In El Capitan 10.11.6, all folders and sidebar items are opening in new windows. HELP!

UPDATE: I deleted two Finder prefs files per these instructions:

From your Finder "Go" menu > "Go to folder" and enter

~/Library/Preferences/com.apple.finder.plist

Click Go.

Delete the plist file.

Then do the same for com.apple.sidebarlists.plist

Then, restart, or log out and in again.

and now everything is behaving normally. Leaving the original post for anyone else who has this problem.

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In Finder->Preferences->General there USED to be a checkbox labeled "always open folders in a new window," but that is long gone in El Capitan. Any other suggestions for stopping this annoying behavior?

Oddly enough, if I select a folder in a window and type cmd-O or cmd-down-arrow, the folder opens in the same window. But double-clicking always opens it in a new window.

Might there be a Terminal command that would allow me direct access to whatever prefs file stores this setting?

Thanks!

DM