How to check if an UI Element has reached the top

There is a page which has a scroll viewer and some content which is dynamic in nature. In the middle of the page there is a grid. Whenever the user scrolls the page and grid reaches the top of the page i want a notifier. Basically i want to make the grid sticky on top whenever it reaches top of the page. Is there any way we can achieve this in Windows phone application. I dont want to calculate the offset because the content between top of the page and grid is dynamic.

This used to be tricky to do but thanks to the new Windows Composition API, it's now fairly simple.

Let's say I have a ScrollViewer named MainScroll which hosts a Grid called StickyGrid and I want to make the latter sticky once it hits the top.

There's the code with comments to explain what it does.

MainScroll.SizeChanged += (s, e) =>
{        
    // Let's first get the offset Y for the main ScrollViewer relatively to the sticky Grid.
    var transform = ((UIElement)MainScroll.Content).TransformToVisual(StickyGrid);
    var offsetY = (float)transform.TransformPoint(new Point(0, 0)).Y;

    // Get Composition variables.
    var scrollProperties = ElementCompositionPreview.GetScrollViewerManipulationPropertySet(MainScroll);
    var stickyGridVisual = ElementCompositionPreview.GetElementVisual(StickyGrid);
    var compositor = scrollProperties.Compositor;

    // Basically, what the expression 
    // "ScrollingProperties.Translation.Y > OffsetY ? 0 : OffsetY - ScrollingProperties.Translation.Y"
    // means is that -
    // When ScrollingProperties.Translation.Y > OffsetY, it means the scroller has yet to scroll to the sticky Grid, so
    // at this time we don't want to do anything, hence the return of 0;
    // when the expression becomes false, we need to offset the the sticky Grid on Y Axis by adding a negative value
    // of ScrollingProperties.Translation.Y. This means the result will forever be just OffsetY after hitting the top.
    var scrollingAnimation = compositor.CreateExpressionAnimation("ScrollingProperties.Translation.Y > OffsetY ? 0 : OffsetY - ScrollingProperties.Translation.Y");
    scrollingAnimation.SetReferenceParameter("ScrollingProperties", scrollProperties);
    scrollingAnimation.SetScalarParameter("OffsetY", offsetY);

    // Kick off the expression animation.
    stickyGridVisual.StartAnimation("Offset.Y", scrollingAnimation);
};

Here is a working demo on GitHub.