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The details part can be automatically opened when the row is selected, can be always visible, or can be always collapsed.
public DataGridRowDetailsVisibilityMode RowDetailsVisibilityMode { get; set; }
Public Property RowDetailsVisibilityMode As DataGridRowDetailsVisibilityMode
The details can be expanded by code using the property DetailsVisibility in the DataGridRow, or manually by pressing the toggle button located inside the row headers.