Need Xamarin CollectionView Behavior to Run Again

Question

I created a behavior that sizes my collectionview rows to prevent the collectionview from invoking its own scrolling behavior and for preventing too much space after the last row.

XAML

<CollectionView ItemsSource="{Binding}" Margin="-55,0,0,0">

    <CollectionView.ItemsLayout>
        <LinearItemsLayout Orientation="Vertical" ItemSpacing="0"/>
    </CollectionView.ItemsLayout>

    <CollectionView.Behaviors>
        <core:CollectionViewRectangleVerticleHeightBehavior/>
    </CollectionView.Behaviors>

    <CollectionView.ItemTemplate>
        <DataTemplate>
            ....
        </DataTemplate>
    </CollectionView.ItemTemplate>
</CollectionView>

Class file (not code behind)

public class CollectionViewRectangleVerticleHeightBehavior : Behavior<CollectionView>
{
    bool hasHeightChanged = false;

    public CollectionView CollectionViewObject { get; private set; }

    protected override void OnAttachedTo(CollectionView bindable)
    {
        base.OnAttachedTo(bindable);
        CollectionViewObject = bindable;
        bindable.BindingContextChanged += Bindable_BindingContextChanged;
        bindable.SizeChanged += Bindable_SizeChanged;
    }

    ...

    private void Bindable_SizeChanged(object sender, EventArgs e)
    {
        if (!hasHeightChanged)
        {
            if (CollectionViewObject.BindingContext is Grouping<string,Estimate> grp)
            {
                CollectionViewObject.HeightRequest = (grp.Count * (32 + grp.Count == 1 ? 0 : 66 + 50));
                hasHeightChanged = true;
            }
        }
    }

    ...
}

It works fine with the initial data given to ItemsSource. However, when I add records to the ObservableCollection in the viewmodel, the behavior is not run again thereby causing a scroll bar to appear.

How do I force the CollectionView to "redraw" itself so that the Behavior is run again? Ideally, an event would fire if the scrollbar is about to appear and a expose a method to invoke the behavior for the current row. I don't find anything like that in reality.