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 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.

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.

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.