ADR-0018: UI contributions are declarative

• Status: Accepted, not implemented (the general plugin platform is deferred)
• Date: 2026-06-24
• Deciders: jka
• Decision: D18

Context

A plugin that draws its own Tk widgets would not work in the Ck TUI, and a plugin author should not need to know which frontend is running. This is the hardest part of any plugin system that spans more than one frontend.

Decision

Plugins contribute structured, semantic UI, such as "add a navigation section showing this tree", "add a status item", "decorate these ranges", or "add a panel with this list". They never supply raw Tk or Ck widget code. Each frontend renders the same declarative contribution in its own toolkit.

Consequences

• Plugins stay portable and GUI/TUI parity is preserved by construction.
• The concrete schema and vocabulary for declarative UI are still open.
• The model is unsuitable for behaviour that runs on every keystroke. Editing modes were therefore given their own frontend-code seam instead (ADR-0038).
• The dock-site system (ADR-0035) is the intended render target for a plugin-contributed panel.