text_input element

Single-line first. The cursor is a new concept — Frame needs to track it.

• Add cursor: {row, col} | nil to Frame. Writer emits cursor-show / cursor-position after diff, or cursor-hide if nil.

• text_input element opts: :value (caller-owned, model holds state), :placeholder, :focusable (required), :on_change (msg to send), :max_length, :style, :placeholder_style, :password (mask with •).
• Runtime injects key events to the focused text_input's :on_change with shape {msg, {:input_event, kind, payload}} where kind is :insert | :delete | :move | :submit. App owns the buffer.
• Helper module Harlock.TextBuffer (pure functions: insert/3, delete_backward/2, move_cursor/2, etc.). App calls it from update/2. Keeps the element dumb, model honest.
• Keys: printable chars insert, Backspace deletes-back, Delete deletes-forward, Left/Right move, Home/End jump, Enter submits.

Defer to v0.3: multi-line, IME, word movement (Ctrl-Left/Ctrl-Right), selection.

viewport element

Generic scrollable container.

• viewport(child: el, height: n, offset: model.offset, on_scroll: msg).
• Renders child into a virtual Frame of {requested_w, large_h}, then blits the visible slice into the real region.
• Emits scroll messages on PgUp/PgDn/Up/Down when focused.
• App owns the offset (same TEA discipline as text_input).

Presentation track (parallel with v0.2 implementation)

Tooling itself ships in v0.2-prep (above). What's left here is the external-facing surface that gates adoption:

• Fill mix.exs: description, package (licenses, links, maintainers), source_url, homepage_url, docs config (main: "readme", extras).
• Replace README.md: 30-second pitch, screenshot/GIF, install snippet, minimum counter example, status table linking to ROADMAP, "why not X" (vs Owl, Ratatouille, ratatui-via-port).
• Asciinema cast of sysmon embedded in README.

v0.3 — "shows well in demos" (~3 weeks after v0.2)

Layout: real :min and :max ✓

Shipped. The solver:

1. Computes a lower bound per slot: :length(n)→n, :percentage(p)→p%, :min(n)→n, :fill(_)→0, :max(_)→0.
2. If lower bounds exceed total → truncate from tail and warn (unchanged over-constraint behavior).
3. Distributes the remainder across flexible slots. :fill(weight) carries its weight; :min and :max each carry weight 1.
4. Clamps :max violators to their cap, returns excess to remaining, iterates. Convergence is bounded by length(constraints) — each pass either freezes ≥1 slot or terminates.
5. If :max caps leave space unallocated ([max: 10, max: 10] in a 30-cell region), the trailing region is simply not used — children don't overflow caps to fill space.

Standard widgets ✓

Shipped — all composable from existing primitives, dumb renderers with app-owned state:

• progress(:value, :max, :width, :style, :fill_style) — single-line bar with █ for the filled portion.
• spinner(:tick, :frames, :style) — single cell. Caller increments :tick from a Sub.interval subscription.
• tabs(:items, :active, :focusable, :style, :active_style) — horizontal tab bar; the body for the active tab is rendered separately by the app. Pair with Harlock.Tabs.apply_key/3 in update/2 for Left/Right/Home/End navigation.
• statusbar(:left, :right, :style) — pinned-row helper, left/right alignment with style-filled middle.
• keybar(:bindings, :style, :separator, :right) — formats [?q, "quit"] → [q] quit, with arbitrary separators and an optional right-aligned addendum.

Viewport element ✓

Shipped. App-owned scroll state, render-then-clip pipeline:

• viewport(child:, offset:, content_height:, scrollbar:) — required :offset and :content_height (app knows what it's scrolling).
• Renderer allocates a width × content_height temporary frame, renders the child into it, blits the visible slice. Cost is O(content × width) per frame — fine for hundreds of rows. Day-one perf test (viewport_perf_test.exs) holds the budget.
• Scroll-into-view is a render-pipeline phase: focusable elements record their bounds via Frame.set_focus_rect/2; the viewport reads its child's tall_frame.focus_rect and snaps the effective offset so the focused element stays visible. Model offset is untouched — this is render-time-only adjustment.
• Cursor positions (set by text_input) are remapped from tall-frame coords to dst coords when the cursor falls in the visible window; hidden otherwise.
• Optional cosmetic scrollbar: single column on the right edge, thumb proportional to visible_h / content_h.
• Harlock.Viewport.apply_key/4 translates scroll keys (:up | :down | :page_up | :page_down | :home | :end) into a new clamped offset. Scroll keys are explicit — app calls the helper from update/2, no runtime interception.

