Hello, I've been a Mac owner for a grand total of 2 days on a 2012 mini running 10.8.1, so bear with me. I'm currently porting some .NET application code to interface with Cocoa via MonoMac, and I've encountered a weird behavior with NSOutlineView where sometimes expanding a nested node in the tree will cause the root node to collapse immediately afterward. Initially I thought this might just be my Mono code, but I've also reproduced the behavior with Apple's DragNDropOutlineView demo, and with the project navigator in Xcode. I've found a semi-reliable way to produce the behavior is to rapidly click the arrow of sub-node to expand and collapse it, along with some subtle pointer movement. Eventually it will trigger the root node to collapse. Even more interesting, I've managed to crash Xcode several times just by doing this. Another semi-reliable way to reproduce it is click two different node arrows in a very short time period. Now, I don't expect users to intentionally mash mouse buttons until things explode, but in my own application I've had this occur several times just via casual node expansion. Since I dynamically build and destroy the tree as nodes are expanded and collapsed, the root node collapsing is a really big problem for me. Has anyone encountered this before? Is it a known issue? Are there any suggestions for working around it?