Default drag adorner does not display when multiple items are selected

[gtaylor1981]

If multiple items are selected (in a ListBox for example) then the default drag adorner does not display.

What steps will reproduce this issue?

Set DragDrop.UseDefaultDragAdorner="True" on a ListBox with SelectionMode="Extended" then drag multiple items.

This can be seen directly in ListBoxSamples.xaml at ListBox > Horizontal > ListBox bound to a collection.
Dragging a single item shows the default drag adorner, dragging multiple items does not.

Expected outcome

Expectation would be to see a similar drag adorner as for a single item, but with multiple items.

The code for this seems to be present in DragDrop.cs however it is not used because the useDefaultDragAdorner flag is negated there. If the negation is removed, the code works as expected.

Environment

• GongSolutions.WPF.DragDrop v2.2.0
• Windows 10 (1709)
• Visual Studio 2019 v16.6.2
• .NET Framework 4.6