ADR-0009: The responsive layout rule is a shared pure function

• Status: Accepted, not implemented (the TUI is deferred, ADR-0112)
• Date: 2026-06-24
• Deciders: jka
• Decision: D9

Context

Both frontends must decide how side panes arrange themselves as the available width changes. If each frontend encodes the rule itself, the GUI and the TUI will drift apart.

Decision

The collapse rule, (width, panes) → layout, is a pure function in the core that both frontends call. Only the rendering differs: Tk geometry managers in the GUI, Ck geometry managers in the TUI. ADR-0014 defines the tiers the function returns.

Consequences

• The layout policy exists once and can be tested without a display.
• The function has not been built: the GUI currently arranges its panes through the dock-site layout of ADR-0035, and the TUI that would consume the shared rule is deferred. The decision stands for when a second frontend exists.
• The Ck spike found that Ck reports resizes through <Expose> rather than <Configure>, and that a re-layout must be guarded by an actual width change to avoid a repaint loop. A TUI rendering this policy inherits that rule.