             else { return }
         
         isDragging = true
+        
+        assert(previousTranslation == nil)
         
         //horizontalScroller?.alwaysVisible = true
         //verticalScroller?.alwaysVisible = true

            }
        }
    }
    
    private func beginDragging() {
        

                
            case .changed:
                
                let translation = panGestureRecognizer.translation(in: self)
                
                let delta: CGPoint
                
                if let previousTranslation = self.previousTranslation {
                    
                    delta = CGPoint(x: translation.x - previousTranslation.x,
                                    y: translation.y - previousTranslation.y)
                    
                } else {
                    
                    delta = translation
                }
                
                self.previousTranslation = translation
                
                drag(by: delta)
                
