diff --git a/crates/ui-egui/src/editor.rs b/crates/ui-egui/src/editor.rs
index 767f134..4aada29 100644
--- a/crates/ui-egui/src/editor.rs
+++ b/crates/ui-egui/src/editor.rs
@@ -251,9 +251,10 @@ pub fn column_completion(sheet: &gridcraft_engine::model::Sheet, at: CellRef, ty
         match sheet.value(CellRef::new(row, at.col)) {
             gridcraft_engine::core::Value::Empty => false,
             gridcraft_engine::core::Value::Text(t) => {
-                if t.to_lowercase().starts_with(&lower) && t.len() > typed.len() {
+                let folded = t.to_lowercase();
+                if folded.starts_with(&lower) && folded.len() > lower.len() {
                     match &found {
-                        Some(f) if !f.eq_ignore_ascii_case(&t) => ambiguous = true,
+                        Some(f) if f.to_lowercase() != folded => ambiguous = true,
                         None => found = Some(t.to_string()),
                         _ => {}
                     }
@@ -281,6 +282,13 @@ pub fn column_completion(sheet: &gridcraft_engine::model::Sheet, at: CellRef, ty
     if ambiguous { None } else { found }
 }
 
+/// The part of `full` after the `typed` prefix, counted in characters: case folding can change
+/// a character's byte length ("ß" is two bytes, "ẞ" three).
+pub fn completion_rest<'a>(full: &'a str, typed: &str) -> Option<&'a str> {
+    let start = full.char_indices().nth(typed.chars().count())?.0;
+    full.get(start..)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/crates/ui-egui/src/formula_bar.rs b/crates/ui-egui/src/formula_bar.rs
index 083fa9c..7f1ad87 100644
--- a/crates/ui-egui/src/formula_bar.rs
+++ b/crates/ui-egui/src/formula_bar.rs
@@ -336,8 +336,7 @@ pub fn editor_widget(app: &mut SheetApp, ui: &mut egui::Ui, id: egui::Id, font:
     }
     // Column AutoComplete: the rest of the suggested entry, shown selected after the caret.
     if let Some(full) = &ed.completion
-        && full.len() > ed.text.len()
-        && let Some(rest) = full.get(ed.text.len()..)
+        && let Some(rest) = crate::editor::completion_rest(full, &ed.text)
     {
         let pos = resp.rect.left_top() + egui::vec2(output.galley.rect.width(), 0.0);
         let g = ui.painter().layout_no_wrap(
diff --git a/crates/ui-egui/tests/interaction.rs b/crates/ui-egui/tests/interaction.rs
index 7307156..e2447e9 100644
--- a/crates/ui-egui/tests/interaction.rs
+++ b/crates/ui-egui/tests/interaction.rs
@@ -537,6 +537,25 @@ fn column_autocomplete_skips_ambiguous_prefixes_and_numbers() {
     assert_eq!(completion(&h), ("12".into(), None), "a number is not completed");
 }
 
+#[test]
+fn column_autocomplete_folds_case_beyond_ascii() {
+    let mut s = blank();
+    s.execute("range.setValues", json!({"range": "A1", "values": [["Ébène"], ["Ébano"], ["ébano"], ["STRAẞENBAHN"]]})).unwrap();
+    s.execute("selection.set", json!({"cell": "A5"})).unwrap();
+    let mut h = harness(s);
+    text(&mut h, "éba");
+    assert_eq!(completion(&h), ("éba".into(), Some("ébano".into())), "Ébano and ébano are one entry, Ébène doesn't match");
+    h.state_mut().cancel_edit();
+    text(&mut h, "éb");
+    assert_eq!(completion(&h).1, None, "Ébène and Ébano differ past the prefix");
+    h.state_mut().cancel_edit();
+    // "ß" is two bytes, "ẞ" three: the proposed rest starts after the fifth character, not the sixth byte.
+    text(&mut h, "straß");
+    assert_eq!(completion(&h), ("straß".into(), Some("STRAẞENBAHN".into())));
+    let painted = h.output().shapes.iter().any(|c| matches!(&c.shape, egui::Shape::Text(t) if t.galley.job.text == "ENBAHN"));
+    assert!(painted, "the rest \"ENBAHN\" is drawn after the caret");
+}
+
 #[test]
 fn column_autocomplete_can_be_turned_off() {
     assert!(gridcraft_ui_egui::UiState::default().autocomplete);