Fix an issue where a tracking scroll content stops and the panel does…

…n't move

Resolve #530

Sources/Core.swift

@@ -66,6 +66,7 @@ class Core: NSObject, UIGestureRecognizerDelegate {
     private var stopScrollDeceleration: Bool = false
     private var scrollBounce = false
     private var scrollIndictorVisible = false
+    private var scrollBounceThreshold: CGFloat = -30.0
 
     // MARK: - Interface
 
@@ -337,7 +338,10 @@ class Core: NSObject, UIGestureRecognizerDelegate {
                     if surfaceView.grabberAreaContains(gestureRecognizer.location(in: surfaceView)) {
                         return false
                     }
-                    return allowScrollPanGesture(for: scrollView)
+                    guard state == layoutAdapter.mostExpandedState else { return false }
+                    // The condition where offsetY > 0 must not include here. Because this condition prevents the panel
+                    // pan gesture recognizer when a user start scrolling content from more than 0 offset.
+                    return allowScrollPanGesture(of: scrollView) { offsetY in offsetY <= scrollBounceThreshold  }
                 default:
                     return false
                 }
@@ -456,21 +460,24 @@ class Core: NSObject, UIGestureRecognizerDelegate {
                     }
                 } else {
                     if state == layoutAdapter.mostExpandedState {
+                        let allowScroll = allowScrollPanGesture(of: scrollView) { offsetY in
+                            offsetY <= scrollBounceThreshold || offsetY > 0
+                        }
                         switch layoutAdapter.position {
                         case .top, .left:
-                            if velocity < 0, !allowScrollPanGesture(for: scrollView) {
+                            if velocity < 0, !allowScroll {
                                 lockScrollView(strict: true)
                             }
-                            if velocity > 0, allowScrollPanGesture(for: scrollView) {
+                            if velocity > 0, allowScroll {
                                 unlockScrollView()
                             }
                         case .bottom, .right:
                             // Hide a scroll indicator just before starting an interaction by swiping a panel down.
-                            if velocity > 0, !allowScrollPanGesture(for: scrollView) {
+                            if velocity > 0, !allowScroll {
                                 lockScrollView(strict: true)
                             }
                             // Show a scroll indicator when an animation is interrupted at the top and content is scrolled up
-                            if velocity < 0, allowScrollPanGesture(for: scrollView) {
+                            if velocity < 0, allowScroll {
                                 unlockScrollView()
                             }
                         }
@@ -804,7 +811,7 @@ class Core: NSObject, UIGestureRecognizerDelegate {
             } else {
                 let pinningOffset = contentOffsetForPinning(of: scrollView)
 
-                // `scrollView.contentOffset` can be a value in [-30, 0) at this time by `allowScrollPanGesture(for:)`.
+                // `scrollView.contentOffset` can be a value in [-30, 0) at this time by `allowScrollPanGesture(of:condition:)`.
                 // Therefore the initial scroll offset must be reset to the pinning offset. Otherwise, the following
                 // `Fit the surface bounds` logic don't working and also the scroll content offset can be invalid.
                 initialScrollOffset = pinningOffset
@@ -1103,16 +1110,15 @@ class Core: NSObject, UIGestureRecognizerDelegate {
         }
     }
 
-    private func allowScrollPanGesture(for scrollView: UIScrollView) -> Bool {
-        guard state == layoutAdapter.mostExpandedState else { return false }
+    private func allowScrollPanGesture(of scrollView: UIScrollView, condition: (_ offsetY: CGFloat) -> Bool) -> Bool {
         var offsetY: CGFloat = 0
         switch layoutAdapter.position {
         case .top, .left:
             offsetY = value(of: scrollView.fp_contentOffsetMax - scrollView.contentOffset)
         case .bottom, .right:
             offsetY = value(of: scrollView.contentOffset - contentOffsetForPinning(of: scrollView))
         }
-        return offsetY <= -30.0 || offsetY > 0
+        return condition(offsetY)
     }
 
     // MARK: - UIPanGestureRecognizer Intermediation