Corner-radius handle

The corner-radius handle is on-canvas chrome that edits a node's corner radius in place. It is a property handle: a drag on it changes the radius, never the node's size or position — the distinction from the transform handles (resize, rotate), which are the surface's default chrome and edit geometry. The interaction seam it rides — the hit tiers, the preview→commit phase model, and the stateless-chrome rule — is owned by surface.md; this document specifies only what the corner-radius handle adds.

Cardinality

The same property is presented at two cardinalities:

• Uniform (single handle) — one handle edits a single scalar radius applied to all four corners, for node types (and shapes) whose radius is one value.
• Per-corner (four handles) — four handles, one at each corner (the four intercardinal anchors), each editing that corner's own radius, for node types that expose four independent corner radii.

A per-corner handle's anchor (which corner) is part of its identity and feeds both the placement rule and the drag mapping.

Placement

A corner handle sits on the arc of the rounded corner, at the arc's center, so it visually tracks the roundness being edited. When the radius is small the arc center falls too close to the node corner to grab, so placement switches:

• Arc-center placement — at or above a threshold radius, the handle sits at the arc's center: offset inward from the node corner by the (scaled) radius on both axes.
• Margin-offset placement — below the threshold, the handle sits at a fixed inset from the node corner (lower-bounded so it clears the knob), decoupled from the tiny radius so it stays reachable.

The switch is a function of the current radius versus the threshold; crossing it moves the handle but changes nothing about the value. The inset is screen-space and constant in size across zoom.

Drag and clamp

A drag resolves to a signed scalar delta along the corner's inward diagonal: the dominant of the drag's two screen axes, signed from the anchor so that dragging inward (toward the node center) increases the radius and dragging outward decreases it. The uniform handle uses the same rule against a single axis.

The committed radius is clamped to [0, min(width, height) / 2] — non-negative, and no larger than half the node's smaller side (a larger radius is geometrically meaningless). The clamp applies on every preview, so the value the user sees never exceeds the node.

Link vs. split

The per-corner handle carries a modifier that chooses whether a drag edits one corner or all four:

• Link (default, no modifier) — while the four corners are currently equal, dragging any one corner drives all four together, keeping them uniform. A uniformly rounded rectangle stays uniform.
• Split (modifier held, or corners already unequal) — the drag edits only the anchored corner. Holding the modifier forces split even when the corners are equal; once they are unequal, split is implied and the modifier is no longer needed.

The rule: link iff (corners uniform ∧ modifier absent); otherwise split. "Round all corners the same" is the zero-effort path; "round one differently" is the deliberate, modifier-gated path — with no mode toggle.

Readout

While a drag is active, a decorative label shows the live radius next to the handle being dragged, and only that handle. It is pure chrome — no hit region — and vanishes on commit or abort, exactly like the measurement readout (measurement.md).

Deferred

• Multi-selection. Specified for a single bound node; editing corner radius across a heterogeneous multi-selection is out of scope.
• Rotated-node placement. Placement assumes an axis-aligned node frame; the local-frame placement for a rotated node follows the transform handles' rotated treatment (surface.md) and is deferred with it.

Contracts

• CRAD-1 Binding presence: the handle is present in the hit registry iff the selection is a single node whose type carries a corner radius; absent otherwise.
• CRAD-2 Radius only: a drag's committed delta lands in the corner radius, never in the node's transform (position/size/rotation) — a property edit, not a geometry edit.
• CRAD-3 Clamp on preview: the radius lies in [0, min(width, height) / 2] on every previewed value, at every zoom.
• CRAD-4 Placement switch: arc-center placement at or above the threshold radius, margin-offset below it; crossing the threshold relocates the handle without changing the value.
• CRAD-5 Link/split: with the four corners uniform and no modifier held, a corner drag drives all four equally; with the modifier held, or the corners already unequal, it drives only the anchored corner.
• CRAD-6 Drag direction: dragging inward increases the radius and outward decreases it, with the sign derived from the anchored corner.