Add it to diagram's parent panel instead to the diagram:

<Grid Name="grid">
    <ScrollViewer ...>
        <diag:Diagram ... />
    </ScrollViewer>

    <Button Content="HUD"
        HorizontalAlignment="Right"
        VerticalAlignment="Bottom"
        Margin="0,0,16,16"
        />
</Grid> 

The margins are necessary when aligning to bottom or right to avoid overlapping the scrollbars.

I hope that helps,
Stoyan

Hi,

The code above will place the button at the bottom right corner. Are you asking how to style the button with semi-transparency and other HUD-like effects?

Stoyan

Hi,

You can ensure there's no blank space left in the scroll viewer (or other parent controls) like this:

var r = diagram.Bounds;
r.Height = Math.Max(r.Height, scrollViewer.ActualHeight);
diagram.Bounds = r; 

You could also call that code from the ScrollViewer.SizeChanged handler to fill any added space if the scroll viewer can resize while the application runs.

If on the other hand you'd like to keep the empty space but move the button to the diagram's corner, set button's bottom margin to scrollViewer.ActualHeight - diagram.Bounds.Height.

I hope that helps,
Stoyan