Debian testing can't tab to desktop?

[Chevelle]

Hello, I upgraded a Mate Bookworm plus Backports to Debian Testing.  
When I press alt-control-tab, I cycle between the Top Panel and Bottom 
Panel, but not the desktop.  On the Bottom Panel is a button that says 
Show Desktop, I press space on that, but it doesn't make any 
difference.  Any idea how to bring back the desktop?