Thanks for the fix, good catch.

Merged in david_gileadi/unity-ui-extensions/fix-panel-dimensions (pull request #13)

Calculate the panel dimensions in Start to avoid scale issues

Scripts/Layout/HorizontalScrollSnap.cs

@@ -16,6 +16,7 @@ namespace UnityEngine.UI.Extensions
             _isVertical = false;
             _childAnchorPoint = new Vector2(0, 0.5f);
             _currentPage = StartingScreen;
+            panelDimensions = gameObject.GetComponent<RectTransform>().rect;
             UpdateLayout();
         }
 

Scripts/Layout/VerticalScrollSnap.cs

@@ -16,6 +16,7 @@ namespace UnityEngine.UI.Extensions
             _isVertical = true;
             _childAnchorPoint = new Vector2(0.5f,0);
             _currentPage = StartingScreen;
+            panelDimensions = gameObject.GetComponent<RectTransform>().rect;
             UpdateLayout();
         }