Port Corona::freeEdges usages away from Containment::screen
lastScreen refers to the assigned index, primary, secondary and so on. screen is a rather complicated method of which screen the containment is on, potentially -1 if a screen is attached.
freeEdges or config lookup doesn't require the screen to be attached. It should use the more explicit property.
This MR leaves the only usages of "screen" to be relating to only paths relating to screen sizes.