Sidebar: optional date headers (Today / Yesterday / Previous 7 days) for active threads

[sergiiopm]

Problem

I work across about 8 projects at once, so I keep all of them visible instead of filtering. Threads pile up, and a lot of them are ones I want to come back to in a few days. Right now the only way to keep track of them is pinning, and with 7 pinned threads the pins already take more than half of the sidebar.

What I'm missing is a sense of when. When I'm looking for "the thread I was on yesterday", I scan every row and read every time label.

Idea

An opt-in setting that splits active threads under date headers:

• Today
• Yesterday
• Previous 7 days
• Older

Headers fold like Settled and Snoozed already do. Pinned, Snoozed and Settled stay as they are.

This only works if the list follows activity, which is what #6964 asks for (I left the details there). So I see it as a step on top of that: with "sort by last message I sent" on, the same order just gets headers. Without that sort, headers don't make sense, so the option would depend on it.

Scope I have in mind

• Off by default, nothing changes unless you turn it on
• Web/desktop only, mobile unchanged
• Computed on the client from the timestamp the row already shows (latestUserMessageAt, falling back to updatedAt), so nothing new stored on the server or on threads
• Folded state saved per client, the open thread stays visible even inside a folded group, and search still covers everything
• Day boundaries use local midnight and update when the day changes

Questions

• Would you rather have this as part of the sort option from [Feature]: Allow Sidebar V2 threads to be sorted by recent activity #6964, or as its own toggle (e.g. on the new Beta page from feat(web): boring sidebar groups threads into project folders #21039)?
• Are these four buckets fine, or would you prefer fewer (e.g. Today / Earlier)?

Happy to build it with screenshots and a recording once there's a direction.