Tables: Tab and Shift+Tab skip cells hidden under a merge

Tab moved into the cells covered by a merged cell, editing text that is
never drawn. It now goes to the next visible cell; Tab in the last
visible cell still adds a row, and Shift+Tab in the first cell stays.

crates/engine/src/keys.rs

@@ -285,15 +285,22 @@ impl Session {
         let Some(t) = st.selection.text.clone() else { return Ok(Value::Null) };
         let Some((r, c)) = t.cell else { return Ok(Value::Null) };
         let Some(deckcraft_model::ShapeKind::Table(tb)) = st.shape(t.shape).map(|s| s.kind.clone()) else { return Ok(Value::Null) };
-        let (nr, nc) = (tb.n_rows(), tb.n_cols());
+        let (nr, nc) = (tb.n_rows(), tb.n_cols().max(1));
         let idx = r * nc + c;
-        if !back && idx + 1 >= nr * nc {
-            // Tab in the last cell adds a row.
-            self.execute("table.insertRowBelow", &json!({"id": t.shape, "row": r}))?;
-            return self.execute("text.edit", &json!({"id": t.shape, "cell": [r + 1, 0]}));
-        }
-        let ni = if back { idx.saturating_sub(1) } else { idx + 1 };
-        self.execute("text.edit", &json!({"id": t.shape, "cell": [ni / nc.max(1), ni % nc.max(1)]}))?;
+        // Cells hidden under a merged cell are skipped.
+        let visible = |i: &usize| tb.cell(i / nc, i % nc).is_some_and(|x| !x.is_covered());
+        let next = if back { (0..idx).rev().find(visible) } else { (idx + 1..nr * nc).find(visible) };
+        let ni = match next {
+            Some(ni) => ni,
+            None if !back => {
+                // Tab in the last cell adds a row.
+                let last = nr.saturating_sub(1);
+                self.execute("table.insertRowBelow", &json!({"id": t.shape, "row": last}))?;
+                return self.execute("text.edit", &json!({"id": t.shape, "cell": [last + 1, 0]}));
+            }
+            None => idx,
+        };
+        self.execute("text.edit", &json!({"id": t.shape, "cell": [ni / nc, ni % nc]}))?;
         self.execute("text.selectAll", &json!({}))
     }
 }

crates/engine/src/tests.rs

@@ -676,6 +676,25 @@ fn rotated_tables_hit_the_cell_under_the_pointer() {
     assert_eq!(text_sel(&s).and_then(|t| t.cell), Some((1, 1)));
 }
 
+#[test]
+fn tab_moves_between_cells_skipping_merged_ones() {
+    let mut s = session();
+    let id = table_at(&mut s, &[100.0, 100.0, 100.0], json!([["a", "b", "c"], ["d", "e", "f"]]));
+    s.execute("table.merge", &json!({"id": id, "from": [0, 0], "to": [0, 1]})).unwrap();
+    s.execute("text.edit", &json!({"id": id, "cell": [0, 0]})).unwrap();
+    let cell = |s: &Session| text_sel(s).and_then(|t| t.cell);
+    let shift = Mods { shift: true, ..Default::default() };
+    s.key("Tab", Mods::default()).unwrap();
+    assert_eq!(cell(&s), Some((0, 2)), "the cell under the merge is skipped");
+    s.key("Tab", shift).unwrap();
+    assert_eq!(cell(&s), Some((0, 0)));
+    s.key("Tab", shift).unwrap();
+    assert_eq!(cell(&s), Some((0, 0)), "Shift+Tab in the first cell stays");
+    s.execute("text.edit", &json!({"id": id, "cell": [1, 2]})).unwrap();
+    s.key("Tab", Mods::default()).unwrap();
+    assert_eq!((cell(&s), table_of(&s, id).n_rows()), (Some((2, 0)), 3), "Tab in the last cell adds a row");
+}
+
 #[test]
 fn right_to_left_paragraphs() {
     let mut s = session();