feat: update scroll navigation

Sources/Navigator/EPUB/EPUBNavigatorViewController.swift

@@ -869,6 +869,7 @@ open class EPUBNavigatorViewController: UIViewController,
         }
 
         view.backgroundColor = settings.effectiveBackgroundColor.uiColor
+        paginationView.isVerticalChapterTransition = settings.scroll && !settings.verticalText
     }
 
     // MARK: - User interactions
@@ -1150,6 +1151,15 @@ extension EPUBNavigatorViewController: EPUBSpreadViewDelegate {
     func spreadViewDidTerminate() {
         reloadSpreads(force: true)
     }
+
+    func spreadView(_ spreadView: EPUBSpreadView, didRequestChapterNavigationForward forward: Bool) {
+        guard !paginationView.isAnimatingChapterTransition else { return }
+        if forward {
+            _ = goForward(animated: true, completion: {})
+        } else {
+            _ = goBackward(animated: true, completion: {})
+        }
+    }
 }
 
 extension EPUBNavigatorViewController: EditingActionsControllerDelegate {

Sources/Navigator/EPUB/EPUBReflowableSpreadView.swift

@@ -397,6 +397,20 @@ final class EPUBReflowableSpreadView: EPUBSpreadView {
         NSObject.cancelPreviousPerformRequests(withTarget: self, selector: #selector(notifyPagesDidChange), object: nil)
         perform(#selector(notifyPagesDidChange), with: nil, afterDelay: 0.3)
     }
+
+    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
+        guard viewModel.scroll else { return }
+
+        let insets = scrollView.contentInset
+        let atBottom = scrollView.contentOffset.y + scrollView.bounds.height >= scrollView.contentSize.height + insets.bottom - 1
+        let atTop = scrollView.contentOffset.y <= -insets.top + 1
+
+        if atBottom && velocity.y > 1.0 {
+            delegate?.spreadView(self, didRequestChapterNavigationForward: true)
+        } else if atTop && velocity.y < -1.0 {
+            delegate?.spreadView(self, didRequestChapterNavigationForward: false)
+        }
+    }
 }
 
 /// Determines the Readium CSS stylesheets to use depending on the publication languages and

Sources/Navigator/EPUB/EPUBSpreadView.swift

@@ -41,6 +41,9 @@ protocol EPUBSpreadViewDelegate: AnyObject {
 
     /// Called when WKWebview terminates
     func spreadViewDidTerminate()
+
+    /// Called when the user scrolls past the edge of the chapter in scroll mode.
+    func spreadView(_ spreadView: EPUBSpreadView, didRequestChapterNavigationForward forward: Bool)
 }
 
 class EPUBSpreadView: UIView, Loggable, PageView {

Sources/Navigator/Toolkit/PaginationView.swift

@@ -297,6 +297,8 @@ final class PaginationView: UIView, Loggable {
 
         if currentIndex == index {
             scrollToView(at: index, location: location, completion: completion)
+        } else if isVerticalChapterTransition && animated {
+            navigateVertically(to: index, location: location, completion: completion)
         } else {
             fadeToView(at: index, location: location, animated: animated, completion: completion)
         }
@@ -343,6 +345,63 @@ final class PaginationView: UIView, Loggable {
             size: scrollView.frame.size
         ), animated: false)
     }
+
+    // MARK: - Vertical Chapter Transition
+
+    /// When true, chapter-to-chapter transitions use a vertical slide animation
+    /// (scroll == true && verticalText == false).
+    var isVerticalChapterTransition: Bool = false {
+        didSet {
+            guard oldValue != isVerticalChapterTransition else { return }
+            // Disable horizontal paging scroll when in vertical scroll mode so
+            // the WebView's own scroll view handles all touch input.
+            scrollView.isScrollEnabled = !isVerticalChapterTransition
+        }
+    }
+
+    var isAnimatingChapterTransition = false
+
+    private func navigateVertically(to index: Int, location: PageLocation, completion: @escaping () -> Void = {}) {
+        isAnimatingChapterTransition = true
+
+        let height = bounds.height
+        let isForward = location != .end
+        let slideOutY: CGFloat = isForward ? -height : height
+        let slideInY: CGFloat = isForward ? height : -height
+
+        let snapshot = snapshotView(afterScreenUpdates: false)
+        snapshot?.frame = bounds
+        if let snapshot = snapshot { addSubview(snapshot) }
+
+        // New chapter loads off-screen behind the snapshot.
+        scrollView.transform = CGAffineTransform(translationX: 0, y: slideInY)
+
+        // Phase 1: slide out current content immediately — no waiting.
+        UIView.animate(withDuration: 0.2, delay: 0, options: .curveEaseIn) {
+            snapshot?.transform = CGAffineTransform(translationX: 0, y: slideOutY)
+        }
+
+        // Phase 2: slide in new chapter as soon as it finishes loading.
+        setCurrentIndex(index, location: location) { [weak self] in
+            guard let self = self else { return }
+            UIView.animate(
+                withDuration: 0.25,
+                delay: 0,
+                usingSpringWithDamping: 1.0,
+                initialSpringVelocity: 0.8,
+                options: [],
+                animations: {
+                    self.scrollView.transform = .identity
+                },
+                completion: { [weak self] _ in
+                    guard let self = self else { return }
+                    snapshot?.removeFromSuperview()
+                    self.isAnimatingChapterTransition = false
+                    completion()
+                }
+            )
+        }
+    }
 }
 
 extension PaginationView: UIScrollViewDelegate {