Taskbar buttons stopped working

Hello,

I'm perplexed. Until today clicking on an item in the taskbar flipped the relevant window to the top, or brought up a preview if the button was associated with multiple windows.

Today clicking on a button for anything other than the top window just causes the button to blink for a while. The associated window is not brought to the top -- I have to close the open windows to get the windows below.

I didn't change anything intentionally, but clearly something changed.

I greatly appreciate any help you can provide.