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.