openchamber/packages/ui/src/components/chat/composer/DOCUMENTATION.md

# Composer

The chat composer: the prompt language, the editor that renders it, and
everything between typing and sending.

`ChatInput.tsx` (one directory up) is the orchestrator. It holds the composer's
own state and wires these modules together; it should not grow logic that
belongs to one of them.

`ChatContainer.tsx` keeps one `ChatInput` mounted while a new-session draft
becomes its first session. Draft-only UI first fades for 120ms while the editor
stays in place. The parent then moves the editor to its final session position
with a 180ms transform-only FLIP animation. Reduced-motion mode skips these
transitions. `session-ui-store.ts` marks sessions materialized from a submitted
draft, so selecting an existing session while a draft is open switches without
animation. Do not restore separate draft and session composer branches:
remounting the editor loses focus and interrupts the transition. Keep the
existing mobile fixed-position rules unchanged.

## Layers

| Directory | Owns |
|---|---|
| `language/` | What the text *means*: `@` references, `/` and `#` tokens, markdown, and which picker a caret asks for |
| `editor/` | The CodeMirror view that renders the language and owns the caret |
| `state/` | Composer state with a lifecycle: drafts, mobile shell, history, popup placement, draft targeting |
| `submit/` | Turning what the user has into what gets sent |
| `attachments/` | Files: paths, drop payloads |
| `ui/` | Presentation |
| `text.ts` | How inserted text meets the text already there |

## The prompt language

`language/` is the single source of truth for composer syntax. Everything that
needs to know what a token means — highlighting, send-time resolution, and the
autocomplete triggers — goes through it.

**This is the invariant that matters most in this module.** Before it existed,
the `@` rule was written four times with divergent cleanup and the `/` rule
three times with different valid character sets, so a token could be painted as
a reference and then not resolve as one. Adding a construct meant finding every
copy.

- `mentions.ts` — `@` references. The `start..end` span is the reference
  itself and is what gets highlighted; in `see @a/b.ts,` the comma is sentence
  punctuation, not part of the file being referenced. Mentions are plain
  editable text: deleting a character edits the token and reopens the mention
  picker, the same way `/skill` tokens behave — not an atomic delete.
- `prefixTokens.ts` — `/command`, `/skill`, `#snippet`. Scanning is deliberately
  generous; **membership in the command, skill or snippet registry is the
  authority**, not the pattern. An unknown `/token` stays plain prose.
- `triggers.ts` — which picker a caret position asks for. Exactly one can be
  active, with precedence `command > skill > snippet > mention`.
- `tokenize.ts` — one pass producing every highlight range. Adding a construct
  to the language means adding it here, once.

## The editor

`editor/` wraps CodeMirror. The document is a plain string: `getValue()` is
exactly what gets sent, so nothing downstream serializes a rich document model
back into a prompt.

The document is not, however, the string it was given: CodeMirror normalizes
line endings, so a `\r\n` pair becomes one break and the document ends up
shorter than the inserted string. **Never derive a caret position from the
length of text you are inserting** — a caret past the end makes `dispatch`
throw, the transaction never applies, and the un-normalized text stays in React
state to crash again on the next restore. Every edit that moves the caret goes
through `replaceWithCaret` (`editor/documentEdits.ts`), which measures the
change instead of the string.

The composer previously painted a transparent `<textarea>` over a mirror
`<div>`. That restricted highlighting to styles which do not change glyph
advance width — colour, background, underline — because anything else made the
mirror drift out from under the caret. Bold and italic were impossible, and the
overlay was disabled outright on mobile, where wrapped text drifted anyway.
**Those constraints are gone**; adding a width-affecting style is now a
question of design, not of feasibility.

Selection rendering: every device runs CodeMirror's `drawSelection()` — it
keeps typing on the drawn-selection code path, and removing it makes
CodeMirror enforce cursor association on the native selection, which iOS
answers with severe input lag. **That much is not platform-specific and must
not be undone.** What differs is who paints the selection, and
`composerSelectionExtension` (`editor/theme.ts`) picks that per platform.

When CodeMirror 6.43.9's iOS predicate does not match,
`composerNativeSelectionExtension` layers over `drawSelection()`: it re-shows
the native selection, and — only while a range is selected — the native caret,
hiding the painted layers those replace. The native selection is the one that
shows for two reasons: the painted layer sits behind the content, so tokens
with their own background (inline code, fences) cover it completely; and the
platform's selection drag handles attach to the visible native selection and
take their colour from the caret, so a transparent caret means invisible
handles. The range-only caret scoping is load-bearing — a native caret visible
while typing makes the browser re-render its caret UI after every keystroke,
felt as severe input lag.

