[brucesantos]

Summary

Every send rendered the pending message with its own avatar, author, and "Sending…" header, then collapsed it into the author group on acknowledgement. The timeline shifted on every message, and a message sent within the same second as the previous one rendered above it first.

• Grouping (timelineItems.ts, MessageRow.tsx): pending rows group exactly like acknowledged ones. A grouped pending row dims its body (60% opacity) and keeps "Sending…" for screen readers only, so the acknowledgement changes opacity, never height. The first message of a group keeps the inline "Sending…" beside its timestamp, as Fix pending message feedback #3543 intended, since nothing shifts there. This also applies to the thread panel.
• Ordering (channelWindowStore.ts): compareRelayOrder treats a pending send as the newest event in its second. Before this, the optimistic id sorted against hex ids and put the send above an earlier same-second message.

Not covered:

• Once acknowledged, same-second messages follow the relay's id tie-break, so they can still swap (Desktop: messages sent within the same second can swap places when acknowledged #8192).
• The relay event is signed with its own created_at (send_channel_message), not the optimistic row's second. A send right at a second boundary can therefore still move on acknowledgement if that crosses same-second neighbours, the grouping window, or a day divider.
• The avatar blink on new groups is separate (Desktop: avatars blink (blank → gray → photo) whenever a new message group renders #8193).

Related issue

Fixes #8194. Related: #3543, #8192, #8193.

Testing

• Unit: updated buildTimelineItems: pending messages group like acknowledged ones. Added test_live_pending_send_projects_after_same_second_window_events, which follows the same live-overlay path as the send mutation. Both fail without the fix.
• E2E: message-feedback-snapshots.spec.ts asserts that a pending continuation has no author header, announces "Sending…", and keeps the same row height through acknowledgement. It fails on main.
• Ran: desktop unit suite (6,975 passed), tsc --noEmit, and messaging.spec.ts plus the feedback spec (93/94; the one failure, "editing a thread root in single-panel view", is unrelated and passed 5/5 when run alone).
• Manual: rapid sends in a DM on the desktop app.

Before / after, ⅓ speed:

before-after-pr.mp4

buzz-review-completed

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[github-actions]

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