renderer/metal: clear cell contents bottom-up

src/renderer/Metal.zig

@@ -1897,10 +1897,10 @@ fn rebuildCells2(
     // Go row-by-row to build the cells. We go row by row because we do
     // font shaping by row. In the future, we will also do dirty tracking
     // by row.
-    var row_it = screen.pages.rowIterator(.right_down, .{ .viewport = .{} }, null);
+    var row_it = screen.pages.rowIterator(.left_up, .{ .viewport = .{} }, null);
-    var y: terminal.size.CellCountInt = 0;
+    var y: terminal.size.CellCountInt = screen.pages.rows;
     while (row_it.next()) |row| {
-        defer y += 1;
+        y = y - 1;
 
         // If we're rebuilding a row, then we always clear the cells
         self.cells.clear(y);

src/renderer/metal/cell.zig

@@ -128,6 +128,14 @@ pub const Contents = struct {
     }
 
     /// Clear all of the cell contents for a given row.
+    ///
+    /// Due to the way this works internally, it is best to clear rows
+    /// from the bottom up. This is because when we clear a row, we
+    /// swap remove the last element in the list and then update the
+    /// mapping for the swapped element. If we clear from the top down,
+    /// then we would have to update the mapping for every element in
+    /// the list. If we clear from the bottom up, then we only have to
+    /// update the mapping for the last element in the list.
     pub fn clear(
         self: *Contents,
         y: usize,
@@ -135,7 +143,12 @@ pub const Contents = struct {
         const start_idx = y * self.cols;
         const end_idx = start_idx + self.cols;
         const maps = self.map[start_idx..end_idx];
-        for (maps) |*map| {
+        for (0..self.cols) |x| {
+            // It is better to clear from the right left due to the same
+            // reasons noted for bottom-up clearing in the doc comment.
+            const rev_x = self.cols - x - 1;
+            const map = &maps[rev_x];
+
             var it = map.array.iterator();
             while (it.next()) |entry| {
                 if (!entry.value.set) continue;