With the trackpad on a Mac notebook if your fingers make even a slight unintentional horizontal movement when scrolling it causes a selection to be started / made in the control that has the focus. With text controls such as the stc if the selection is not undone and another horizontal movement is made it starts a text drag and drop action which can be problematic. In a tree control such as the one in the demo if a slight unintentional horizontal movement is made while scrolling it causes an item to be selected. This is not as bad as the hard crashes that were present in 2.8.4.2 but is a fairly problematic from a usage point of view.
This was not present in 2.8.4.0, but I did also notice this issue in the 2.8.5 preview build
Thanks,
Cody Precord