If you press delete key when the scene variable screen has focus but not a field and one of the left side items is selected, it will delete the selected item. Even without any extensions left to delete, it asks if you want to delete the selection. (Windows app using version 5.3.201)
I frequently press the delete key expecting it to delete the selected variable. This probably explains where my scene went the other day. Luckily, it was just a test project.
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