feat(composer): hand-rolled contenteditable editor engine

[rpvilo]

PR 2 of 3 — the replacement editor engine, stacked on #32.

Composer.Textarea is now a hand-rolled contenteditable over a flat {text|chip}[] segment model; the TipTap implementation stays mountable as Composer.TextareaLegacy for side-by-side comparison until PR 3 deletes it.

Design invariants: the browser owns text editing (native typing/IME; model syncs from the DOM via read → diff → commit); programmatic ops are the only imperative DOM writes; engine state lives outside React (version-counter subscription, chip visuals via portals into engine-owned spans).

• segments.ts — pure kernel: splice/slice, mapPosition (PM bias semantics), diffFlatText, planBeforeInput allow-list policy, snapshot converters (engine-portable under the ComposerSnapshot brand)
• trigger-tracker.ts — sticky-token port of the PM plugin (spaces in query, dismissedAt suppression)
• editor-dom.ts — DOM read-back (padding-<br> pact, NBSP normalization, badge-span transparency), chip-span reuse keeps React portals stable
• use-composer-editor.tsx — engine factory + hook; full RegisteredEditor; IME composition guard incl. keyCode-229 Enter protection; blocked native undo (parity: the TipTap setup shipped no History extension)
• New props: maxLength (chip = 1), required, name (hidden-input FormData mirror), forwarded natives (spellCheck, enterKeyHint, inputMode, dir, aria-*); baked-in role="textbox"/aria-multiline

Accepted divergences: onValueChange emits "\n" per soft break (legacy: "\n\n"); copy/cut emit chip-markdown; in-editor text drag-drop denied in v1; IME maxLength enforced at composition commit.

Verified: 156/156 tests (68 new), tsc --noEmit clean, biome clean, package builds. Store/command-list/popover/container untouched.

Manual checklist (before merge)

Compare against legacy by pointing the styled ComposerTextarea in components/ai/composer.tsx at ComposerPrimitive.TextareaLegacy.

• Basics: typing, Enter submit, Shift+Enter soft break, placeholder toggle, autoFocus, disabled
• Commands: @ opens; badge + "Type to filter" hint; spaces in query keep it open; Escape dismissal suppression; ArrowLeft/Right caret trap; click-outside closes; Tab/Enter select → chip + trailing space
• Chips: backspace/forward-delete remove whole chip; arrows skip as one unit; range-select delete; copy/cut/paste round-trip (chip markdown); paste files → attachments
• IME (macOS Safari + Chrome): Hiragana + Pinyin composition; Enter confirming a conversion must not submit; badge behavior around composition; long-press accents; dictation
• Ask-user: arrows navigate options + editor blurs; type-to-answer refocuses; single-select clears on typing
• Form: name mirrors serialized text into FormData; required blocks empty submit
• Multi-line: type past max-h cap → scrolling; trailing empty line reachable

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Project	Deployment	Actions	Updated (UTC)
intentface-chat	Ready	Preview, Comment	Jul 10, 2026 1:06pm

[rpvilo]

Added the native-textarea callback surface: onFocus/onBlur/onKeyDown/onKeyUp/onPaste/onCopy/onCut now route to the editable element (not the wrapper) and run before the engine — preventDefault in onKeyDown/clipboard handlers overrides the engine's handling, exactly like a native textarea. Checklist addition: verify a consumer onKeyDown that preventDefaults Enter blocks the submit.

[rpvilo]

Two more additions from testing:

1. Soft break dismisses an active trigger token (the screenshot bug): the tracker's sticky range was absorbing inserted newlines, dragging the badge across lines and keeping the popup open. Now: Shift+Enter while the popup is open dismisses the token Escape-style (with re-entry suppression) and then inserts the break; the tracker additionally enforces that a token can never span a line break (covers pasted newlines too).

2. submitOn prop — "enter" (default, current behavior) or "shift-enter" (Enter breaks, Shift+Enter sends). The mapping lives in the pure interpretEditorKey kernel; the send chord is suppressed while the command popup is open in both modes.

Checklist additions:

• Repro the screenshot case: @query → Shift+Enter → popup closes, break inserts, badge doesn't fragment across lines; caret back into the token does not reopen it
• submitOn="shift-enter": Enter breaks, Shift+Enter sends; with popup open, Enter selects and Shift+Enter soft-breaks

[rpvilo]

Fix for the reported disappearing-editor bug (98fe977): line-break parity invariant. Browsers add/remove <br>s freely during native edits — emptying the editor can leave no <br> (the unpadded editable collapses to zero height, hiding the caret and the placeholder overlay, which sizes to it → looks like the whole Textarea vanished), and typing before the padding <br> strands it mid-content where later reads count it as a phantom newline (the rapid-backspace line breaks). Now every native sync compares the DOM's <br> count to the canonical rendering and repaints from the model on divergence. Plain typing never diverges, so the fast path is untouched; IME guard applies.

Checklist addition:

• Repro: mentions back-and-forth + rapid backspace to empty — the editor line, caret, and placeholder must survive; no phantom line breaks accumulate

[rpvilo]

Fix for the two-line-editor screenshot (hasContent port bug): legacy computed text.trim().length > 0 || !instance.isEmpty — the second half made any structure count as content. My port replaced it with a chips check, silently dropping the whitespace-only case: a doc holding just "\n" (Shift+Enter on empty, or newline residue from delete flows) reported hasContent: false, so the placeholder painted over a legitimately two-line editor. Now hasContent = documentLength(doc) > 0 — exact legacy parity and native-textarea parity (placeholder hides on any character, including a newline). Repro for the checklist: Shift+Enter in the empty composer → placeholder must disappear, caret on line two, backspace restores the placeholder.

[rpvilo]

Heads-up: this PR's squash landed on feature/composer-editor-seam (its base at merge time), not main — the engine never actually shipped. Superseded by the re-land PR from feature/composer-editor, which also carries the post-merge bug fixes.