I want to have a continuous Cell, which is not cropped to the width of the corresponding table column. Returning YES to the - (BOOL)outlineView:(NSOutlineView *)outlineView isGroupItem:(id)item delegate method takes no effect.
This screenshot visualizes my problem:

According to the documentation,
outlineView:dataCellForTableColumn:item:will be called a first time with the table column parameter set tonil. If you return a cell in that case (as opposed to returningnil), it will be used to draw group rows.