NSOutlineView indentation in Lion

I have an NSOulineView that does not properly indent the expansion =
markers of secondary items on Lion. Basically, secondary expandable rows =
are not indented, all the expansion markers are to the far left and the =
text starts right behind it, also for secondary items (children). =
Non-expandable rows are properly indented as expected. The outlineview =
is in source list style. Also, the outlineview has =
indentationMarkerFollowsCell set to YES. On earlier OS systems this all =
works as expected, i.e. children are all indented according to their =
level, including the expansion markers. I cannot find anything in the =
docs or the release notes saying that Lion has a different behavior in =
this respect.

Does anyone else see this, and know how to fix this? Is this a (known) =
bug?

thanks
Christiaan