Main feed area — drag the panel's left edge to resize.
Data-bound thread panel: parent + replies rendered channel-style via MessageGroup, ComposerField reply input (mentions included — a thread's composer used to be MiniComposer, which excludes them outright), Recap above the composer (≥4 replies), and caller-owned width via use-panel-resize (rAF drag, no setState per move). Esc closes; focus lands in the composer on open. Attach is wired here to a fake upload seam (issue #1164): the "+" appears only when the host wires one, staged files ride onSend, and Send sits flush right because this surface has no Schedule split button to leave room for.
Main feed area.
The read-only branch (the viewer's own deleted root) with the same upload seam wired: a MiniComposer that genuinely disables, and deliberately no attach button — nothing can be posted to a frozen thread.
Main feed area.
The mention picker itself, pre-seeded with "@": agent (HOLOH, Scout) and people candidates, same picker the main channel/DM composer offers.
Main feed area — the chip that opened this thread is still running.
A live job's thread (issue #893): the composer names what a reply does instead of implying an instant effect — gone once the job settles or on any non-job thread.