Mouse events (parser) ✓

Parser landed. Emits {:mouse, action, button | nil, col, row, mods} for SGR encoding (CSI < button;col;row M|m). Actions: :press | :release | :drag | :move | :wheel_up | :wheel_down. Buttons: :left | :middle | :right | :extra4 | :extra5.

Runtime enabling + hit-test routing — deferred. The runtime does not write \e[?1006h by default and does not route events to elements. Apps that need mouse input can write the enable sequence themselves and match on raw (col, row) in update/2. Hit-test infra (frames carry element bounds, runtime resolves cursor → element) waits on a real use case shaping the API.

Modified arrows ✓

CSI 1;<mod><letter> for arrows + Home/End with shift/alt/ctrl/meta. CSI <n>;<mod>~ for modified PageUp/PageDown/Insert/Delete and F1-F12.

Kitty keyboard protocol (parser) ✓

Parser landed:

• Detection response CSI ? <flags> u → {:capability, :kitty_keyboard, flags}.
• Key events CSI <code>[:<shifted>:<base>][;<mod>[:<type>]] u. Event-type 1=press → {:key, ...}, 2=repeat → {:key_repeat, ...}, 3=release → {:key_release, ...}. Kitty private-range codepoints (57344-57375) map to functional-key atoms.

Enabling the protocol (writing CSI > <flags> u to push state) is a runtime decision deferred until a real use case appears.

Telemetry instrumentation ✓

Shipped. Events:

• [:harlock, :frame, :render, :start | :stop | :exception] — span wrapping view/1 + tree walk + diff emission. Stop measurements include duration; metadata includes app, dirty, rows, cols.

• [:harlock, :input, :dispatch, :start | :stop | :exception] — span wrapping the runtime mailbox → update/2 return path. Metadata includes app, event, focused.

• [:harlock, :cmd, :dispatch] — one-shot when a cmd is handed to the task supervisor. Metadata: %{kind: :fun | :batch | :map | :none}.
• [:harlock, :cmd, :complete] — when a cmd task returns. Measurements: %{duration: native}. Metadata: %{status: :ok | :error}.
• [:harlock, :reader, :tty_lost] — one-shot on EOF (ssh disconnect, terminal close).

Harlock.Telemetry documents the full catalog. :telemetry is a hard dep (tiny library, no transitive deps).

v0.4 — "absorb the boilerplate" ✓ (shipped 2026-05-18)

The v0.4 plan was re-scoped against docs/feedback-v0.3.md and docs/v0.4-plan.md: instead of growing the widget roster, v0.4 made the widgets that already shipped cost less to wire. The original "polish & adoption" sub-sections below are kept as-is for provenance — entries marked ✓ landed, others moved to v0.5.

Focus-aware widget key routing (R2) ✓

The headline. Focusable viewport, tabs, and text_input elements get their navigation keys auto-routed by the runtime; the app's update/2 receives a synthesised {:harlock_scroll | _select | _edit | _submit, focus_id, payload} instead of raw {:key, …}. Opt out per-element with handle_keys: false. The four routed-message tuples are public API and documented in Harlock.App's moduledoc. Applied to examples/showcase.exs: the per-field text-input dispatch clause in update/2 went 21→7 lines (-67%); the manual Viewport.apply_key/4 scroll dispatch went 7→3 lines. No widget calls apply_key/_ from update/2 in showcase any more.

End-to-end README example ✓

examples/overview.exs (also embedded inline in the README between the Counter snippet and Installation) is a runnable ~70-line app covering focus traversal, a selectable table, a focusable viewport with R2 auto-routing, and a Cmd round-trip. test/examples/overview_test.exs Code.require_files it so the README snippet can't rot silently.

