Thanks for bringing up this topic as there are some facts that need to be clarified.
The native sticky header behavior in Mango translates the letter visible in the header following the scrolling position of the jump list, specifically of the new-coming data group. This means that, to implement the same behavior in RadJumpList, we need to track the scrolling position of the ScrollViewer (used in RadJumpList) per pixel and translate the contents of our sticky header accordingly. As you may already now, with Mango Microsoft introduced some major improvements in the way the ScrollViewer works. A new Manipulation Thread was introduced which performs the scrolling and thus offloads the UI thread to do UI virtualization (as you may recall there were many complaints about the poor scrolling performance on the previous version of Windows Phone - NoDo). Part of the performance improvements is also the approach of not constantly updating all ScrollViewer properties (ScrollOffset etc.) on the UI thread. This makes it very difficult to track the scrolling position and thus implement exactly the same Sticky Header behavior as seen on the native JumpList (I guess native APIs are used there).
You can read more about these performance improvements and their limitations here:
I hope this is helpful.
the Telerik team
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