Retain layout sizes by pixel axis

src/widget/position/span.rs

@@ -15,16 +15,14 @@ impl Widget for Span {
         let mut cursor = UiScalar::rel_min();
         let mut lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let len = match painter.size_hint(child, axis) {
+            let mut span = UiSpan::new(cursor, UiScalar::rel_max());
+            if self.dir.sign == Sign::Neg {
+                span.flip();
+            }
+            let region = UiRegion::from_axis(axis, span, UiSpan::FULL);
+            let len = match painter.known_len(child, axis, region) {
                 Some(len) => len,
-                None => {
-                    let mut span = UiSpan::new(cursor, UiScalar::rel_max());
-                    if self.dir.sign == Sign::Neg {
-                        span.flip();
-                    }
-                    let region = UiRegion::from_axis(axis, span, UiSpan::FULL);
-                    painter.place(child, region).len(axis)
-                }
+                None => painter.place(child, region).len(axis),
             };
             cursor.abs += len.abs + self.gap;
             cursor.rel += len.rel;