diff --git a/crates/core/src/addr.rs b/crates/core/src/addr.rs
index 8a64910..2bdfe79 100644
--- a/crates/core/src/addr.rs
+++ b/crates/core/src/addr.rs
@@ -30,14 +30,14 @@ impl CellRef {
     }
     /// Moved by `(dr, dc)`, `None` when the result leaves the sheet.
     pub fn offset(&self, dr: i64, dc: i64) -> Option<CellRef> {
-        let r = self.row as i64 + dr;
-        let c = self.col as i64 + dc;
+        let r = (self.row as i64).checked_add(dr)?;
+        let c = (self.col as i64).checked_add(dc)?;
         if (0..MAX_ROWS as i64).contains(&r) && (0..MAX_COLS as i64).contains(&c) { Some(CellRef::new(r as u32, c as u32)) } else { None }
     }
     /// Moved by `(dr, dc)` and clamped to the sheet.
     pub fn offset_clamped(&self, dr: i64, dc: i64) -> CellRef {
-        let r = (self.row as i64 + dr).clamp(0, MAX_ROWS as i64 - 1);
-        let c = (self.col as i64 + dc).clamp(0, MAX_COLS as i64 - 1);
+        let r = (self.row as i64).saturating_add(dr).clamp(0, MAX_ROWS as i64 - 1);
+        let c = (self.col as i64).saturating_add(dc).clamp(0, MAX_COLS as i64 - 1);
         CellRef::new(r as u32, c as u32)
     }
     /// Parses `A1`, `$A$1`, `xfd1048576` (case-insensitive). `None` if malformed or out of range.
@@ -272,6 +272,21 @@ mod tests {
         assert_eq!(a.iter().count(), 9);
     }
 
+    #[test]
+    fn extreme_offsets() {
+        let cell = CellRef::new(1, 1);
+        for delta in [i64::MIN, i64::MAX] {
+            assert_eq!(cell.offset(delta, 0), None);
+            assert_eq!(cell.offset(0, delta), None);
+            assert_eq!(cell.offset(delta, delta), None);
+        }
+        assert_eq!(cell.offset_clamped(i64::MAX, i64::MAX), CellRef::new(MAX_ROWS - 1, MAX_COLS - 1));
+        assert_eq!(cell.offset_clamped(i64::MIN, i64::MIN), CellRef::new(0, 0));
+        assert_eq!(cell.offset_clamped(i64::MAX, i64::MIN), CellRef::new(MAX_ROWS - 1, 0));
+        assert_eq!(cell.offset_clamped(i64::MIN, i64::MAX), CellRef::new(0, MAX_COLS - 1));
+        assert_eq!(cell.offset(2, -1), Some(CellRef::new(3, 0)));
+    }
+
     #[test]
     fn offsets() {
         assert_eq!(CellRef::new(0, 0).offset(-1, 0), None);
diff --git a/crates/engine/src/cmd/edit.rs b/crates/engine/src/cmd/edit.rs
index bb54fe0..2dba66f 100644
--- a/crates/engine/src/cmd/edit.rs
+++ b/crates/engine/src/cmd/edit.rs
@@ -1,6 +1,6 @@
 //! Cell entry, selection, undo/redo, clipboard, clear, fill, find & replace.
 
-use gridcraft_core::{CellRef, MAX_COLS, MAX_ROWS, RangeRef, Value};
+use gridcraft_core::{CellRef, MAX_ROWS, RangeRef, Value};
 use gridcraft_model::{Cell, Formula, StyleId};
 use serde_json::{Value as Json, json};
 
@@ -408,9 +408,9 @@ fn selection_move(s: &mut Session, p: &Json) -> Result<Json> {
         if !extend && let Some(m) = sheet.merge_at(from) {
             // Leaving a merged cell moves from its edge.
             if dr > 0 {
-                t = CellRef::new((m.end.row + dr as u32).min(MAX_ROWS - 1), from.col);
+                t = CellRef::new(m.end.offset_clamped(dr, 0).row, from.col);
             } else if dc > 0 {
-                t = CellRef::new(from.row, (m.end.col + dc as u32).min(MAX_COLS - 1));
+                t = CellRef::new(from.row, m.end.offset_clamped(0, dc).col);
             }
         }
         t
diff --git a/crates/engine/tests/extreme_selection_moves.rs b/crates/engine/tests/extreme_selection_moves.rs
new file mode 100644
index 0000000..f6f5737
--- /dev/null
+++ b/crates/engine/tests/extreme_selection_moves.rs
@@ -0,0 +1,40 @@
+use gridcraft_core::{CellRef, MAX_COLS, MAX_ROWS};
+use gridcraft_engine::Session;
+use serde_json::json;
+
+#[test]
+fn selection_move_clamps_extreme_displacements() {
+    let mut session = Session::new();
+    session.new_workbook();
+    for (dr, dc, expected) in [
+        (i64::MAX, 0, CellRef::new(MAX_ROWS - 1, 1)),
+        (0, i64::MAX, CellRef::new(1, MAX_COLS - 1)),
+        (i64::MAX, i64::MAX, CellRef::new(MAX_ROWS - 1, MAX_COLS - 1)),
+        (i64::MIN, i64::MIN, CellRef::new(0, 0)),
+        (i64::MAX, i64::MIN, CellRef::new(MAX_ROWS - 1, 0)),
+        (i64::MIN, i64::MAX, CellRef::new(0, MAX_COLS - 1)),
+        (2, -1, CellRef::new(3, 0)),
+    ] {
+        session.execute("selection.set", json!({"cell": "B2"})).unwrap();
+        session.execute("selection.move", json!({"dr": dr, "dc": dc})).unwrap();
+        assert_eq!(session.doc().unwrap().selection.active, expected);
+    }
+}
+
+#[test]
+fn selection_move_from_a_merged_cell_clamps_extreme_displacements() {
+    let mut session = Session::new();
+    session.new_workbook();
+    session.execute("selection.set", json!({"range": "B2:C3"})).unwrap();
+    session.execute("home.mergeCells", json!({})).unwrap();
+    for (dr, dc, expected) in [
+        (i64::MAX, 0, CellRef::new(MAX_ROWS - 1, 1)),
+        (0, i64::MAX, CellRef::new(1, MAX_COLS - 1)),
+        (4_294_967_297, 0, CellRef::new(MAX_ROWS - 1, 1)),
+        (1, 0, CellRef::new(3, 1)),
+    ] {
+        session.execute("selection.set", json!({"cell": "B2"})).unwrap();
+        session.execute("selection.move", json!({"dr": dr, "dc": dc})).unwrap();
+        assert_eq!(session.doc().unwrap().selection.active, expected);
+    }
+}