oh-my-pi/docs/tui-core-renderer.md

TUI core renderer — the append-only contract

What you are dealing with before you touch the rendering engine. This is the companion to tui-runtime-internals.md: that doc maps the flow (input → component tree → render); this doc explains the render contract, why it is shaped this way, and the invariants you must not violate. Scope is the core engine only:

• packages/tui/src/tui.ts — frame pipeline, commit ledger, window math, emitters, cursor placement.
• packages/tui/src/terminal.ts — ProcessTerminal, capability probes, private-CSI reassembly.
• packages/tui/src/terminal-capabilities.ts — TERMINAL profile, sync-output / DECCARA / image detection.
• packages/tui/src/stdin-buffer.ts — escape-sequence reassembly.
• packages/tui/src/utils.ts — width/slice/wrap (the width model).
• packages/tui/src/kitty-graphics.ts + components/image.ts — inline images.
• packages/tui/src/deccara.ts — rectangular-fill optimizer.

Application-layer renderers (transcript, tool calls, session tree, editor, widgets) are out of scope — they live in packages/coding-agent. The one app-layer file that is load-bearing for this contract is transcript-container.ts, which implements the commit-boundary seam described below.

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1. The one thing to understand first

The renderer cannot observe the terminal's scroll position (ConPTY's probe lies; POSIX has no API at all). The previous engine tried to guess when it was safe to rewrite native scrollback, and every policy choice over that unobservable variable traded one failure family for another (yank ↔ flash ↔ corruption ↔ invisible-until-resize — see the git history of this file for the full war journal). The current engine removes the guess entirely: native scrollback is append-only.

We keep the transcript on the normal screen (native scrollback, native selection, transcript persists after exit). The engine maintains one ledger:

• committedRows (C) — frame rows [0, C) have been physically scrolled into terminal history. They are immutable: the engine never rewrites them, and components must never change them.
• windowTopRow (W) — the frame row mapped to grid row 0. The visible window is frame rows [W, W + height), repainted in place with relative cursor moves.
• commit boundary (B) — reported by the component tree per frame (NativeScrollbackLiveRegion): B = commitSafeEnd ?? liveRegionStart ?? frame.length. Rows below B may still re-layout and must not enter history.

Per ordinary frame: W = max(C, L − height), C' = max(C, min(B, W)), and the only bytes that ever touch history are the chunk frame[C, C') written at the scrollback seam. Scrollback therefore equals frame[0..C) — every row exactly once, in order, with its content at commit time. There is nothing to guess, nothing to defer, and nothing to reconcile: the scroll position is irrelevant because ordinary updates never rewrite anything a scrolled reader could be looking at.

What this costs (the accepted tradeoffs)

• A block that has scrolled past the window top cannot reflow in place. Blocks stay in the live region (below B) until they are final; a late mutation of committed content is ignored (the stale committed copy stays in history).
• A component tree that reports no seam gets shell semantics: whatever scrolls off is final. Shrinking such a frame into its committed prefix re-anchors the window and leaves the stale copy in history (§3).
• Inside multiplexers, a resize leaves the pane history wrapped at the old width (same as any shell output).

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2. The frame pipeline (what you are editing)

#doRender per frame:

1. Compose the frame (render(width)), collecting liveRegionStart / commitSafeEnd from the root children (absolute row indices).
2. Audit the committed prefix (findCommittedPrefixResync, skipped on geometry frames). Components must never re-layout rows below C, but real flows violate it (a TTSR rewind truncating a streamed block, an image-cap demotion shrinking a committed image) and the violation must not become content loss. The detector samples the prefix tail (up to 8 non-blank rows in the last 24, SGR-stripped): an in-place edit or restyle disturbs only the touched rows (≤1 mismatch ⇒ aligned ⇒ ignored — stale styling in history is the accepted artifact), while any insertion/deletion shifts every row below it including the tail (⇒ re-anchor C at the first changed row and recommit from there: history keeps the stale copy and gains a fresh one — duplication, never loss).
3. Classify: fullPaint (first paint, clearScrollback session replace, or geometry change outside a multiplexer — all user gestures) or update.
4. Window math as in §1. Two special rules:
   • Overlays freeze commits (C' = C): composited rows must never enter history; the hidden gap backfills via the chunk after the overlay closes.
   • Shrink into the committed prefix (L ≤ C): re-anchor W = max(0, L − height), reset C = min(B, W), keep the stale history above (no gesture, no erase).
5. Extract the cursor marker (strip-first: markers never reach the terminal, the prefix ledger, or the audit), prepare lines (width fitting), slice the window, composite overlays into the window slice only (screen coordinates — an overlay never touches the frame or the ledger).
6. Emit:

