Please add an option to allow to select folders on middle click. Right now, the middle click behaviour of folders and files is different, causing a bit of confusion. Middle click also is a much more convenient way to select folders than hovering over the icon and clicking the '+' which appears, especially in 'details' view with the smallest icon size.
Middle-click opens the item in a new tab, which is a common and expected behavior for tab-based programs. If you don't like the default single-click-to-open behavior (I don't like it either) you can return to the typical left-click-select behavior using System Settings > Input > Mouse > "Double-click to open files and folders (select icons on first click)" This setting is rather buried and not in the right place, IMHO. We are tracking that with https://bugs.kde.org/show_bug.cgi?id=377310