I am using Arduino 1.8.13 on Windows 10 but this problem has existed for as long as I can recall.
When I open a window within the Arduino IDE such as the Library Manager, Board Manager or Preferences windows and go to another open program (e.g. Firefox), I am unable to use Alt + Tab to bring the Arduino IDE windows to the front again; I have to use the mouse cursor to select the desired window from the taskbar. Does anyone else experience this? I use Alt + Tab a lot, and figure most competent users do also make good use of this shortcut.