Having a problem with folders inside the project window in CS6 using Apple 10.11 El Capitan. The twirly arrow will not work. The only way to open a folder is to click the folder name. You can no longer drag files in and out of the folder reliably. Clicking a file in the root folder will sometimes move it inside a highlighted folder. Haven't found all the weirdness yet but something is amiss.
Answer: damaged pref file:
Re-create the Adobe application preferences file to eliminate problems that a damaged preferences file can cause.
The main preferences file is called "Adobe [Product Name] [Product Version] Prefs.psp."
On Mac OS, the preference file is found at:
/Users/[user name]/Library/Preferences/Adobe [Product Name] [Product Version] Settings/Adobe [Product Name] [Product Version] Prefs.psp