Ensure grids have at least the given amount of rows (#3644)

crates/typst/src/layout/grid/layout.rs

@@ -847,15 +847,26 @@ impl CellGrid {
             }
         }
 
+        // If the user specified cells occupying less rows than the given rows,
+        // we shall expand the grid so that it has at least the given amount of
+        // rows.
+        let Some(expected_total_cells) = c.checked_mul(tracks.y.len()) else {
+            bail!(span, "too many rows were specified");
+        };
+        let missing_cells = expected_total_cells.saturating_sub(resolved_cells.len());
+
         // Fixup phase (final step in cell grid generation):
         // 1. Replace absent entries by resolved empty cells, and produce a
         // vector of 'Entry' from 'Option<Entry>'.
-        // 2. If any cells were added to the header's rows after the header's
+        // 2. Add enough empty cells to the end of the grid such that it has at
+        // least the given amount of rows.
+        // 3. If any cells were added to the header's rows after the header's
         // creation, ensure the header expands enough to accommodate them
         // across all of their spanned rows. Same for the footer.
-        // 3. If any cells before the footer try to span it, error.
+        // 4. If any cells before the footer try to span it, error.
         let resolved_cells = resolved_cells
             .into_iter()
+            .chain(std::iter::repeat_with(|| None).take(missing_cells))
             .enumerate()
             .map(|(i, cell)| {
                 if let Some(cell) = cell {

tests/typ/layout/grid-5.typ

@@ -28,3 +28,13 @@
   ],
   align(top)[B],
 )
+
+---
+// Ensure grids expand enough for the given rows.
+#grid(
+  columns: (2em, 2em),
+  rows: (2em,) * 4,
+  fill: red,
+  stroke: aqua,
+  [a]
+)