Well ... no joy there. I backed it up, then emptied the folder & restarted Poser. Except for the fact that the panes I'd previously left floating were now docked, nothing changed. And when I went back to the Preferences folder to check, sure enough it had put new copies of both Poser UI Prefs_1680x1050.xml and Poser UI Prefs_1920x1080.xml in there.

I'm at my wits end. Maybe a reinstall is the only way to fix this.