A scroll that has not been scrolled and whose content fits asked for its
content box as `Part::From(UiSpan::FULL)`. A `Part::From` span is in window
lengths, so `rel(1.0)` in one is the whole window rather than the whole box,
and the content landed in a window-tall box anchored at the viewport's
start -- 50 px low for a 300 px viewport in a 400 px window.
Saying the whole of the box as `Part::All` is the one expression that cannot
mean anything else, and it is also the place the child was already asked in,
so the placement becomes a no-op.
iris-ai
e8a5792dcb