Theming (full token set) ✓

The four-token minimum (header, focus, selection, border) landed in v0.2. v0.4 extends to the full palette and ships the ergonomics around it:

%Theme{
  primary: :cyan,
  accent: :magenta,
  muted: %Style{fg: {:rgb, 128, 128, 128}},
  error: :red,
  border: %Style{fg: :white},
  focus: %Style{reverse: true},
  header: %Style{bold: true},
  selection: %Style{bg: :cyan}
}

• Add the remaining tokens (primary, accent, muted, error) and any per-widget tokens surfaced during v0.3 widget work.
• Built-in themes: :default, :dark, :high_contrast.
• Auto-downgrade truecolor → 256 → 16 based on Caps.
• App still configures via Harlock.run(MyApp, init_arg, theme: theme); Harlock.Theme.get/1 already exists from v0.2.

Style cascade ✓

• box propagates :border_style to title. ✓ (was already true in v0.3 — draw_title shares the border style parameter; verified during Phase 3.)
• table accepts :header_style, :row_style, :alt_row_style, :selected_style, :focus_style (all default to theme). ✓ :alt_row_style is the only behavioural addition; the rest are pure overrides that preserve v0.3 output when unset.
• Style.merge/2 with proper inheritance (child overrides parent; unspecified attrs inherit). ✓ (already had these semantics in v0.2; verified during Phase 3.)

:default byte-identical to v0.3 ✓

Pinned in test/harlock/golden_frame_test.exs. Hash captured by running the same canonical app under a git worktree at tag v0.3.0, not by observing the v0.4 implementation — so the test proves parity, not self-locking.

v0.5 — widgets on the new contract ✓ (shipped as v0.5.0)

The work originally listed under v0.4 that was deferred so it could ship as the first consumer of the R2 routing contract instead of being built against the v0.3 manual-dispatch idiom. Building them inside v0.4 would have meant writing their key handling against the old API and rewriting it the moment R2 landed; v0.5 lets them arrive as native R2 widgets from day one.

That reason is now spent: R2 shipped in v0.4, and textarea proved the contract on the hardest case in v0.4.2–v0.4.3. What remains is a closed list — menu, select, tree, and the undo helper — and the milestone closes when those are done. Subscriptions and box(focus_proxy:) moved to v0.6 to keep that boundary meaningful; a widget set has no natural end otherwise, and this one needs one.

Build order is menu → select → tree: menu is the smallest R2 consumer and establishes the pattern, select adds the overlay open state on top, and tree comes last because it is the only recursive one and the only one adding a routed tuple to the public contract.

tree / menu / select widgets

• tree(:nodes, :expanded, :focused, :on_toggle, :on_select) — recursive node display with expand/collapse on Right/Left or Enter. Auto-routed via {:harlock_select, focus_id, node_id} and a new {:harlock_toggle, focus_id, node_id}.
• menu(:items, :on_select) — vertical list, arrow navigation, Enter selects. Auto-routed via {:harlock_select, focus_id, id}.
• select(:items, :value, :on_change) — dropdown (uses overlay for the open state). Same routed-select shape.

Each gets its own apply_key/n pure helper plus a per-type clause in the runtime's route_to_widget/4; no new mechanism, just three new consumers.

All three shipped. Two things the plan above did not anticipate, both worth keeping:

select needed a deferred-draw layer. overlay/1 establishes z-order by holding both layers as children, which a dropdown cannot do — it is anchored to a control buried in the layout, and every sibling drawn after that control would bury the panel. So the renderer records floats during the walk and draws them after it, in push order. That ordering is also the stack nested panels would need, without anything tracking depth. Placement flips rather than clips: above the control near the bottom margin, shifted left near the right one.

tree keeps the tree as data. Harlock.Tree.visible/2 projects to the flat list of visible rows and the renderer draws rows, never recursing. Rows carry ancestors_last — the last-child flag of every ancestor — not just a depth, because the guides need to know whether each ancestor column wants a continuation bar or a blank, and only the projection can see siblings. Expansion is keyed by node id; index keys would move the selection whenever a node above collapsed.

Lazy children are the interesting part: :children is a list or one of :unloaded / :loading, so expanding a node over a filesystem or a remote node is a side effect. The app receives {:harlock_toggle, …}, marks the node loading, returns a Cmd, and swaps the result in when it arrives — which is the clearest demonstration in the widget set of why the model is not ceremony.

