Gerry gave the solution, to elaborate on taht -
The TextBlock is a read-only control, and that's why you can't edit the content when you double-click on it -
MS Learn | TextBlock Class[
^]
To use editing, you should use an editable control such as 'TextBox' in your 'CellEditingTemplate' -
<DataGridTemplateColumn IsReadOnly="False" Header="Comment" MinWidth="80">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<TextBlock
Background="LightGray"
Margin="5"
MinHeight="35"
TextAlignment="Center"
Text="{Binding Comment, Mode=OneWay}"
IsEnabled="True"
/>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
<DataGridTemplateColumn.CellEditingTemplate>
<DataTemplate>
<TextBox
Background="LightSalmon"
Margin="5"
MinHeight="35"
TextAlignment="Center"
Text="{Binding Comment, UpdateSourceTrigger=PropertyChanged}"
IsEnabled="True"
/>
</DataTemplate>
</DataGridTemplateColumn.CellEditingTemplate>
</DataGridTemplateColumn>
The 'TextBox' in the 'CellEditingTemplate' allows for your user input, and the 'UpdateSourceTrigger=PropertyChanged' makes sure that the source property gets updated as your user types.