Replace placement calls with region nodes

tests/unsettled.rs

@@ -216,8 +216,8 @@ fn swapping_two_children_lands_where_growing_them_that_way_does() {
 
 /// Eight widgets, shrunk from 80. The scroll decides how wide to make its
 /// content from what the content says, and hands that box down through a
-/// pass-through; the span under it was placed once, in that box, so nothing
-/// at its own edge says the box was its own answer.
+/// pass-through; the span under it was given that box once, so nothing at its
+/// own edge says the box was its own answer.
 fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget<Span>; 2]) {
     let words = "Wrapping shapes one source into as many lines as the box leaves room for,";
     let text = wtext(words).size(16).wrap(true).add(&mut h.rsc);
@@ -277,7 +277,7 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
 }
 
 #[test]
-fn a_span_placed_once_in_a_box_its_answer_decided() {
+fn a_span_given_the_box_its_answer_decided_matches_a_cold_layout() {
     let mut warm = Harness::new((640, 900));
     let (ids, spans) = plant_scrolled(&mut warm, false);
     warm.frame();