{:harlock_toggle, focus_id, node_id} is the only routed message the whole set added. menu and select reuse {:harlock_select, …} and {:harlock_submit, …}.

Multi-line text_area ✓ (shipped in v0.4.2)

Shipped as textarea/1 + Harlock.TextArea. Hard and soft line breaks, vertical motion, display-row Home / End, logical-line kills, and Enter inserting a newline rather than submitting. Horizontal editing delegates to Harlock.TextBuffer, so both widgets share one key-binding set.

Word wrap is opt-in per element (wrap: true) and breaks at word boundaries, measuring display cells so CJK wraps where it actually reaches the edge. When wrapping, ↑ / ↓ and Home / End follow display rows; kills stay logical-line relative, because a wrap boundary isn't in the text.

One deviation from the plan above, deliberate: the cursor stayed a flat grapheme index rather than widening to {line, col}. Cross-line motion then falls out for free — moving left from a line start lands on the newline ending the previous line, and deleting there joins them — and the routed-edit message is byte-identical to the one text_input produces, so apps write one clause for both. The visual-vs-logical distinction the plan anticipated lives in visual_rows/2 instead of in the cursor type, which keeps it out of the public message contract.

Goal-column memory landed in v0.4.3. The column is held in runtime state, keyed by focus and discarded on focus change, so it stays out of the routed message contract — apply_key/6 threads it, and vertical motion is the only thing that pays for computing it.

Undo / redo as an app-held helper ✓

A textarea without undo is the gap users hit first, and more sharply than in a single-line input where retyping is cheap. It cannot live in the widget: the app model owns :value and may rewrite it — loading a file, clearing a form, applying a Cmd result — without the widget seeing it, so a buffer-held history desyncs and starts restoring text the user never typed.

So it ships as a pure helper the app holds in its model and threads explicitly: bounded snapshots of {cursor, text} with coalescing, ring-capped around 100 entries. Snapshots rather than a command stack — for a buffer this size they are simpler and fast enough.

Coalescing is the part that has to be right, and it is contract, not implementation detail. A run of insertions collapses into one step, and the run breaks on a newline, on any cursor jump, and on a delete following an insert. Get it wrong and a user types a paragraph, presses undo expecting to lose a word, and loses the paragraph — worse than shipping no undo at all, because it destroys work rather than merely failing to restore it.

Shipped as Harlock.UndoStack, with those three break conditions written into its moduledoc as contract rather than left as tuning. A cursor jump closes the run without becoming a step of its own: restoring a caret position while leaving the text alone would spend an undo press on nothing the user thinks of as a change. examples/notes.exs wires it to Ctrl-Z / Ctrl-R.

v0.6 — the event-source seam (next)

Split out of v0.5 so that milestone could close on the widget set alone. Nothing here is a widget, and holding tree / menu / select back until Phoenix.PubSub integration was also done would have left finished work unpublished for no reason.

Sub currently has one kind, :interval, which is a timer — the runtime wakes itself up. Everything else worth subscribing to is the opposite shape: an external source pushes, and the runtime has to receive without the source knowing anything about rendering. That is one seam, and the milestone is organised around building it once rather than five times.

The ordering below is deliberate. Sub.telemetry and Sub.logger come first not because they are the most requested but because they are the same seam with two genuinely different consumers, which is the only way to find out whether the abstraction holds before four more callers depend on it.

Sub.telemetry — the integration that pays for the seam

:telemetry is how the entire serious Elixir ecosystem already reports on itself: Phoenix, Ecto, Oban, Broadway, Finch, Bandit. Integrate with that one thing and a Harlock app gets an ops UI for all of them. Attaching [:ecto, :repo, :query] and [:oban, :job, :stop] should be a working dashboard in about twenty lines.

Sub.telemetry([:ecto, :repo, :query], &{:query, &1.query_time})

Three constraints that shape the design rather than decorate it:

• Handlers execute in the emitting process. The handler must do the minimum — build a message and send it — or it slows down the very system it is measuring. All aggregation happens on the runtime side.
• Handlers are global. They must detach when the app stops, or a restarted app leaks a handler that sends to a dead pid.
• Attach a named module function, not a closure. :telemetry warns about anonymous handlers for real performance reasons; Harlock's own test suite prints that warning today, which is the mistake to avoid making publicly.

