What a handle carries when its screen has a screen below it: the back
method the framework adds itself, never declared by the screen. It is absent
from a tab and an item by kind; a handle reached from nothing — the launch
root — has no back at all (ScreenRootHandle omits it), while an edge's
handle requires it. A result whose path is not in the type — a retrace, a
restore — may be either, so its back is optional (ScreenBackMode).
What a handle carries when its screen has a screen below it: the
backmethod the framework adds itself, never declared by the screen. It is absent from a tab and an item by kind; a handle reached from nothing — the launch root — has nobackat all (ScreenRootHandleomits it), while an edge's handle requires it. A result whose path is not in the type — a retrace, a restore — may be either, so itsbackis optional (ScreenBackMode).