Enable Metering window in App Setup/APP.
Click on the Mute Groups or Routing icon and the window opens. Click it again and it closes.
Click on the Meters icon and the window opens. Click it again and it flashes reopen.
I understand why, the metering window is floating, could be behind something, and the button recalls it. Perhaps the small fixed windows should close via a close button, same as all the other windows. If you're not used to the old behavior, it is not obvious how to close the small fixed windows.