Window containment describe boundaries in which window exists. There are two types of containment.

By default, containment is inherited from the taskbar window is bound to, from it's "windowsContainment" options, but it can also be set explicitly using window "containment" option.

Windows on the top taskbar has "containment" set to "viewport", where windows on the bottom taskbar has "containment" set to "visible". Resize and drag the windows around to get the difference.