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/collab/remote.rs

@@ -8,6 +8,7 @@ use gpui::{
 use super::peers::Peer;
 use crate::{
     forms::TextInput,
+    primitives::caret_bar,
     theme::{ActiveTheme, Radius, TextSize},
     typography::LEADING,
 };
@@ -61,10 +62,9 @@ impl RenderOnce for RemoteCursor {
                     return;
                 }
                 window.with_content_mask(Some(ContentMask { bounds }), |window| {
-                    window.paint_quad(fill(
-                        gpui::Bounds::new(caret.origin, size(bar, caret.size.height)),
-                        color,
-                    ));
+                    let scale = window.scale_factor();
+                    let bar = caret_bar(caret.origin, bar, caret.size.height, scale);
+                    window.paint_quad(fill(bar, color));
                 });
                 let font = gpui::Font {
                     weight: FontWeight::MEDIUM,

src/forms/text/element.rs

@@ -8,7 +8,10 @@ use gpui::{
 };
 
 use super::{Highlight, Layout, TextInput};
-use crate::theme::ActiveTheme;
+use crate::{
+    primitives::{caret_bar, caret_width},
+    theme::ActiveTheme,
+};
 
 pub(crate) struct TextElement {
     input: Entity<TextInput>,
@@ -214,7 +217,10 @@ impl Element for TextElement {
     ) -> Prepaint {
         let theme = cx.theme();
         let (selection_color, caret_color) = (theme.colors.selection, theme.colors.focus);
-        let caret_width = theme.caret_width().to_pixels(window.rem_size());
+        let caret_width = caret_width(
+            theme.caret_width().to_pixels(window.rem_size()),
+            window.scale_factor(),
+        );
         let input = self.input.read(cx);
         let (text, placeholder, runs) = shown_runs(input, window, cx);
         let font_size = window.text_style().font_size.to_pixels(window.rem_size());
@@ -262,7 +268,10 @@ impl Element for TextElement {
         };
         let caret = caret_at
             .filter(|_| focused && caret_on && selection.is_empty())
-            .map(|at| Bounds::new(bounds.origin + at - scroll, size(caret_width, line_height)));
+            .map(|at| {
+                let at = bounds.origin + at - scroll;
+                caret_bar(at, caret_width, line_height, window.scale_factor())
+            });
         self.input.update(cx, |input, _| input.scroll = scroll);
         Prepaint {
             lines,

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;

src/terminal/paint.rs

@@ -12,6 +12,7 @@ use super::{
     links::Target,
     view::Terminal,
 };
+use crate::primitives::caret_bar;
 
 /// How a terminal paints: font and cell, ink, and the colors of what it marks.
 pub(crate) struct Look {
@@ -161,10 +162,11 @@ pub(crate) fn grid(
                     (CursorShape::HollowBlock, _) | (_, false) => {
                         outline(Bounds::new(origin, cell), look.caret, BorderStyle::Solid)
                     }
-                    (CursorShape::Beam, true) => fill(
-                        Bounds::new(origin, size(look.caret_width, cell.height)),
-                        look.caret,
-                    ),
+                    (CursorShape::Beam, true) => {
+                        let scale = window.scale_factor();
+                        let bar = caret_bar(origin, look.caret_width, cell.height, scale);
+                        fill(bar, look.caret)
+                    }
                     (CursorShape::Underline, true) => fill(
                         Bounds::new(
                             origin + point(Pixels::ZERO, cell.height - look.caret_width),

tasks/github.md

@@ -16,7 +16,7 @@
 # GitHub round, 2026-10-08
 
 - [x] G10 CI: `cargo fmt` on the files the last commits left unformatted
-- [ ] G11 Issue #7: a caret is a whole number of device pixels wide at a device pixel, so it never shows thin then thick; tested
+- [x] G11 Issue #7: a caret is a whole number of device pixels wide at a device pixel, so it never shows thin then thick; tested
 - [ ] G12 Issue #7: an IME asks for its candidate box past the laid-out text; the box clamps to the last layout, as `bounds_for` does; tested
 - [ ] G13 Issue #12: a floating option list occludes, so the wheel scrolls the list and not the page under it; tested
 - [ ] G14 Issue #7: BlockEditor scrolls in its own `ScrollArea` with its scrollbar and keeps the caret in view as blocks are added; tested