Caret · whole device pixels at a device pixel: gpui snapped a 1.5px bar's edges apart, so it painted 1 or 2 pixels by where it stood (#7)

src/primitives/caret.rs

@@ -0,0 +1,65 @@
+use gpui::{Bounds, Pixels, Point, size};
+
+fn snap(logical: Pixels, least: f32, scale: f32) -> Pixels {
+    Pixels::from((logical.as_f32() * scale).round().max(least) / scale)
+}
+
+/// The width a caret paints at: whole device pixels, at least one.
+pub(crate) fn caret_width(width: Pixels, scale: f32) -> Pixels {
+    snap(width, 1.0, scale)
+}
+
+/// A caret bar at `origin`, `width` wide, snapped whole to device pixels so it keeps one width.
+pub(crate) fn caret_bar(
+    origin: Point<Pixels>,
+    width: Pixels,
+    height: Pixels,
+    scale: f32,
+) -> Bounds<Pixels> {
+    Bounds::new(
+        Point::new(snap(origin.x, f32::MIN, scale), origin.y),
+        size(caret_width(width, scale), height),
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use gpui::{point, px};
+
+    use super::caret_bar;
+
+    /// gpui's paint snapping: each edge alone, half toward zero.
+    fn device(logical: f32, scale: f32) -> f32 {
+        let value = logical * scale;
+        (value.abs() - 0.5).ceil().copysign(value)
+    }
+
+    #[test]
+    fn a_caret_keeps_one_device_width_wherever_it_stands() {
+        for scale in [1.0, 1.25, 1.5, 2.0] {
+            let widths: Vec<f32> = (0..200)
+                .map(|step| {
+                    let bar = caret_bar(
+                        point(px(10.0 + step as f32 * 0.037), px(3.3)),
+                        px(1.5),
+                        px(18.0),
+                        scale,
+                    );
+                    device(bar.right().as_f32(), scale) - device(bar.left().as_f32(), scale)
+                })
+                .collect();
+            let first = widths[0];
+            assert!(first >= 1.0, "at {scale}x the caret shows");
+            assert!(
+                widths.iter().all(|width| *width == first),
+                "at {scale}x the caret took widths {:?}",
+                widths.iter().fold(Vec::new(), |mut seen, width| {
+                    if !seen.contains(width) {
+                        seen.push(*width);
+                    }
+                    seen
+                })
+            );
+        }
+    }
+}

src/primitives/mod.rs

@@ -1,4 +1,5 @@
 mod backdrop;
+mod caret;
 mod disclosure;
 mod divider;
 mod files;
@@ -17,6 +18,7 @@ mod tooltip;
 mod tests;
 
 pub use backdrop::{Backdrop, Place};
+pub(crate) use caret::{caret_bar, caret_width};
 pub use disclosure::Disclosure;
 pub use divider::Divider;
 pub use files::IconTheme;