I don't know what this new feature is called exactly or where in System Preferences the control panel is. There's a new feature - probably related to some 'tap drag' or 'sticky/lazy key' something or another. What happens is the first time you click the shift or command modifier keys to do something (eg. make a selection range, spot selections with ⌘-click, opening a link in a new tab with ⌘-click, etc), the modifier key is ignored. A 2nd click works as expected. (The 2nd click has to be within a few moments of the first or it 'resets'.) I also notice a drag lock thing happening after only a first click attempt. This feature is not for me!! Where is it and how do you turn this off? It's like trying to use a broken mouse or a trackpad on a Windows machine! (Neither of which I'm doing. And no my trackpad didn't just fail. Booting from old systems is normal. This is some intentional feature here.) For example, it's impossible to make a range selection (in the Finder in list view) followed by ⌘-click spot selections (all in the same selection - starting with the shift-click range and then adding additional selections with ⌘-click) with this enabled.