Needs a windowed aggregator — counts, rates, and percentiles over a trailing window — because raw events are useless in a table and something has to hold that state between frames.

Sub.logger — the same seam, proving it generalises

A :logger handler that forwards log events into update/2, giving a log viewer with level filtering and metadata search. Small, and it is the demo that needs no explanation.

It exists in this milestone mainly as the second consumer. If the seam built for telemetry also carries logging without special-casing, it will carry pubsub and port too. If it does not, better to learn that here than after the fourth caller.

Same detach-on-shutdown requirement, same do-not-block rule: a handler that blocks slows every log call in the system.

sparkline ✓

A one-line trend widget, because a numeric column is not a dashboard and a progress bar shows a fraction rather than a history. Takes a list of numbers and draws them with block glyphs, auto-scaled. Needed by the telemetry work above rather than desirable alongside it.

Shipped with the glyph ramp as an explicit :glyphs option rather than switching on capabilities: the internal caps struct tracks colour depth, bracketed paste, mouse and kitty, and has no notion of glyph coverage. Adding one would mean guessing — a mono terminal renders block elements perfectly well, so colour depth is not a proxy for it. Passing an ASCII ramp explicitly is honest and follows spinner's :frames.

The remaining Sub kinds — moved to v0.7

v0.6.0 shipped without pubsub, file, signal and port. The milestone's point was the seam, and the seam is done and proven against two deliberately different consumers, so holding a finished release for four additions that need no design work would have been holding it for nothing.

They are listed under v0.7 rather than becoming a v0.6.x series: each adds a public function to Harlock.Sub, and additive-in-a-patch is the tension this project already hit in v0.4.2. They sit alongside the hardening work rather than displacing it — that work is what v1.0 waits on, and it should not keep sliding to make room for small features.

box(focus_proxy: :child_id) ✓

Polish for the R2 visual story. With R2 default-on, :focusable lives on the interactive widget (the viewport, the tabs, the text input) — but the wrapping box is what users see. The focus_proxy: opt lets a box mirror a named child's focus state for visual highlighting only (border style, title style) without itself participating in focus traversal.

Shipped in the shared focus-styling path rather than as a box special case, so any element taking focus styling honours it. Not participating in traversal needed no work: focusable collection keys on :focusable, so a proxy is invisible to Tab by construction rather than by being filtered out afterwards.

examples/overview.exs has dropped the border_style/1 helper it used as a stand-in — a workaround referenced from its own comment, this section, and the v0.4 R2 review.

v0.7 — measurement, data access, and the generic seam ✓ (shipped as v0.7.0)

Not the milestone originally planned under this number. What shipped is the work that turned out to be ready: a benchmark harness, the textarea speedup it justified, windowed table rows, Sub.source/3, and property tests for the layout solver.

Three of those five changed a plan rather than executing one. The benchmark rescoped incremental rewrap downwards; measuring the table showed windowing was never a rendering optimisation; and Sub.source/3 deleted Sub.pubsub from the roadmap instead of implementing it.

Harlock.Bench ✓

Textarea wrap memoisation ✓

Windowed table rows ✓

Sub.source/3 ✓

Layout solver property tests ✓

v0.8 — pre-1.0 hardening (next)

Everything below is what stands between here and a 1.0 API freeze. It is the third time this work has been renumbered — v0.6, then v0.7, now v0.8 — because each time something finishable arrived first. That is defensible individually and a bad trend collectively, so this milestone gets nothing added to it: new features go to v0.9 or later, and 1.0 waits on this list alone.

The rest of the Sub kinds

Additive, and each follows the seam v0.6 established — a linked process owning the registration, a handler that only builds and sends, detach on the way out. No new design work, which is why they did not gate v0.6.0 or v0.7.0.

• Sub.pubsub(pubsub_mod, topic, transform_fn) — dropped, not deferred. Sub.source/3 covers it: Phoenix.PubSub delivers to whichever process called subscribe, and source/3 runs the caller's function inside the subscription's own process for exactly that reason.