Emitter	Bytes	When
#emitFullPaint	clears + frame[0, C') + window rows	gestures only. clearScrollback ⇒ \x1b[2J\x1b[H\x1b[3J; otherwise ED22 (when supported) + \x1b[2J\x1b[H
#emitUpdate scroll-append	\r\n + new bottom rows + changed-row range	the rows leaving the screen are exactly the chunk, content untouched since painted
#emitUpdate in-window diff	relative move + changed-row range rewrite	nothing scrolls, nothing commits (cursor-only when nothing changed)
#emitUpdate seam rewrite	chunk rows + full window rewrite	commit advance, window re-anchor, hidden-gap backfill, mux resize

ED3 (CSI 3 J) is emitted in exactly one place — #emitFullPaint with clearScrollback: true — and is reached only by user gestures: session replace/branch/resume (requestRender(true, { clearScrollback: true })), resize outside a multiplexer, resetDisplay() (Ctrl+L). A gesture pins the user to the tail, so the snap is acceptable; multiplexers never get ED3 (it is a no-op there and a replay would duplicate pane history).

The ordinary update path never emits ED2/ED3 or an absolute cursor home — several terminal families snap a scrolled reader to the bottom on those.

The commit-boundary seam (the load-bearing app contract)

NativeScrollbackLiveRegion (tui.ts) is how a component keeps mutable rows out of history:

• getNativeScrollbackLiveRegionStart() — first row that may still mutate (everything below it, including root chrome rendered after it, stays in the window).
• getNativeScrollbackCommitSafeEnd() — optional deeper boundary: the append-only prefix of the live region (a streaming assistant message's settled rows). Without it, a single live block taller than the window would hold its head out of history until it finalizes.

TranscriptContainer implements this for the coding agent: finalized blocks freeze (their render is snapshotted, so their content can never drift after the engine may have committed it), still-mutating blocks (isTranscriptBlockFinalized?.() === false) anchor the live region, and deriveLiveCommitState derives the commit-safe end of the first live block from two independent signals:

• append-only detection — a block observed growing without visibly rewriting an interior row commits its full body; a rewrite suspends this for VOLATILE_REARM_FRAMES clean frames.
• stable-prefix ratchet — rows that stayed visibly identical for a full STABLE_PREFIX_COMMIT_FRAMES window commit even while the block's tail keeps rewriting (a task tool's static prompt above a ticking progress tree). Without it, one perpetually animating row holds the whole block out of history, so a block taller than the window reads as cut off (head neither committed nor on screen) for the entire run. The ratchet tracks the window-minimum common prefix; a rewrite above the promoted run retreats it to the divergence, and rows that already committed are the engine audit's problem (recommit → duplication, never loss).

Freezing is unconditional — it is the engine's required guarantee, not a per-terminal optimization.

