diff --git a/crates/ui-egui/src/editor.rs b/crates/ui-egui/src/editor.rs
index d3cab75ea..71fd2a170 100644
--- a/crates/ui-egui/src/editor.rs
+++ b/crates/ui-egui/src/editor.rs
@@ -375,7 +375,7 @@ pub fn formula_refs(text: &str) -> Vec<(usize, usize, RangeRef, Option<String>,
     let offset = text.len() - body.len();
     let Ok(toks) = gridcraft_engine::formula::lexer::tokenize(body) else { return vec![] };
     let mut out: Vec<(usize, usize, RangeRef, Option<String>, usize)> = Vec::new();
-    let mut seen: Vec<String> = Vec::new();
+    let mut seen: std::collections::HashMap<String, usize> = std::collections::HashMap::new();
     let mut i = 0;
     while i < toks.len() {
         use gridcraft_engine::formula::lexer::Tok;
@@ -407,13 +407,8 @@ pub fn formula_refs(text: &str) -> Vec<(usize, usize, RangeRef, Option<String>,
                 continue;
             };
             let key = body.get(start..end).unwrap_or("").replace('$', "").to_ascii_uppercase();
-            let idx = match seen.iter().position(|k| *k == key) {
-                Some(k) => k,
-                None => {
-                    seen.push(key);
-                    seen.len() - 1
-                }
-            };
+            let next = seen.len();
+            let idx = *seen.entry(key).or_insert(next);
             out.push((start + offset, end + offset, range, sheet, idx % REF_COLOR_COUNT));
             continue;
         }
diff --git a/crates/ui-egui/src/grid/quadrant.rs b/crates/ui-egui/src/grid/quadrant.rs
index 0469c345f..7461c2204 100644
--- a/crates/ui-egui/src/grid/quadrant.rs
+++ b/crates/ui-egui/src/grid/quadrant.rs
@@ -278,6 +278,9 @@ fn paint_cell_decorations(q: &QuadrantPaint<'_>, c: CellRef, rect: Rect, lk: &Lo
     false
 }
 
+/// Most repeats of a fill character laid out in one cell.
+const MAX_FILL_REPEATS: usize = 8192;
+
 fn paint_data_bar(p: &egui::Painter, rect: Rect, frac: f32, rgb: [u8; 3], gradient: bool) {
     let bar = Rect::from_min_size(pos2(rect.left() + 2.0, rect.top() + 2.0), vec2((rect.width() - 4.0) * frac, rect.height() - 4.0));
     let col = theme::color32(rgb);
@@ -432,7 +435,8 @@ fn paint_cell(q: &QuadrantPaint<'_>, painted: &(CellRef, Rect, Look, Option<CfLo
     {
         let base_w = w;
         let cw = p.layout_no_wrap(ch.to_string(), font.clone(), color).size().x.max(1.0);
-        let n = (((avail_w - 2.0 * pad) - base_w) / cw).floor().max(0.0) as usize;
+        // A merged cell's width counts offscreen columns: no screen shows more repeats than this.
+        let n = ((((avail_w - 2.0 * pad) - base_w) / cw).floor().max(0.0) as usize).min(MAX_FILL_REPEATS);
         let mut s = first_line.clone();
         if first_line.is_char_boundary(pos) {
             s.insert_str(pos, &ch.to_string().repeat(n));