[BUG] Sidebar mixes project and standalone chats with no project label after Projects upgrade (follow-up to #87346)

Follow-up to #87346, which was auto-closed for inactivity although the problem is still present.

Same problem here, made worse by the Projects upgrade.

After my classic claude.ai project was upgraded to the new Projects format (Oct 3, 2026):

• The left sidebar shows the project's old chats mixed with standalone chats, with no indication of which project (if any) each one belongs to, and no way to group or filter by project.
• Inside the upgraded project, the old chats appear as threads together with the new threads, but standalone (orphan) chats never appear there, and the new threads never appear in the sidebar.

Result: there is no single place where I can see everything, and no way to tell which sidebar chats are orphans without opening them one by one. Before the upgrade, chats were grouped by project.

Requests:

1. Group or filter the sidebar by project, or at least show a project label on each chat.
2. Document what the upgrade does to classic project chats, and whether it can be reverted.

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