fix(ui): fix multi-level header suppression during scrolling

Implemented `show_sublabel` to ensure the first rendered entry in a
scrolled viewport always shows its full group labels.

Add regression tests for scrolling behavior in multi-level headers.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

src/ui/grid.rs

@@ -217,8 +217,7 @@ impl<'a> GridWidget<'a> {
                 let label = if layout.col_cats.is_empty() {
                     layout.col_label(ci)
                 } else {
-                    let show = ci == 0 || data_col_items[ci][..=d] != data_col_items[ci - 1][..=d];
-                    if show {
+                    if show_sublabel(ci, col_offset, d, &data_col_items) {
                         data_col_items[ci][d].clone()
                     } else {
                         String::new()
@@ -340,9 +339,7 @@ impl<'a> GridWidget<'a> {
                         let label = if layout.row_cats.is_empty() {
                             layout.row_label(ri)
                         } else {
-                            let show =
-                                ri == 0 || data_row_items[ri][..=d] != data_row_items[ri - 1][..=d];
-                            if show {
+                            if show_sublabel(ri, row_offset, d, &data_row_items) {
                                 data_row_items[ri][d].clone()
                             } else {
                                 String::new()
@@ -649,6 +646,15 @@ pub fn compute_visible_cols(
     count.max(1)
 }
 
+/// Decide whether the multi-level header sub-label at depth `d` for the data
+/// entry `idx` should be rendered, or blanked because it repeats the previous
+/// visible entry's prefix. `first_rendered` is the index of the first entry
+/// actually on screen (the row/col scroll offset): that entry always shows its
+/// full labels so a scrolled viewport keeps its group context.
+fn show_sublabel(idx: usize, first_rendered: usize, d: usize, items: &[&Vec<String>]) -> bool {
+    idx == first_rendered || items[idx][..=d] != items[idx - 1][..=d]
+}
+
 // Re-export shared formatting functions
 pub use crate::format::{format_f64, parse_number_format};
 
@@ -678,7 +684,7 @@ fn truncate(s: &str, max_width: usize) -> String {
 mod tests {
     use ratatui::{buffer::Buffer, layout::Rect, widgets::Widget};
 
-    use super::GridWidget;
+    use super::{show_sublabel, GridWidget};
     use crate::formula::parse_formula;
     use crate::model::cell::{CellKey, CellValue};
     use crate::ui::app::AppMode;
@@ -1060,4 +1066,49 @@ mod tests {
         assert!(text.contains("2024"), "expected '2024' in:\n{text}");
         assert!(text.contains("2025"), "expected '2025' in:\n{text}");
     }
+
+    // ── Multi-level header repeat suppression ─────────────────────────────────
+
+    /// Items: [A,x], [A,y], [B,z] — multi-level header tuples.
+    fn sublabel_items() -> Vec<Vec<String>> {
+        [["A", "x"], ["A", "y"], ["B", "z"]]
+            .iter()
+            .map(|t| t.iter().map(|s| s.to_string()).collect())
+            .collect()
+    }
+
+    /// Bug (improvise-jjx): suppression compared against the entry at
+    /// `idx - 1` even when that entry is scrolled off-screen. The first
+    /// RENDERED entry must always show its full labels, otherwise scrolling
+    /// blanks the group label at the top of the viewport.
+    #[test]
+    fn first_rendered_entry_always_shows_sublabel_when_scrolled() {
+        let items = sublabel_items();
+        let refs: Vec<&Vec<String>> = items.iter().collect();
+        // Scrolled so index 1 ([A,y]) is the first visible entry. Its level-0
+        // prefix matches off-screen index 0 ([A,x]) — it must still show "A".
+        assert!(
+            show_sublabel(1, 1, 0, &refs),
+            "first rendered entry must show its label even when its prefix matches the off-screen entry above"
+        );
+    }
+
+    #[test]
+    fn repeated_prefix_below_first_rendered_entry_is_suppressed() {
+        let items = sublabel_items();
+        let refs: Vec<&Vec<String>> = items.iter().collect();
+        // Unscrolled: index 1 repeats index 0's level-0 prefix → suppressed.
+        assert!(!show_sublabel(1, 0, 0, &refs));
+        // Leaf level differs → shown.
+        assert!(show_sublabel(1, 0, 1, &refs));
+    }
+
+    #[test]
+    fn changed_prefix_is_always_shown() {
+        let items = sublabel_items();
+        let refs: Vec<&Vec<String>> = items.iter().collect();
+        assert!(show_sublabel(0, 0, 0, &refs));
+        assert!(show_sublabel(2, 0, 0, &refs)); // B != A
+        assert!(show_sublabel(2, 1, 0, &refs)); // scrolled, still differs
+    }
 }