Unread marks and the New divider are per browser, so a new sign-in shows nothing new #677

Unread state lives in the browser, so a new sign-in (another browser, device or profile) shows no New divider and no unread mark for messages that arrived while you were away. Only mention counts, which the host keeps, survive.

What was seen

Two-Mac re-check on Canary 0.0.54.93 (build 7119e57, includes #645).

• Sam (owner of b-chan) closed Ace. Nate posted Plain note from Nate, ignore me and @samechocolixir can you check the README later? in b-chan. Sam signed in again in a fresh browser: the sidebar row was plain # b-chan before and after a reload, and opening it showed no New divider. Only the mention was tinted.
• The same happened the other way round in pair-hello-2 (Nate's channel): the sidebar had 1 direct mention, but opening the channel showed no divider, and Sam's plain message looked like old ones.
• In one continuous browser session it works: Sam opened b-chan, switched to b-hello, Nate posted, and on returning b-chan showed the New divider above Nate's messages.

useAttention keeps seen in localStorage (ace:attention:<host>:<user>), and a missing entry starts with since: Date.now(), so a new browser treats everything earlier as seen.

Expected: what you've seen in a channel follows you, not the browser, so a new window or device still shows where new messages start.

Map: ^903. Follows #602.

Found in the 2026-10-22 two-Mac re-check.