Replace placement calls with region nodes

src/widget/position/span.rs

@@ -21,8 +21,8 @@ pub struct Span {
 impl Widget for Span {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
-        // A length for every child before any is placed: from its own hint
-        // where it has one, and from drawing it where it does not.
+        // A length for every child before their final boxes are chosen: from
+        // a hint where one exists, and from drawing otherwise.
         let mut cursor = UiScalar::rel_min();
         let mut lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
@@ -33,7 +33,7 @@ impl Widget for Span {
             let region = UiRegion::from_axis(axis, span, UiSpan::FULL);
             let len = match painter.known_len(child, axis, region) {
                 Some(len) => len,
-                None => painter.place(child, region).len(axis),
+                None => painter.widget_within(child, region).len(axis),
             };
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
@@ -47,24 +47,28 @@ impl Widget for Span {
         // beside 300 px is full at 600 and overfull at 400. The answer is
         // the same on either side of the length the fixed parts alone fill.
         let fixed = 1.0 - total.rel;
-        let shares = total.leftover > 0.0 && fixed * painter.px_len(axis) > total.px;
+        let mut shares = false;
         if total.leftover > 0.0 {
-            let range = if fixed > 0.0 {
+            let current = painter.px_len(axis);
+            let holds = if fixed > 0.0 {
                 let full = total.px / fixed;
+                shares = current > full;
                 match shares {
-                    true => full..=f32::INFINITY,
-                    false => f32::NEG_INFINITY..=full,
+                    true => Holds::exact(full.next_up()..=f32::INFINITY),
+                    false => Holds::exact(f32::NEG_INFINITY..=full),
                 }
             } else if fixed < 0.0 {
                 let full = total.px / fixed;
+                shares = current < full;
                 match shares {
-                    true => f32::NEG_INFINITY..=full,
-                    false => full..=f32::INFINITY,
+                    true => Holds::exact(f32::NEG_INFINITY..=full.next_down()),
+                    false => Holds::exact(full..=f32::INFINITY),
                 }
             } else {
-                f32::NEG_INFINITY..=f32::INFINITY
+                shares = total.px < 0.0;
+                Holds::ANY
             };
-            painter.holds(axis, range);
+            painter.holds(axis, holds);
         }
 
         let mut start = UiScalar::rel_min();
@@ -93,7 +97,7 @@ impl Widget for Span {
             if self.dir.sign == Sign::Neg {
                 region.flip(axis);
             }
-            let placed = painter.place(child, region);
+            let placed = painter.widget_within(child, region);
             if self.ortho == OrthoSize::Children {
                 let used = placed.len(!axis);
                 // Choosing between a fixed and a relative length from the