port bezel theme system into the theme crate

crates/theme/src/layout.rs

@@ -0,0 +1,87 @@
+//! Layout constants, ported from bezel
+//! (`crates/theme/src/theme/layout.rs`, MIT License, Copyright (c) 2026 bezel
+//! contributors). Numbers drive layout, colors are paint: these live as plain
+//! numbers and never depend on which color is painted.
+
+use std::sync::atomic::{AtomicU32, Ordering};
+
+/// The branded base radius behind every radius accessor, as raw `f32` bits.
+static BASE: AtomicU32 = AtomicU32::new(BASE_RADIUS.to_bits());
+
+/// Point the radius accessors at a base. Called when a palette is installed.
+pub(crate) fn set_base_radius(radius: f32) {
+    BASE.store(radius.to_bits(), Ordering::Relaxed);
+}
+
+/// The gap between siblings. Measured on macOS 26: `NSStackView().spacing`,
+/// visual format's `-`, and `constraint(equalToSystemSpacingAfter:multiplier: 1)`
+/// all report 8.
+pub const SPACE: f32 = 8.0;
+
+/// The margin from content to its container's edge. Same measurement,
+/// visual format's `|-`.
+pub const CONTENT_MARGIN: f32 = 20.0;
+
+/// From a column's edge to the glyph of the control at the end of a row,
+/// measured to the glyph rather than the button. Mirrored at the trailing end.
+pub const EDGE: f32 = 12.0;
+
+/// Button, text field and select-trigger height. Measured:
+/// `NSButton`, `NSTextField` and `NSPopUpButton` all report 24 at
+/// `.regular` — the body line box with 4 above and below.
+pub const BUTTON_HEIGHT: f32 = 24.0;
+
+/// The same controls at `.small`.
+pub const CONTROL_HEIGHT_SMALL: f32 = 20.0;
+
+/// Every radius below is a ratio of this, so a brand radius moves the whole
+/// set together and keeps the concentric relationships intact.
+pub const BASE_RADIUS: f32 = 8.0;
+
+/// Message bubble corner radius (2x).
+pub fn bubble_radius() -> f32 {
+    radius(2.0)
+}
+
+/// Floating-surface corner radius — popovers, menus, dialogs (1.5x).
+pub fn surface_radius() -> f32 {
+    radius(1.5)
+}
+
+/// Panel / card corner radius (1.25x).
+pub fn panel_radius() -> f32 {
+    radius(1.25)
+}
+
+/// Button, text field and select-trigger radius (1x).
+pub fn button_radius() -> f32 {
+    radius(1.0)
+}
+
+/// Small control radius (chips, tags, steppers) — a size down from
+/// [`button_radius`], for things that sit inside a control rather than being one
+/// (0.75x).
+pub fn control_radius() -> f32 {
+    radius(0.75)
+}
+
+/// A corner as a multiple of the branded base radius.
+///
+/// Read from a process-wide mirror rather than the theme global for the reason
+/// [`crate::paint::current_appearance`] is: the element builders that round a
+/// corner are free functions with no `cx` in scope, and a radius is one number
+/// for the whole app.
+fn radius(ratio: f32) -> f32 {
+    f32::from_bits(BASE.load(Ordering::Relaxed)) * ratio
+}
+
+/// The concentric child of a surface: a row inset by `inset` inside a
+/// container of radius `outer` keeps its corners parallel to the container's,
+/// rather than looking pasted onto it.
+///
+/// This is SwiftUI's `ContainerRelativeShape` rule done as arithmetic. gpui
+/// has no container shape to inherit at paint time, so the relationship is
+/// stated where the child is *defined* instead of resolved at runtime.
+pub const fn inset_radius(outer: f32, inset: f32) -> f32 {
+    if outer > inset { outer - inset } else { 0.0 }
+}