 test('opened attribute', function() {
          assert.equal(collapse.opened, true);
        });

        test('horizontal attribute', function() {
          assert.equal(collapse.horizontal, true);
        });

        test('default opened width', function() {
          assert.equal(collapse.style.width, 'auto');
        });

        test('set opened to false, then to true', function(done) {
          // This listener will be triggered twice (every time `opened` changes).
          collapse.addEventListen