The feature was apparently missing, as monitors were always fullscreened at the surface best monitor.
Keep using best monitor if the selected monitor is not specified, otherwise move the window to the selected monitor before going fullscreen.
This helper is useful to ensure we are consistent with how we keep a
window clamped to the workarea of a monitor when placing windows on
screen. (This does not affect snap-to-edges).
When creating new windows, it is better if we create them with a slight
offset to where they were created before so that they are visible to the
user separately from what they might be overshadowing.
