add second cursor list

src/renderer/cell.zig

@@ -103,11 +103,12 @@ pub const Contents = struct {
         // form a single grapheme, and multi-substitutions in fonts, the number
         // of glyphs in a row is theoretically unlimited.
         //
-        // We have size.rows + 1 lists because index 0 is used for a special
-        // list containing the cursor cell which needs to be first in the buffer.
+        // We have size.rows + 2 lists because indexes 0 and size.rows - 1 are
+        // used for special lists containing the cursor cell which need to
+        // be first and last in the buffer, respectively.
         var fg_rows = try ArrayListCollection(shaderpkg.CellText).init(
             alloc,
-            size.rows + 1,
+            size.rows + 2,
             size.columns * 3,
         );
         errdefer fg_rows.deinit(alloc);
@@ -118,14 +119,19 @@ pub const Contents = struct {
         self.bg_cells = bg_cells;
         self.fg_rows = fg_rows;
 
-        // We don't need 3*cols worth of cells for the cursor list, so we can
-        // replace it with a smaller list. This is technically a tiny bit of
+        // We don't need 3*cols worth of cells for the cursor lists, so we can
+        // replace them with smaller lists. This is technically a tiny bit of
         // extra work but resize is not a hot function so it's worth it to not
         // waste the memory.
         self.fg_rows.lists[0].deinit(alloc);
         self.fg_rows.lists[0] = try std.ArrayListUnmanaged(
             shaderpkg.CellText,
         ).initCapacity(alloc, 1);
+
+        self.fg_rows.lists[size.rows + 1].deinit(alloc);
+        self.fg_rows.lists[size.rows + 1] = try std.ArrayListUnmanaged(
+            shaderpkg.CellText,
+        ).initCapacity(alloc, 1);
     }
 
     /// Reset the cell contents to an empty state without resizing.
@@ -137,6 +143,7 @@ pub const Contents = struct {
     /// Set the cursor value. If the value is null then the cursor is hidden.
     pub fn setCursor(self: *Contents, v: ?shaderpkg.CellText) void {
         self.fg_rows.lists[0].clearRetainingCapacity();
+        self.fg_rows.lists[self.size.rows + 1].clearRetainingCapacity();
 
         if (v) |cell| {
             self.fg_rows.lists[0].appendAssumeCapacity(cell);