145. A dropdown is as wide as what it drops from

Date: 2026-08-19

Status

Accepted. Adds one parameter to ADR-0104’s measure-then-place, for the one caller that needs it.

Context

select opens a list measured from its content. A field stretched across a form therefore opened a panel as wide as the word “Dark”, hanging off its left-hand end — which reads as a mistake rather than as a menu. Every dropdown on every desktop is at least as wide as the control it drops from.

No measurement of the content can produce that number: it is a fact about the anchor, and the anchor is the caller’s.

Decision

Host.popup(content, anchor, placement, minimumWidth) — a floor, not a width. The content still decides the rest, so an option longer than the field widens the list past it, which is the other half of the same rule.

The floor is applied inside the existing two-pass measurement rather than beside it. Pass one is the content’s natural size, as before; pass two lays it out with a definite width — which is what both the window-width cap and this floor need, and which is what makes the content fill the floor rather than merely be placed in a wider window. Content with a width of its own, which will not stretch, still gets the window the floor asked for: the floor is about where the popup’s edges are, not about what is drawn inside it.

Opt-in per call, and not a property of Placement. It is false for the other two callers: a menu is as wide as its commands and a tooltip as wide as its text, and neither has any business being as wide as the thing it points at. Placement is about position; this is about size.

Consequences

select passes its own painted width, which it already knows because SelectField is Located (ADR-0141). No new plumbing, and no id to anchor by.

The floor is bounded by the window’s width, like the cap it shares a pass with: a popup wider than the window it belongs to is not what any anchor meant.

Host grew a method rather than a default, so every implementation says what it does with the floor. There are two stubs in the test suite and they were the only cost.