After installing 2.7 update, was happy to try implement the Viewport functionality:
we have our diagram document view within a Grid and then ScrollViewer in XAML, e.g.

<UserControl>
<Grid>
<ScrollViewer>
<Diagram>

</Diagram>
</ScrollViewer>
</Grid>
</UserControl>

but the viewport method does not understand that scrollviewer moved, e.g. scrolling happened and viewport still reports the old rectangle which does not correspond to the actual view area ?

strange, when i simplified the issue in code for you to see, it disappeared:

in NodeModifying handler, i had:

Rect size = new Rect(0, 0, diag.Viewport.Width, diag.Viewport.Height);
size.Offset(xoffdrop, yoffdrop); //20,20 only
if (!size.Contains(MousePosition)) //not within view itself
noVea.CancelDrag();

which was cancelling moving of nodes once i moved another type of node way down in zoomed out mode so that the scrollbar moved.

Now that i simplified this to:

if (!diagH.Viewport.Contains(MousePosition))
noVea.CancelDrag();

the canceldrag does not happen prematurely, its always OK

====

maybe either viewport width/height or offset do not work for zoomed out viewports?

but i'm a happy coder atm thx

well it is supposed to duplicate the current viewport rect to a new one against which it checks the mouseposition (if within)

ah yes, now i see: this moved the origin, and made the scroll mistake, thx!