Fix for cart reordering while adding quantity.

[ashharr]

Create a copy of the cartItems:
const newCartItems = [...cartItems]; // for mutating the array in place

now we increase quantity, in the foundProduct and simply add it in the new array using index to prevent reordering.

    if(value === 'inc') {
      foundProduct.quantity += 1;
      newCartItems[index] = foundProduct;  // prevents reordering
      setCartItems(newCartItems)

[graphiiics]

Very nice approach!