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3. Invariants — MUST / NEVER

1. NEVER add a new CSI 3 J (ED3) callsite. ED3 flows only through #emitFullPaint({ clearScrollback: true }), only for gestures, never inside multiplexers.
2. NEVER rewrite a committed row. No emitter may touch frame rows < C, and W ≥ C always (re-showing a committed row on the grid duplicates it for a scrolling reader — the historical corruption family). When a component violates immutability, the audit (§2) degrades to duplication — never silently skip rows, never erase history.
3. Commits are exactly the chunk. Any byte shape that scrolls the screen must scroll only rows accounted for by C' − C — that is what makes scrollback provably frame[0..C).
4. NEVER probe the viewport position or fork on platform in the update path. win32 behaves like POSIX. The probe APIs are gone; do not reintroduce them.
5. Mutable content stays below the commit boundary. App-layer renderers must finalize-before-commit; the engine trusts B and clamps, it does not verify content.
6. Park the hardware cursor at real content bottom, not the padded window bottom, or height shrinks scroll live rows into history and duplicate them per resize step.
7. Cursor writes live inside the synchronized-output frame, before ESU — never as a second frame after it.
8. NEVER throw in the render hot path. Clamp over-wide lines (truncateToWidth); a width mismatch is cosmetic, not fatal.
9. Multiplexers get no destructive clear and no history rewrap on resize — repaint the window in place; pane history keeps its old wrap.
10. Any change to the ledger math, the emitters, or the seam must be validated by the stress harness (§6) across its full scenario matrix, not by a single-terminal smoke test.

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4. Terminal capability detection

TERMINAL (terminal-capabilities.ts) is resolved once at import from TERMINAL_ID plus environment sniffing; detection helpers are pure over (env, platform) and unit-testable.

• shouldEnableSynchronizedOutputByDefault(env, id) → DEC 2026 default. Precedence: user opt-out (PI_NO_SYNC_OUTPUT/PI_TUI_SYNC_OUTPUT=0) → user force-on (PI_FORCE_SYNC_OUTPUT=1/PI_TUI_SYNC_OUTPUT=1) → TERM_FEATURES advertises Sy → WT_SESSION → known direct terminals → off for risky multiplexers and unknowns. Reconciled at runtime by the DECRQM mode-2026 report; a user override still wins.
• detectRectangularSgrSupport(id, env) → DECCARA fills: kitty only, off in multiplexers and under PI_NO_DECCARA.
• supportsScreenToScrollback → kitty's ED22 (used once, on the initial paint, to preserve the pre-existing shell screen).

The old ED3-risk classifier (eagerEraseScrollbackRisk, PI_TUI_ED3_SAFE, submitPinsViewportToTail) is gone: behavior no longer depends on which terminal is rendering, so there is no risk class to detect. Env sniffing now only selects optimizations (sync output, DECCARA, images), where a miss is cosmetic, not corrupting.

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5. Width model

visibleWidth / truncateToWidth / sliceByColumn / wrapTextWithAnsi (utils.ts) all route through one native UAX#11 engine (@oh-my-pi/pi-natives, Rust unicode-width). Bun.stringWidth was dropped deliberately — mixing two width models in measure-vs-slice produced crashes.

• Fast path: printable ASCII is one cell per code unit.
• ZWJ pictographic emoji take the visibleWidthByGrapheme override.
• OSC 66 sized text takes the native path.

Rule: any new measuring code routes through these helpers, and the hot path clamps instead of throwing. Known residual: combining-heavy scripts (Arabic harakat) survive painting verbatim, but ghostty-web's cell readback can migrate non-spacing marks across cells — the stress harness compares those rows with marks stripped (sameLinesAllowingMarkDrift).

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6. The fidelity gate (use it)

packages/tui/test/render-stress-harness.ts drives the renderer's real emitted ANSI into a ghostty-web VirtualTerminal across randomized op sequences and parameterized terminal shapes, and validates the contract with a shadow commit ledger: an independent reimplementation of §1's math, fed only by observed frames (a render wrap) and observed bytes (a write wrap). Per op it asserts:

• the whole tape (scrollback + grid) equals shadowTape + window slice, row for row, including across resizes;
• scrolled readers stay pinned and visible history rows are never rewritten;
• multiplexer pane history grows by exactly the committed chunk;
• sync-output/autowrap bracket discipline, cursor parking, background columns, duplicate accounting.

