Please see attached screenshot with question.
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At the top-right of the folder tree, there's a lock icon. (For anyone reading this who can't see it, you may need to turn on Preferences / Folder Tree / Appearance / Show Folder Tree header.)
Click that and the tree will stay where it is and not follow you as you move to other folders in the file display.
After turning the lock on, you may need to use Settings > Set as Default Lister to save the change, if Opus is configured not to save it automatically. (If you're using Layouts or Lister Styles to open/modify windows, then you may also need to update those.)
Alternatively, you can also add a toolbar on the left of the window which has a favorites list on it if you prefer to use that instead of the folder tree (or favorites menu), which then gives you an always-visible list of favorites while still being able to use the folder tree for other things.
Thanks! LOL I was looking all over in preferences. The lock is much better though because I can imagine people wanting to change that setting on the fly. Love the product.