PR #8181 - superset-sh/superset

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Summary

Chat v3 composer work:

Composer height cap - a long draft no longer pushes the send button out of the pane.
Keyboard - / and ⌘J focus the composer, ⌘U attaches a file, ↑/↓ recall sent prompts, ⌘/Ctrl+Enter sends and steers, ⇧Tab moves into the queue.
Server-side steer - a prompt sent with steer: { expectedTurnId } is steered by the host in the same command that queues it.
Attachments reach the agent - attached files go to the worktree's .superset/attachments and travel as <attachment path="…" type="…" /> tags in the message text, so every harness can read them. The UI parses the tags: images as thumbnails, other files as name badges.

A long draft grew the editor past the pane and clipped the send button.
There was no keyboard path to the composer, to past prompts, or to steering.
The Claude and Codex harnesses never sent attachments to the agent: claudeAdapter turned one into [name], codexAdapter dropped it. Only ACP resolved them.

Composer (packages/chat-ui)

.prompt-input-editor gets max-height: 40vh; overflow-y: auto.
utils/historyNavigation registers ↑/↓ on the Lexical editor. ↑ recalls when the box is empty or the caret is at the very start; after that it walks history while the recalled text is unedited. Past the newest, ↓ restores the draft as EditorState, so chips survive. Once a recalled prompt is edited, the arrows stop navigating so the edit is kept.
The submit payload gains steer: boolean (⌘/Ctrl+Enter); IME composition never submits.

Steering (packages/chat, packages/chat-runtime)

promptInputSchema gains optional steer: { expectedTurnId }. LiveSession.prompt steers the queued item when the running turn still has that id; otherwise it only queues.
The outbox entry carries the flag, so a retry sends it again. The desktop passes the running turn id and has no client-side steer logic.

Desktop chat

FOCUS_CHAT_INPUT and CHAT_ADD_ATTACHMENT were registered but unhandled; the active pane's composer now handles them through useHotkey. / stays a plain listener (a registry entry holds one binding per platform) with the same IME/AltGr guards.
⇧Tab from the editor focuses the newest queued row: ↑/↓ move, Enter steers, ⌫ removes, E edits, Esc returns.
History comes from the timeline (utils/promptHistory, bookkeeping blocks and attachment tags removed) inside useStableList, so the memoised Composer does not re-render on every item.
Attachments: uploadPastedFiles (moved from TerminalPane to usePaneRegistry/utils) writes the files; utils/attachmentTags formats and parses the tags; thumbnails load through filesystem.readFile, capped at 5 MB, falling back to a name badge.

Shortcut	Action
`/`	Focus the composer (active pane, focus not in a field)
`⌘J` / `Ctrl+Shift+J`	Focus the composer (rebindable)
`⌘U` / `Ctrl+Shift+U`	Attach a file (rebindable)
`↑` / `↓`	Recall previous / next sent prompt
`⌘`/`Ctrl` + `Enter`	Send and steer (sends normally when idle)
`⇧Tab`, then `↑` `↓` `Enter` `⌫` `E` `Esc`	Queue: move, steer, remove, edit, back

Verified in the dev app over CDP:

60-line draft: editor 401px (40% of the window), send button inside the pane
Pasted PNG + "What color is this image?": file in .superset/attachments, thumbnail shown without the raw tag, claude-code agent ran Read .superset/attachments/red-square.png and answered "Red"
⌘J from the page body focuses the composer
↑ then ↓ restores a draft that starts with a /autoplan chip
(walkthrough page) / focus, ⇧Tab + ↑ + Esc, ⌘Enter steer order, history recall, layout of images and file badges

Still to check:

packages/chat: 42 pass. packages/chat-runtime: 181 pass, including two new LiveSession tests (steer for the running turn interrupts it and runs next; steer for another turn only queues).
packages/chat-ui: 4 pass (new historyNavigation tests, headless Lexical). The package had no tests before; it now has a test script and excludes tests from tsc, like packages/chat.
apps/desktop (usePaneRegistry): 232 pass, including attachmentTags, promptHistory, uploadPastedFiles.
tsc --noEmit for chat, chat-runtime, chat-ui: clean. apps/desktop typecheck: no errors in changed files.
bun run lint on changed files: clean. sherif: clean.

Steer on the host: the client-side "send, wait for the echo, then steer" needed three guards and still had a race between the check and the action.
Attachments as text tags: one plain path works for every harness with no adapter changes, and the files live in the workspace where agents already read.
⇧Tab into the queue: ⌘↑/⌘↓ is the macOS jump-to-start/end shortcut. ⇧Tab was the brainstorm-page pick and keeps roving focus for screen readers.
Assistant text colour unchanged: an earlier commit set it to full foreground; it is reverted here and left to settle with Avi (#8163).

Thumbnails crop to a square and cannot be enlarged yet.
A recalled slash command such as /review opens the command menu, so the next ↑ moves in that menu.
The 40vh cap is window-based, not pane-based.
Attachments stay in the worktree until it is deleted (gitignored).
steerInputSchema in the protocol is still unused.

prompt input gains an optional steer. Older clients never send it, so behaviour is unchanged. An older host strips the unknown key (non-strict zod object) and queues the message without steering.
No host-service procedure was added or changed. Older messages with structured attachment parts still render as name badges.

New Features
- Navigate previous prompts with the Up and Down arrow keys, with drafts restored when returning to the editor.
- Steer a running chat turn with a new prompt using the keyboard.
- Navigate, edit, remove, and select queued prompts with keyboard shortcuts.
- Attachments appear in chat messages as image previews or file labels.
- Focus the prompt editor and manage attachments with keyboard shortcuts when the chat pane is active.
Improvements
- The prompt editor scrolls when its content exceeds the available height.