See this Screencast for a short video of the issue at runtime: https://www.screencast.com/t/YXm6xOtO
Here's the code to reproduce:
<grid:RadDataGrid ItemsSource="{x:Bind ViewModel.DataItems, Mode=OneWay}"
UserGroupMode="Disabled"
UserFilterMode="Disabled"
UserColumnReorderMode="None"
UserSortMode="None"
AutoGenerateColumns="False"
GridLinesVisibility="Horizontal">
<grid:RadDataGrid.Columns>
<grid:DataGridNumericalColumn Header="1"
PropertyName="Key"
SizeMode="Auto" />
<grid:DataGridTextColumn Header="2"
PropertyName="First"
SizeMode="Auto" />
<grid:DataGridTextColumn Header="3"
PropertyName="Second"
SizeMode="Auto" />
<grid:DataGridTextColumn Header="Stretchy"
PropertyName="Stretch"
SizeMode="Auto" />
<grid:DataGridTextColumn Header="5"
PropertyName="SecondLast"
SizeMode="Auto" />
<grid:DataGridTextColumn Header="6"
PropertyName="Last"
SizeMode="Auto" />
<grid:DataGridNumericalColumn Header="7"
PropertyName="Count"
SizeMode="Auto" />
</grid:RadDataGrid.Columns>
</grid:RadDataGrid>