Show which messages are new when opening a channel, and highlight mentions #602

Opening a channel with new messages doesn't show which ones are new, and a message that mentions you isn't highlighted.

What was seen

• B came back to b-chan after A's mention: "Inside b-chan the new messages appear as a new group… There's no 'New' divider, unread line, highlight or jump-to-mention. I could only tell what was new from the timestamp."
• While B was in b-chan when the mention arrived: "the bubble wasn't highlighted… no unread line". Only the green chip with their name marked it.
• Grouping made it worse: three messages sent at 1:20, 1:23 and 1:24 sat under one "iamnbutler 1:24 PM" header, and the earlier times were no longer shown.

The sidebar badge worked live: a green "1" labelled "1 direct mention" on b-chan's row while B was elsewhere.

Not filed: badge across reload, plain unread dot

The testers also reported that the mention badge didn't survive a reload and that plain unread messages got no indicator. Both are probably the study rig. apps/app/src/attention.ts keeps what you've seen in localStorage from the profile's first visit (since), and each tester turn ran a fresh Playwright --isolated profile, so earlier mentions were treated as history. The badge also covers the unread dot. This hasn't been confirmed with a persistent profile.

Steps

1. Teammate A posts two messages in B's channel, one @mentioning B, while B is in another channel.
2. B opens the channel.

Expected: a "New" divider above the first unread message, the mention highlighted, and each message's time visible.

Map: ^903, ^386.

Found in the 2026-10-22 two-Mac pair study.