Address the grabber area detection in scroll tracking (#407)

Allows to expand the panel while touching in the grabber area initially.

Author: scenee

Sources/Core.swift

@@ -530,15 +530,20 @@ class Core: NSObject, UIGestureRecognizerDelegate {
             return false
         }
 
-        // When the current and initial point within grabber area, do scroll.
+        // When the current point is within grabber area but the initial point is not, do scroll.
         if grabberAreaFrame.contains(point), !grabberAreaFrame.contains(initialLocation) {
             return true
         }
 
+        // When the initial point is within grabber area and the current point is out of surface, don't scroll.
+        if grabberAreaFrame.contains(initialLocation), !surfaceView.frame.contains(point) {
+            return false
+        }
+
         let scrollViewFrame = scrollView.convert(scrollView.bounds, to: surfaceView)
         guard
-            scrollViewFrame.contains(initialLocation), // When initialLocation not in scrollView, don't scroll.
-            !grabberAreaFrame.contains(point)           // When point within grabber area, don't scroll.
+            scrollViewFrame.contains(initialLocation), // When the initial point not in scrollView, don't scroll.
+            !grabberAreaFrame.contains(point)          // When point within grabber area, don't scroll.
         else {
             return false
         }