Replace placement calls with region nodes

tests/determinism.rs

@@ -23,14 +23,14 @@ impl Widget for BranchesOnMeasurement {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let mut top = UiRegion::FULL;
         top.y.end = top.y.start.offset(40.0);
-        let measured = painter.place(&self.probe, top).len(Axis::X);
+        let measured = painter.widget_within(&self.probe, top).len(Axis::X);
         let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
 
         let mut below = UiRegion::FULL;
         below.y.start = below.y.start.offset(40.0);
         match px > self.threshold {
-            true => painter.place(&self.wide, below),
-            false => painter.place(&self.narrow, below),
+            true => painter.widget_within(&self.wide, below),
+            false => painter.widget_within(&self.narrow, below),
         };
         Size::LEFTOVER
     }