[teamleaderleo]

Leo: "likewise, the chatgpt desktop app, let's do their + symbol's capabiliities for our composer. we want to be having more command palette related capabilities and stff to be able to to be surfaced".

The New Tab + menu now offers Attach Screenshot…, Attach Window… and the Plan/Build row in the app, not only in the component gallery. #26859 added the rows behind two optional NewTabScreen props, capture and onPlusMode, and App passed neither, so the app hid them. This PR passes them:

• capture is the chat composer's existing composer.capture request (captureScreen). A cancelled pick returns no file and keeps the draft.
• onPlusMode(pick) (in newtab/screenActions.ts) sends the page's deferred chat.new for the picked harness, folder and model, the same request the page's mode chip sends first, then chat.mode. The harness switch holds the mode until that chat starts (harnessSwitch.pickMode), and the page's send continues that chat. If the page's chat has already started, only chat.mode goes. A refused mode rejects, so the menu stays open with its error.
• The + target follows the send's own order: a saved environment, then a machine, then no project, then this Mac's folder. A saved local environment picked after another machine or no project therefore offers Plan for its own folder. Plan/Build is offered only when the target is a project on this Mac, and onPlusMode refuses other targets.
• A Plan picked before the place changes is kept. A send to another machine or to no project starts a fresh chat.new, which cancels the deferred switch holding the mode. The page keeps the last mode pick the host took and passes it to onAsk as mode. onAsk sets it on the started chat (chat.mode) before chat.send; if the mode is refused, nothing is sent. Both cases were found by the exact-head reviews of 1d1379dd and bdfd3956.

No native change: both requests already exist in the host.

Stack: #26843 (component) -> #26616 (native catalog) -> #26859 (New Tab Add rows) -> this PR. Base lane/nx-plus-newtab-capabilities at 65e16dad1afc844fce09a2a184b755003f884f8e.

Reference: ChatGPT desktop's + menu (composer-plus-menu-plugins-apps-sites). Copy: the capability rows in +. Keep: our colours and glass, the existing menu styling. Do not touch: the menu component, the model picker.

Verification

• Regression first: cd webviews && bun test test/new-tab-plus-mode.test.ts fails 4/4 at 7b64e082 (the test commit: the screen actions have no onPlusMode) and passes 4/4 at the head.
• Review fixes, red then green:
  • bun test src/agent-session/acpmux/newtab/NewTabScreen.test.tsx -t 'no Plan for a no-project' fails at 47d642da (the Plan row stays after Do not work in a project).
  • bun test test/new-tab-saved-environments.test.tsx fails 3/20 at e95eccf4. It covers a saved local environment after another machine (Plan offered for its folder) and Plan picked before the place changes to another machine or to no project, where the send must go chat.new, chat.mode, chat.send. It passes 20/20 at the head, across 4 runs.
• At the head: NewTabScreen.test.tsx (23), new-tab-plus-mode (4), plusActions.test.ts (4), new-tab-saved-environments (20), new-tab-recent-project (11), new-tab-started-project (10), new-tab-ask-attachments (4), new-tab-ask-model (1) and new-tab-create-actions (2) pass. bun run check, scripts/web-lint.sh, python3 scripts/cmux-next/prepush-gate.py and git diff --check pass.
• Gallery plays, Chromium, light and dark, strict CLS 0: agent-pane.new-tab/attachment-menu and agent-pane.newtab-plus-capabilities ready, planning, files and skill all report layout shift 0 and no anchor movement. The only step problems are frames over 33 ms on a shared Linux host, which the gallery tests exclude as runner timing. One of them is consistent: the skill play's row click takes about 270 ms in both themes. That code is Composer: share New Tab Add capabilities #26859's and unchanged here. The 0.017892 menu-opening shift Composer: share New Tab Add capabilities #26859 reported is gone. It came from the shared anchor's first-frame sizing, which fix(ui): constrain popup height before first placement #26852 fixed and the rebased stack now includes.

Observed / Not run

Run on a Linux host: the tests, checks and gallery plays above. Not run: a native build, or capture in the real app on a fleet Mac. The change is web-only and uses existing host requests.

Before / after

Real gallery captures of the mounted App New Tab page (agent-pane.new-tab, variant attachment-menu, 900×560, synthetic fixture). Before is #26859 at 65e16dad; after is this head. The menu now lists Attach Screenshot… and Attach Window…, which the app wired. Plan/Build appears only once the selected harness advertises a plan mode, which this fixture doesn't; the component gallery's planning variant covers it.

Theme	Before (#26859 65e16dad)	After (43446b78)
Dark
Light

Text under the open menu reads through its glass in these Chromium gallery frames. That is the documented Chromium backdrop behaviour (ui/glass/glass.css), and the same in both columns.

Risk: normal. The change is web-only and wires two existing host requests to rows that already exist. A Plan pick follows the mode chip's own order (deferred chat.new, then chat.mode). A send that starts a fresh chat now waits for that chat and its mode before the prompt, as a send to another machine already waited for its chat.

Changelog

Added: New Tab's + menu attaches a screenshot or window and switches Plan/Build.

Agent session: agent session

[cmux-bot]

cmux status _{(one comment, edited in place; each line is updated by the job that owns it)}

CI fast guards failed on 43446b7809 (https://github.com/manaflow-ai/cmux/actions/runs/40780000462). It does not block the merge; a red guard merged into main breaks it for every open PR.
The log named no failed step; see the run.

Agents: python3 scripts/ci/guard_attribution.py fix applies the mechanical fixes locally.