  Sub.source({:pubsub, "orders"}, fn ->
    Phoenix.PubSub.subscribe(MyApp.PubSub, "orders")
  end)

  A named constructor would have added a phoenix_pubsub dependency to a terminal UI library in order to deliver messages the seam already receives — the case principle 7 exists for. The same shape covers :global groups, registries, GenStage consumers, and anything else that sends to its subscriber.

• Sub.file(path, opts) — watch via :fs if available, polling fallback. Stays first-party: the polling fallback and change-detection are real logic, not a seam.

• Sub.port(cmd, args) — long-running external process, stdout lines as events. Also stays: port lifecycle, partial-line buffering and exit handling are real logic.

• Sub.signal(:sigusr1, msg) — blocked on a verification, not on effort. Keeper's moduledoc claims :os.set_signal(:sigwinch, :handle) "routes {:signal, :sigwinch} messages to the most recent caller". That is the mechanism an app-level signal sub would build on, and it has not been checked against OTP's actual delivery path — signals are documented as going to erl_signal_server, which is global. Until someone confirms how Keeper really receives them on a POSIX host, building a second signal consumer means building on an unverified comment.

Their relative order should be decided by whichever real app needs one first, not guessed here. That is the mistake the v0.6 list originally made: Sub.pubsub led it because it was written first, not because anything needed it — and it turned out not to be needed at all.

Windowed aggregation for metrics

Rates and percentiles over a trailing window, for Sub.telemetry consumers. Deliberately scoped down by examples/dashboard.exs: counts, mean, min and max over a bounded list turned out to be three lines of Enum in the model, with nothing worth abstracting. What a helper is actually for is the part that needs time — per-second rates, and percentiles that cannot be computed incrementally from a running total.

Hardening

• Dialyzer clean at :underspecs + :overspecs. Strict specs on all public functions.

• Property-based tests — done for the layout solver, twelve properties over generated constraint lists: sizes non-negative, one rect per constraint, slots contiguous and non-overlapping, cross axis untouched, :max never exceeded, :length honoured with slack absorbed, :fill proportional to weights, over-constraint truncating rather than crashing, zero regions yielding zero slots, and determinism.

  One of them was wrong before the solver was: "a split always consumes the whole region" fails for [{:max, 0}], because filling the space would violate the cap the caller asked for. The solver is right and the invariant needed the condition, which is the sort of correction generated input produces and example tests do not.

  Running at StreamData's default 100 runs per property. Worth raising if the solver changes.

  The frame-diff half is not done, and needs a prerequisite. "Replay produces an equivalent frame" requires interpreting the ANSI the differ emits, and nothing in the tree can do that — the test backend's writer captures a cell buffer written through the renderer, not a terminal emulator that consumes escape sequences. Either a minimal ANSI interpreter gets written for tests only, or this property gets dropped as costing more than it proves. Deciding which is the actual open item; it is not a matter of writing the property.

• Benchmarks ✓ — Harlock.Bench measures render and diff over five canonical scenarios, reporting percentiles rather than a mean because a frame budget is about the slow frames. Public rather than dev-only, so app authors can measure their own views.

  First baseline, 200×80, 100 samples, one dev machine — useful for comparison against itself, not as an absolute:

  scenario	p50	p99
  text_rows	3 481µs	4 403µs
  nested_boxes	8 499µs	10 035µs
  table_rows	11 878µs	12 697µs
  tree_expanded	12 083µs	13 312µs
  textarea_wrapped	19 148µs	22 425µs

  Three things this establishes, all of which were assumptions before:

  A wrapped textarea already misses a 60 Hz budget. ~16 700µs per frame is the target and p50 is 19 148µs. Scaling confirms the cause is rewrapping the whole value: n=50 → 4 198µs, n=200 → 20 275µs, n=800 → 56 115µs, roughly linear in content. That is the number the incremental-rewrap item below now has to beat, rather than a suspicion.

  An unchanged frame is not free. Diffing two identical 200×80 frames costs 3 072µs at p50 — comparable to rendering text_rows outright, because the comparison walks all 16 000 cells with no early-out. The dirty-flag runtime only diffs when something changed, so this is not currently a live cost, but it caps how cheap a redraw can ever be.

  The layout solver is not the bottleneck. 24 levels of nested boxes cost less than half a wrapped textarea, which redirects optimisation effort away from where it might otherwise have gone first.

