Strange Behavior suddenly with Lightroom Classic going into Full-screen & stealing Focus

I've been using Lightroom on my Macbook successfully since May.  A few days ago something weird started happening:

1. Open Lightroom.
2. Go into Develop mode for a particular photo.
3. Change focus to any other app.
4. Change focus back to Lightroom.

When I do this, Lightroom suddenly goes into Full Screen Preview mode on my Macbook screen ... without me telling it to. To get out of this, I can usually press "F" twice and it returns to regular Develop mode.  Sometimes though, even doing that doesn't work so I have to stop and restart Lightroom in order to be able to edit photos.

This is super annoying.  I've tried everything to reset Lighroom back to the original, including starting it with Cmd-Opt-Del and then clicking on the Lighroom icon.  Even that "global reset" doesn't fix the problem.

Any ideas how to fix this?

Robert

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