Stepper field (external piece)

stepper(value) is a numeric stepper field: a text field with increment/decrement arrows, bound two-way to any Binding<f64> (a Signal<f64>, a day-model Field, or an app’s own binding over a selection). Each arrow click, and each settled edit (Return, focus loss), lands through write_commit, so under an undo stack every click is one undoable step.

Authoring

use day_piece_stepper::stepper;

let width = Signal::new(1.0f64);

labeled(
    tr("stroke_width"),
    stepper(width)
        .range(0.0..=64.0)     // typed and stepped values both clamp (default 0..=100)
        .step(1.0)             // one arrow click (default 1)
        .decimals(0)           // fraction digits shown (default 0)
        .key("insp-stroke-w"), // the field's dayscript id, both idioms
)

• .native() / .composed() / .idiom(…) pin one idiom; the default is Automatic: native where an arm exists (support() answers Native on appkit, gtk, qt), composed everywhere else. Native is literal: pinned on a toolkit with no renderer it draws Day’s visible placeholder.
• .key(…) names the field for dayscript on both idioms (the composed row’s wrapper is a layout node no toolkit realizes, so Decorate::id on the piece would tag nothing).

Driving and asserting from dayscript

The native leaf accepts input: (typed text, parsed and clamped), set_value: (ValueChanged previews, ValueCommitted commits), and mirrors its state into the probe through day-core’s set_probe_value hook, so assert_text sees the display form (fmt_value’s fixed-decimal text) and assert_value the number, exactly like a built-in control. The composed idiom is a real text field and needs nothing special. One script drives both idioms by the same key.

Verification status

The AppKit, GTK and Qt arms plus the composed row are exercised by Day-Sketch’s walkthrough (the stroke-width row of its inspector). There is no XAML or web arm yet. Both platforms have native spin controls (a WinUI NumberBox, <input type="number">), and arms for them are welcome, but the composed row already behaves correctly there, so they are an upgrade rather than a gap.