The draw.io editor with a workspace behind it. Diagrams belong to an account and keep a version history; each one can be shared through read-only or read-write links carrying an expiry, a revoke button and a last-used stamp. Concurrent editing is protected by a soft lock, and a save that arrives without it is never discarded — it is kept as a marked version. The editor itself is unmodified draw.io in a pinned container, so an upgrade is a tag change; the price, stated rather than hidden, is that there is no live co-editing.
Who it is for: Whoever needs a diagram to outlive the machine it was drawn on, and to reach somebody as a link instead of an attachment.
Source code: https://github.com/that-ugly-cat/draw