Description
Is it by design, that, if the dd:DragDrop.IsDragSource attached property is set to True on a Datagrid it ceases to be editable, even if:

• IsReadOnly="False"
• the DataGrid's Itemsource is modifiable

To Reproduce
A simple way to reproduce this behavior is to apply two small modifications to the
DataGridSamples View XAML file in the Showcase test application:

Setting IsReadOnly to False on one of the DataGrid controls, like so:

<DataGrid x:Name="LeftBoundDataGrid"
    ...
    IsReadOnly="False"
    ...

does not make their cells editable, even though the properties of the DataGridRowModel all have valid setters.

If, however one sets dd:DragDrop.IsDragSource to False:

<DataGrid x:Name="LeftBoundDataGrid"
    ...
    dd:DragDrop.IsDragSource="False"
    ...

or removes this attached property altogether, the DataGrid becomes editable, as expected.

Environment

• OS: Windows 11 Pro, 24H2
• Target .NET Version: 9.0
• Package Version: 4.0.0 (Latest)

I sincerely hope, this is not a duplicate. I didn't find this behavior mentioned in the documentation or in any of the DataGrid-related issues.
(Finally: Although a bug report is probably not the proper place for this, I would nevertheless like to express my admiration for the sophistication of your code as well as my gratitude for making it available as open source!)