Place a scroll's fitting content in the viewport, not in the window

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.

tests/cases/scroll.rs

@@ -145,3 +145,19 @@ fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
     h.set_root(clipper);
     h.frame();
 }
+
+/// Content that fits sits in the viewport, not in a box of the window's
+/// length anchored at the viewport's start. `Part::From` takes window
+/// lengths, so a `rel(1.0)` span in one is the window, and only a scroll
+/// filling the window would land right.
+#[test]
+fn content_that_fits_is_placed_in_the_viewport_and_not_in_the_window() {
+    let mut h = Harness::new((400, 400));
+    let head = rect(Color::RED).height(100).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).height(50).add(&mut h.rsc);
+    let scroll = Scroll::new(inner.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    h.set_root((head, scroll).span(Dir::DOWN));
+
+    assert_corners!(h, scroll, (0, 100), (400, 400));
+    assert_corners!(h, inner, (0, 225), (400, 275));
+}