Take a span child's length from its hint, and ask it once in its slot

A share child was drawn in the measuring room and again in its slot, and
one record holding two questions made every local change under it defer
to the span. Where a rule or a hint gives the length along the span, the
first ask answers nothing the rule does not, so the child is asked once,
in its slot; the widgets that always report the whole of their box now
say so. A hint with a fraction resolves against the frame and pins it.

The dump rig prints every cold layout so a change to it shows in a diff.

core/src/ui/painter.rs

@@ -259,17 +259,24 @@ impl<'a> Painter<'a> {
         declared_lens(self.rsc.widgets(), id.id())
     }
 
-    /// What a child says its length is without being drawn, if it can say.
-    /// Asking counts as reading its size.
+    /// What a child says its length is without being drawn, if it can say,
+    /// as the length its draw would report: a fraction in it is resolved
+    /// against this widget's frame, which is the frame a child asked with
+    /// nothing narrowed gets. Asking counts as reading its size.
     pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
         let widgets = self.rsc.widgets();
         // A rule is the answer where there is one: it wins over whatever the
         // widget would draw, so it has to win over what the widget says too.
-        let hint = widgets.size_rules(id.id()).axis(axis).exact().or_else(|| {
-            widgets
-                .get_dyn(id.id())
-                .and_then(|widget| widget.size_hint(axis))
-        });
+        let hint = widgets
+            .size_rules(id.id())
+            .axis(axis)
+            .exact()
+            .or_else(|| {
+                widgets
+                    .get_dyn(id.id())
+                    .and_then(|widget| widget.size_hint(axis))
+            })
+            .map(|hint| hint.within_len(self.frame.axis(axis)));
         #[cfg(feature = "layout-diagnostics")]
         diag::hint_read(id.id(), self.id, axis, hint);
         match hint {
@@ -277,6 +284,11 @@ impl<'a> Painter<'a> {
                 #[cfg(feature = "layout-diagnostics")]
                 diag::bump(Counter::HintHits);
                 self.depend_on(id);
+                // A fraction was just resolved against this frame, so what
+                // this draw does with it is a function of the frame's length.
+                if hint.rel != Rel::ZERO {
+                    self.frame_own_len[axis as usize] = Some(self.frame.axis(axis));
+                }
                 Some(hint)
             }
             None => {