• Incremental rewrap for textarea — rescoped by what the benchmark actually showed, and now smaller than it looked.

  A single render called visual_rows/2 three times over: directly, then again inside visual_position/3, then again inside scroll_to_reveal/5. Removing that repetition needed no invalidation logic at all — visual_rows/2 keeps a one-entry memo keyed by the value and width themselves, so a hit requires the key to match and there is nothing to get stale. Editing replaces the binary, the next call misses, and that is correct.

  n paragraphs	redraw unchanged	while editing
  50	2 457µs	3 686µs
  200	5 734µs	10 444µs
  800	6 656µs	21 196µs

  Against the pre-memo 19 148µs at n=200, that is 1.8x on the editing path and 3.3x on redrawing unchanged content. n=200 now fits a 60 Hz budget; n=800 does not.

  What remains is bounded by profiling, not guesswork. At n=200 wrapping is 2 457µs of a 10 444µs render — 24% — and the rest is cell writing, which is bounded by region size rather than content. So a line-break index with per-line invalidation can only take a quarter off a mid-sized document. Its value grows with size, though, because wrapping scales with content while drawing does not: at n=800 wrapping is closer to half the cost. That makes it worth doing for large documents and not worth doing for typical ones — which is the opposite of the priority it had before it was measured.

• Windowed data access for table ✓ — :rows now accepts a fn offset, limit -> rows function as well as an enumerable, with :offset app-owned because keyset pagination has no row index to auto-centre on.

  The benchmark corrected the reason for doing it. Render cost is flat in row count already — 10 137µs at 200 rows, 8 908µs at 20 000 — because drawing is bounded by the region, and the length/1 / find_index/2 / drop/2 traversals the list path performs are trivial beside cell writing. So this was never a rendering optimisation.

  What it actually buys is not acquiring rows nobody will see. A list-backed table over a query issues a query for every row; a windowed one asks for viewport-many. The benchmark could not show that because it pre-built an in-memory list, which is exactly the kind of measurement that confirms the wrong thing.

  No Ecto.Queryable adapter, deliberately — see the principle below. A query-backed table is a few lines of app code over the window function, and building it in-tree would put a database dependency inside a terminal UI library.

• Documentation: every public function has @doc + example. Module guides (guides/getting_started.md, guides/widgets.md, guides/testing.md, guides/embedding.md).

• Examples expansion: filemgr (two-pane), todo (text_input + list), log_viewer (viewport + filter), git_branch_picker (tree).

• Caps refinement: detect terminfo entries properly; fallback table for common terminals. Document the Caps API as public.

• Public API freeze candidate: walk every @moduledoc false, decide stable-public vs internal-forever. Move stable parts to @moduledoc proper.

v1.0 — stable API

• Public API frozen per the @moduledoc decisions above.
• All v0.7 hardening complete.
• Announcement post + Reddit/Elixir Forum thread.
• Minimum supported: Elixir 1.19+, OTP 26+ — matching the elixir: "~> 1.19" requirement mix.exs already ships.

Out of scope (probably forever)

• Windows native console. The TTY layer is POSIX-only and that's fine. WSL works.
• Rendering anything other than monospace cells. No Sixel, no Kitty graphics protocol, no images. Different project.
• Web export. Different project.

Stretch / "would be cool"

• Harlock.LiveView — a Phoenix LiveView hook that renders the same element tree to HTML for remote viewing. Same model, two backends. Probably v1.x.
• Hot reload of the app module during dev. Possible because TEA is pure — re-invoke view/1 after code change. Needs careful supervisor handling.
• Recording / replay — log every event + final frame, replay deterministically for bug reports. Useful for headless testing too.

Notes for whoever picks this up

• The runtime is the spine. Don't add state to it casually — every field is load-bearing for focus/render/sub lifecycle. New features that need process state usually want their own GenServer, not a runtime field.
• The renderer is pure. Keep it that way. If you find yourself wanting IO.puts in there, you're doing it wrong.
• Always test the crash path. smoke_crash/0 is the template — kill a linked process mid-render, assert the terminal is restored. New IO paths get the same treatment.
• Read the App.Supervisor comment block before changing supervisor config. The rest_for_one + :temporary runtime + Keeper-first ordering is the entire correctness argument for clean teardown.