Starting a day or so ago, when I left-click a filename in a lister, the click toggles the selected/n on-selected status of the file, rather than letting me edit the filename. If s single-click several non-contiguous files, the files are cumulatively selected. In other words, DO acts as if I were control-clicking. Single clicking a directory in the directory tree works fine, but in the right pane clicking a directory misbehaves as described above.
Rebooting the machine did not help. No other programs are misbehaving.
You can configure Power Mode so the left mouse button works the same as in Details Mode, while still configuring other things differently. Don't be put off looking at Power Mode just because of the default configuration.
Equally, though, if you just want everything to act like Details mode you might as well use Details mode itself. (That's what I do. Power mode is more flexible but I'd only end up making it the same as Details mode, so I just use Details mode.)