Code editor: columns sit where gpui lays them out

Carets, selections, presses and the input method box measured the line
as columns times one 'm', so a wide character (CJK, emoji) put the caret
mid-glyph. editor::measure lays out the drawn line; rulers and indent
guides stay on the column grid; whitespace marks hide the rest.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/editor/measure.rs

@@ -0,0 +1,76 @@
+//! Where a drawn line's columns fall, as gpui lays it out.
+
+use std::sync::Arc;
+
+use gpui::{Hsla, LineLayout, Pixels, SharedString, TextRun, Window, font};
+
+use super::layout::Shown;
+
+/// A drawn line laid out: wide characters take their own width.
+pub(crate) struct Measured {
+    text: SharedString,
+    layout: Arc<LineLayout>,
+    advance: Pixels,
+}
+
+impl Measured {
+    /// `shown` in the code font `family` at `size`.
+    pub fn new(
+        shown: &Shown,
+        family: SharedString,
+        size: Pixels,
+        advance: Pixels,
+        window: &Window,
+    ) -> Self {
+        let base = font(family);
+        let run = |len: usize, style: Option<&gpui::HighlightStyle>| {
+            let mut font = base.clone();
+            if let Some(style) = style {
+                font.weight = style.font_weight.unwrap_or(font.weight);
+                font.style = style.font_style.unwrap_or(font.style);
+            }
+            TextRun {
+                len,
+                font,
+                color: Hsla::default(),
+                background_color: None,
+                underline: None,
+                strikethrough: None,
+            }
+        };
+        let (text, mut runs, mut at) = (shown.text.clone(), Vec::new(), 0);
+        for (range, style) in shown.styles.iter().filter(|(range, _)| !range.is_empty()) {
+            if range.start > at {
+                runs.push(run(range.start - at, None));
+            }
+            runs.push(run(range.len(), Some(style)));
+            at = range.end;
+        }
+        if at < text.len() {
+            runs.push(run(text.len() - at, None));
+        }
+        let layout = window.text_system().layout_line(&text, size, &runs, None);
+        Self {
+            text,
+            layout,
+            advance,
+        }
+    }
+
+    /// Where drawn column `column` starts; past the end in whole advances.
+    pub fn x(&self, column: usize) -> Pixels {
+        match self.text.char_indices().nth(column) {
+            Some((at, _)) => self.layout.x_for_index(at),
+            None => {
+                let past = column - self.text.chars().count();
+                self.layout.width + self.advance * past as f32
+            }
+        }
+    }
+
+    /// The drawn column whose start is nearest `x`.
+    pub fn column(&self, x: Pixels) -> usize {
+        let at = self.layout.closest_index_for_x(x).min(self.text.len());
+        self.text[..at].chars().count()
+    }
+}