     
     internal func _toggleShowingOfNavigationBackItem(shows: Bool, duration: TimeInterval = 0.5, animated: Bool) {
         if let _horizontal = _horizontalConstraintOfBackItem {
-            removeConstraint(_horizontal)
+            _containerView.removeConstraint(_horizontal)
         }
         var _horizontal: NSLayoutConstraint
         if shows {
-            _horizontal = _navigationBackItem._view.leadingAnchor.constraint(equalTo: leadingAnchor)
-        } else {
-            _horizontal = _navigationBackItem._view.trailingAnchor.constraint(equalTo: leadingAnchor)
+            _horizontal = _navigationBackItem._view.leadingAnchor.constraint(equalTo: _containerView.leadingAnchor)
+        } else {
+            _horizontal = _navigationBackItem._view.trailingAnchor.constraint(equalTo: _containerView.leadingAnchor)
         }
         _horizontal.isActive = true
         _horizontalConstraintOfBackItem = _horizontal

            }
        }
    }
    // MARK: - Private Properties.
    fileprivate var _normalBackgroundColor: UIColor?
    fileprivate lazy var __titleAlignmentLabel: UILabel = _createGeneralAlignmentLabel()
    fileprivate lazy var _titleItemsContainerView: UIView = _createGeneralContainerView()
    fileprivate lazy var _itemsContainerView: UIView = _createGeneralContainerView()

    
    // Initializer:
    private func _initializer() {
        // Set up the height of the tab navigation bar.
        _setupConstraintOfSelf()
        // Set up container views:
        _setupContainerViews()
        _setupPreviewGesture()