When CodeMirror 6.43.9's exact iOS predicate matches,
`composerIOSSelectionExtension` leaves selection-handle geometry and appearance
to CodeMirror. CodeMirror puts the handles in `.cm-selectionLayer`, normally at
`z-index: -1`; the extension raises that layer above the content so opaque
token backgrounds cannot cover them, and leaves it transparent to touch.
The handle dots extend 8px past their range; matching scroller padding and
negative margin expand the clip area without moving the text or changing the
composer height. iOS still paints its taller system selection overlay even
when CSS makes `::selection` transparent. The extension therefore suppresses
CodeMirror's synthetic selection rectangles on iOS while leaving its handles,
cursor path and `nativeSelectionHidden` facet active. Otherwise the grey system
highlight and themed rectangle overlap with visibly different heights.
Do not add a second custom layer or custom handles here: overlapping translucent
rectangles make selection darker at their seams and imitated handles drift from
the geometry WebKit actually manipulates. What iOS avoids is installing the
native-selection workaround above: explicitly restoring native paint and caret
makes WebKit re-measure them after every decoration redraw, and the composer
rebuilds every decoration on every keystroke. That cost is felt worst during
IME composition.

The non-iOS native selection tint comes from `--primary`, not the selection
token: themes define `--interactive-selection` with its own alpha, so mixing it
with transparent again is nearly invisible. The iOS system overlay owns its
visible selection fill.

The content element keeps the existing correction policy: on in the mobile UI,
off elsewhere. CodeMirror also reads the attribute and reverts Apple and
Android's insert-period-on-double-space only when its value is exactly `off`.
`editor/autocorrect.ts` uses the HTML standard's
[ASCII case-insensitive `autocorrect` keywords](https://html.spec.whatwg.org/multipage/interaction.html#attr-autocorrect)
to keep desktop word correction off while avoiding that CodeMirror-only
revert. Its platform checks deliberately match CodeMirror's own browser flags.

`composerLanguage.ts` retokenizes the whole document on every change. The
composer holds a prompt, not a source file: it is short enough that a full pass
is cheaper and far simpler than incremental mapping, and it keeps the editor
and the send path reading the same grammar.

## Ordering rules worth knowing

- `editor/ComposerEditor.tsx` forwards a click on the composer's padding by
  focusing the view *before* setting the selection: CodeMirror reveals its
  drawn caret through a class it only writes while applying an update, so the
  selection has to be the update that follows the focus.
- `submit/buildOutgoingMessage.ts` flattens queued messages, the composer text,
  context drafts and linked references into OpenCode's one-primary-plus-parts
  shape. The oldest queued message becomes primary. **Every attached context
  item (inline comments, terminal selections, browser annotations, PR context,
  linked issue/PR) becomes its own synthetic text part carrying structured
  metadata** built by `lib/messages/contextParts.ts`; the timeline reads that
  metadata back to render context blocks. PR instructions precede the PR diff.
  Queueing a message leaves context drafts in their store on purpose — the send
  that later delivers the queue consumes them.
- `state/useComposerDraft.ts` — a draft belongs to a (runtime, directory,
  session) identity. Writes are debounced while typing but forced at every edge
  where the page may stop running, because a pending timer is not a saved
  draft. Two orderings are load-bearing: the debounced write is skipped once
  while a draft is being restored, and a deleted draft's empty signature is
  recorded before a queued write could resurrect it.
- `state/useDraftTarget.ts` — the draft can target a directory that does not
  exist yet (a worktree being created). It must survive not appearing in the
  branch list, or the selector snaps back to the project root mid-creation.
- `ui/DraftTargetSelectors.tsx` owns the controlled project/worktree picker
  state and registers its application shortcuts locally. The selectors only
  consume their shared prefix while the draft target UI is mounted.

## Mobile

`state/useMobileComposerShell.ts` and `state/useMobileViewportPin.ts` are
mostly not state machines but corrections for specific platform behaviors:
mobile browsers dismissing the keyboard before a tap's click lands, iOS
refusing programmatic focus outside a gesture, WebKit leaving the layout
viewport panned after the keyboard hides, overlay chains handing off through a
frame where nothing is open.

**Every timeout and `flushSync` in them has a reason recorded next to it, and
none of them is verifiable outside a real device.** Change them only against
hardware.

## Testing

The package has no DOM test environment, so coverage stops at the state and
logic layers: the language, the submit assembly, path and drop handling, text
splicing, message history, and the CodeMirror language extension at the
`EditorState` level.

Rendering, focus, keyboard behavior, IME and WKWebView are **not covered by
tests** and are verified by hand. Do not report a change to them as validated
on the strength of type-check and unit tests.

Run tests per file (`bun test <path>`): `mock.module` is process-global, so
suites that install module mocks are order-dependent.