问题
I've noticed that setting indentationPerLevel to something smaller than 16 doesn't work with NSOutlineView. It always seems to indent by at least 16 pixels. I've tried to set indentationPerLevel to 4, 8, and 12 but the visible indentation is always 16 pixels, although getIndentationPerLevel returns the size I've set it to. The visible indentation, however, always seems to be at least 16 pixels.
An exception is setting indentation to 0. This works and removes the indentation. Setting it to 4, 8, or 12, however, will always result in an indentation of 16 pixels. So could it be that 16 is a minimum here?
来源：https://stackoverflow.com/questions/36063085/is-there-a-minimum-indentationperlevel-of-16