I opened Dolphin as root (using the Open As Root context menu) and opened in a folder, everything selection went to Single Click. I check my System Settings > Workspace Behavior > General Behavior and it was set to Double Click. It took me a while to figure out that I need to launch System Settings using sudo systemsettings and set the Double Click option for root.

This is a common setting for people to prefer double click on folders and files to open. Dolphin should have this in the settings for each user including when opened as root.