Appearing abruptly, LR now places a "Close" radio button in the center of my iMac screen, as shown in the attached. The Close button appears to do nothing when clicked, is not movable, and is present in every LR window, as well as in windows of every app that' open. Closing LR dismisses the button.
A Preference reset might get rid of this, but it seems odd.
OSX 10.11.6, iMac LR Classic CC