#23139 cmux-next composer: a chat's unsent pictures and files come back when it is reopened or restored

Leo asked that a chat's unsent draft (its text, pictures and attached files) survive reopening the chat, and archiving or deleting it and then restoring it, not only a quit (#22999).

acpmux already keeps the draft's text in the session's meta, so this adds the chips:

• Native (AgentPaneDraftAttachments):
  • Each chat's chips are kept in app support under agent-draft-attachments/<sha256 of session id>/. Each chip's bytes are their own file, listed in draft.json. The file helpers are shared with the relaunch wake (AgentPaneAttachmentFiles).
  • Writes for one chat run in order on a per-session queue.
  • Limits: 100 chats (the oldest are pruned) and 64 MB per chat.
  • The page reaches it through two new ops on AgentPageProvider: chat.draftAttachments (write; answers the ids whose bytes the host holds) and chat.draftAttachments.get.
• Page (keptAttachments.ts):
  • Reads the chat's chips when the chat is shown.
  • Writes them 300 ms after they stop changing. A chip's bytes go once; after that only its id goes.
  • Flushes on pagehide, on hidden, and on unmount.
  • Sending clears the kept chips, and a write still pending is dropped then, so sent chips never come back.
  • A harness switch carries the chips over.
  • Quick chat keeps nothing.

Tests:

• webviews/test/composer-kept-attachments.test.tsx covers:
  • reopen;
  • archive/restore;
  • send clearing;
  • paste-then-send inside the wait;
  • a refusal putting chips back;
  • stale held ids;
  • flush on hide.
• ChatDraftAttachmentsTests covers:
  • a round trip through a fresh store on the same folder (relaunch);
  • clearing on send;
  • the byte cap.
• The fleet ran ChatDraftAttachmentsTests, AgentPaneRelaunchWakeTests and ChatScrollRestoreTests at this head.

Known limits:

• A brand-new chat with no session yet keeps nothing on tab close. Its text isn't kept either.
• After a harness switch, the old chat keeps a copy of the chips that moved.
• Two pages on one chat are last-write-wins.

Independent subagent review: PASS (the five findings from round one are fixed; two nits are fixed here: a refused write goes again, and the page's JSON is checked before encoding).

🤖 Generated with Claude Code

Agent session: cosmic-gray

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Summary by cubic

Keeps a chat's unsent pictures and files (its composer chips) across reopening the chat, archiving/deleting and restoring it, and relaunches. Previously only the draft text survived; now the chips are stored per chat on disk and restored when the chat is shown again.

• Host side (AgentPaneDraftAttachments): stores each chat's chips under agent-draft-attachments/<hashed session id>/; chip contents are separate files listed in draft.json.
• Page side (keptAttachments.ts): writes chips 300 ms after they settle, flushes on page close/hide, and clears them on send so sent chips never return.
• Adds chat.draftAttachments and chat.draftAttachments.get ops on AgentPageProvider.
• Shares the file-moving logic with the relaunch wake (AgentPaneAttachmentFiles).
• Refactors AgentPaneRelaunchWake to use the shared attachment file helpers.
• Limits: 100 chats kept (oldest pruned), 64 MB per chat. Quick chats keep nothing.

Known limits

• A brand-new chat with no session keeps nothing on tab close.
• After a harness switch, both old and new chats hold copies.
• Two pages open on the same chat conflict (last write wins).