UI: status messages elide in the middle, the command line's placeholder fits its width

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/ui-egui/src/elide.rs

@@ -0,0 +1,84 @@
+//! Text that does not fit: elided in the middle, so both ends of a name stay readable
+//! (`F2:side:a5#[F2…]|F2:side:a6`), with the full text left to a hover.
+
+use std::borrow::Cow;
+
+use egui::{FontId, Ui};
+
+/// The ellipsis put in place of the middle.
+pub const ELLIPSIS: char = '…';
+
+/// `text`, or as much of its start and end as fits in `max_width` around an [`ELLIPSIS`].
+/// `width_of` measures a string as drawn. Ties keep one more character at the start. With no room
+/// even for the ellipsis alone, the ellipsis.
+pub fn middle<'a>(text: &'a str, max_width: f32, width_of: impl Fn(&str) -> f32) -> Cow<'a, str> {
+    if width_of(text) <= max_width {
+        return Cow::Borrowed(text);
+    }
+    let chars: Vec<char> = text.chars().collect();
+    let joined = |kept: usize| -> String {
+        let head = kept.div_ceil(2);
+        let tail = kept - head;
+        let mut s: String = chars.iter().take(head).collect();
+        s.push(ELLIPSIS);
+        s.extend(chars.iter().skip(chars.len() - tail));
+        s
+    };
+    // The widest that fits: the width grows with the characters kept.
+    let (mut lo, mut hi) = (0, chars.len().saturating_sub(1));
+    while lo < hi {
+        let mid = (lo + hi).div_ceil(2);
+        if width_of(&joined(mid)) <= max_width {
+            lo = mid;
+        } else {
+            hi = mid - 1;
+        }
+    }
+    Cow::Owned(joined(lo))
+}
+
+/// The width of `text` on one line in `font`.
+pub fn width(ui: &Ui, text: &str, font: &FontId) -> f32 {
+    ui.painter().layout_no_wrap(text.to_string(), font.clone(), egui::Color32::PLACEHOLDER).size().x
+}
+
+/// [`middle`] measured with `font` as `ui` draws it.
+pub fn middle_in<'a>(ui: &Ui, text: &'a str, font: &FontId, max_width: f32) -> Cow<'a, str> {
+    middle(text, max_width, |s| width(ui, s, font))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// One unit per character.
+    fn chars(s: &str) -> f32 {
+        s.chars().count() as f32
+    }
+
+    #[test]
+    fn fitting_text_is_kept_whole() {
+        assert!(matches!(middle("Seeds", 5.0, chars), Cow::Borrowed("Seeds")));
+    }
+
+    #[test]
+    fn long_text_keeps_both_ends() {
+        let name = "F2:side:a5#[F2:side:a6]|F2:side:a6";
+        let short = middle(name, 15.0, chars);
+        assert_eq!(short, "F2:side…side:a6");
+        assert!(chars(&short) <= 15.0);
+        assert_eq!(middle(name, 16.0, chars), "F2:side:…side:a6");
+    }
+
+    #[test]
+    fn no_room_leaves_the_ellipsis() {
+        assert_eq!(middle("abcdef", 1.0, chars), "…");
+        assert_eq!(middle("abcdef", 0.0, chars), "…");
+        assert_eq!(middle("", -1.0, chars), "…");
+    }
+
+    #[test]
+    fn multibyte_text_splits_on_characters() {
+        assert_eq!(middle("αβγδεζ", 4.0, chars), "αβ…ζ");
+    }
+}