fix(engine): walk stored cells in transitive precedent selection

Bound the all-level walk to the sheet sparse cell store instead of
iterating every position of each precedent area: formulas late in a
wide area are no longer skipped, blank positions are never selected,
and only formula cells extend the frontier. Adds a regression test
(=SUM(A1:Z1) with a formula at Z1) and silences two clippy lints in
the same function.

Co-Authored-By: Claude <noreply@anthropic.com>

crates/engine/src/cmd/formulas.rs

@@ -1,6 +1,6 @@
 //! Formulas tab: AutoSum, Insert Function, names, auditing, calculation.
 
-use gridcraft_core::{CellRef, RangeRef};
+use gridcraft_core::{CellRef, RangeRef, Value};
 use gridcraft_model::{CalcMode, DefinedName, Formula};
 use serde_json::{Value as Json, json};
 
@@ -485,7 +485,6 @@ fn trace_dependents(s: &mut Session, p: &Json) -> Result<Json> {
 /// another sheet, jump there. `all` walks every level (Ctrl+Shift+[ / Ctrl+Shift+]).
 fn select_trace_cells(s: &mut Session, p: &Json, precedents: bool) -> Result<Json> {
     use std::collections::BTreeSet;
-    const CAP: usize = 4096;
     let d = s.doc()?;
     let at = cell_param(s, p, "cell")?.unwrap_or(d.selection.active);
     let sheet = d.wb.active_sheet;
@@ -495,15 +494,23 @@ fn select_trace_cells(s: &mut Session, p: &Json, precedents: bool) -> Result<Jso
     let mut seen: BTreeSet<(usize, CellRef)> = BTreeSet::new();
     let mut frontier = vec![(sheet, at)];
     seen.insert((sheet, at));
-    loop {
-        let Some((ks, kc)) = frontier.pop() else { break };
+    while let Some((ks, kc)) = frontier.pop() {
         if precedents {
             for a in d.calc.graph.precedents_of((ks, kc)) {
                 if all {
-                    for c in a.range.iter().take(CAP) {
+                    // Walk the sheet's stored cells inside the precedent area: blanks are
+                    // never selected and only formula cells extend the frontier, so a
+                    // formula late in a large range is not skipped.
+                    let Some(sh) = d.wb.sheet(a.sheet) else { continue };
+                    for (c, cell) in sh.cells.iter() {
+                        if !a.range.contains(c) || matches!(cell.value, Value::Empty) {
+                            continue;
+                        }
                         if seen.insert((a.sheet, c)) {
                             found.insert((a.sheet, c.a1()));
-                            frontier.push((a.sheet, c));
+                            if cell.formula.is_some() {
+                                frontier.push((a.sheet, c));
+                            }
                         }
                     }
                 } else {
@@ -538,7 +545,6 @@ fn select_trace_cells(s: &mut Session, p: &Json, precedents: bool) -> Result<Jso
     if si != sheet {
         params["sheet"] = json!(si);
     }
-    drop(d);
     s.execute("selection.set", params)
 }
 

crates/ui-egui/tests/trace_navigation.rs

@@ -72,6 +72,23 @@ fn ctrl_shift_bracket_walks_every_level() {
     );
 }
 
+#[test]
+fn transitive_walk_reaches_formulas_late_in_a_wide_area() {
+    // =SUM(A1:Z1) has a wide precedent area; a formula stored late in it must still be
+    // walked, and blank positions must not be selected (#1865 review).
+    let mut s = Session::new();
+    s.new_workbook();
+    s.execute("cell.set", json!({"cell": "Z1", "input": "5"})).unwrap();
+    s.execute("cell.set", json!({"cell": "AB1", "input": "=SUM(A1:Z1)"})).unwrap();
+    s.execute("cell.set", json!({"cell": "AD1", "input": "=AB1*2"})).unwrap();
+    let mut h = harness(s);
+    h.state_mut().session.execute("selection.set", json!({"range": "AD1"})).unwrap();
+    key(&mut h, Key::OpenBracket, Modifiers::CTRL | Modifiers::SHIFT);
+    let r = ranges(&h);
+    assert!(r.contains(&"Z1".to_string()), "the formula late in the area is reached, got {r:?}");
+    assert_eq!(r.len(), 2, "only the stored cells (AB1, Z1) are selected, no blanks, got {r:?}");
+}
+
 #[test]
 fn no_precedents_leaves_selection_untouched() {
     let mut h = harness(chain());