On 10.8 - 10.10 you could multi select item by drawing a rect with your mouse on NSCollectionView but on El Captain It does not work.
I have modified the sample xamarin sample ( Just change in the Xib to Multiselect) and this works on 10.8 and I presume 10.10 but not 10.11
Attached Sample ScreenShot Mouse Selection on 10.8 ( See Faint Mouse Rect)
Attached Sample MacDatabase Project.
I have been playing around with delegates and mouse Rects but don't seem to be finding the issue.
Anyone have any ideas ?