Run it — plus render-regressions.test.ts, streaming-scrollback-defer.test.ts, and the issue-*-repro.test.ts files — before changing ledger math, emitters, or the seam. A change that passes one terminal and one seed is not verified.

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7. Capability probes & stdin reassembly

ProcessTerminal fuses capability queries with a bare DA1 (CSI c) sentinel so a non-answering terminal is detected when DA1 returns first. Replies can arrive split across a stdin flush, so:

• #privateCsiResponseBuffer accumulates \x1b[?… partials while a sentinel is outstanding, rejoins on the terminator byte, then runs the handlers on the complete reply. A new \x1b mid-reassembly or >256 bytes abandons the partial so real keys still reach input.
• #da1SentinelOwners is a typed FIFO discriminated by kind so a keyboard DA1 cannot be mistaken for an OSC 11 / DECRQM / graphics-probe sentinel.
• DECRQM probes (2026/2048/2031) drive runtime feature gating.

Rule: any new probe must own a typed sentinel and survive a split reply (feed the reply byte-by-byte in a test and assert nothing leaks to input).

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8. Inline images & memory

Kitty images are transmit-once, place-many (kitty-graphics.ts). ImageBudget keeps only the most-recent N images live; when the cap is exceeded the demoted image's pixels are deleted by id (a=d,d=I) and its visible rows re-render as the text fallback through the ordinary window diff — no destructive replay. A demoted placement already committed to history simply loses its pixels (committed rows are immutable), and the text fallback is height-preserving once a graphic has rendered (reserved rows + fallback line), so demotion never shrinks the block and never shifts committed content below it.

Rule: never re-emit full base64 per frame. Kitty Unicode placeholders are default-on only for kitty/ghostty (PI_NO_KITTY_PLACEHOLDERS / PI_KITTY_PLACEHOLDERS).

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9. Escape hatches (env vars)

Var	Effect
PI_NO_SYNC_OUTPUT=1	Disable DEC 2026 BSU/ESU wrappers (autowrap discipline stays on).
PI_TUI_SYNC_OUTPUT=0|1 / PI_FORCE_SYNC_OUTPUT=1	Force sync output off / on.
PI_NO_DECCARA	Disable Kitty DECCARA rectangular-fill optimization.
PI_FORCE_IMAGE_PROTOCOL=kitty|iterm2|sixel|off	Override image protocol detection.
PI_NO_KITTY_PLACEHOLDERS=1 / PI_KITTY_PLACEHOLDERS=1	Force Kitty Unicode placeholders off / on.
PI_HARDWARE_CURSOR=1	Show the real hardware cursor instead of a rendered one.
PI_NOTIFICATIONS=off|0|false	Suppress terminal notifications.
PI_DEBUG_REDRAW=1	Log the chosen render intent + ledger state per frame to the debug log.

Removed with the old engine: PI_TUI_ED3_SAFE (no ED3-risk lever exists), PI_CLEAR_ON_SHRINK (shrinks always clear exactly), PI_TUI_DEBUG (per-render dump superseded by PI_DEBUG_REDRAW ledger logging and the stress harness replay/reduce tooling).

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10. Before you touch the render core — checklist

• Are you about to emit CSI 3 J anywhere other than the gesture-driven clearScrollback full paint? Stop.
• Could any code path rewrite, or re-show on the grid, a frame row below committedRows? Stop.
• Does your byte shape scroll rows that are not the commit chunk? That breaks scrollback == frame[0..C).
• Are you adding a viewport probe, a platform fork, or a terminal-brand branch to the update path? The contract exists so none are needed.
• New mutable UI above the editor? It must report (or live inside) the live-region seam, or it will freeze at first commit.
• Did you run the stress harness and the repro suite across the full scenario matrix — not just one terminal and one seed?
• New probe? Typed sentinel owner + split-reply test.
• New width path? Routed through the shared native engine, clamped (never thrown) in the hot path.