Ask each child once and place its answer by re-expression

A widget draws in the box it is asked in and its answer is placed inside
that box by re-expressing the drawing; nothing is drawn again in a box an
answer chose. The offer machinery, whose job was to tell a measuring draw
from a placing one, goes with the placing draw. A span measures each child
from its cursor and moves fixed children to their slots with place_at; a
share child is asked once more in its decided slot with its frame narrowed
to it. A stack asks non-sizing children in the box its sizing child
decided. A scroll asks its content once and moves it to the scrolled
offset. A local redraw asks the retained question again and puts the
answer back where the parent placed it.

A symbolic length a child pinned composes through Part::Of exactly where
the part is the whole box less pixels, and pins the parent's own length
otherwise; dropping it let a pad reuse a drawing across a narrowed frame
of the same pixel length (shrinker seeds 60, 248 and 384 at depth 5).

Suite 114/114 including the two decided-box pins, fast oracle 11/11,
shrinker 400 seeds at depth 5 over all fifteen cases.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/widget/position/scroll.rs

@@ -12,10 +12,9 @@ pub struct Scroll {
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let container_len = painter.px_len(self.axis);
-        // Measured in the whole viewport, then drawn at the scrolled offset.
-        let whole = UiRegion::FULL;
+        // Asked in the whole viewport, then put at the scrolled offset.
         let answer_len = painter
-            .widget_at(&self.inner, whole, [Place::Fill(Part::All); 2])
+            .widget_at(&self.inner, [None; 2], [Place::Fill(Part::All); 2])
             .len(self.axis);
         let fixed = Len::from_parts(answer_len.rel, answer_len.px).to_px(container_len);
         self.container_len = container_len;
@@ -59,11 +58,10 @@ impl Widget for Scroll {
         }
         // The viewport is the inner's frame, so a fraction it declares or
         // reports is a fraction of what is on screen rather than of the
-        // content box its own answer decided. Where it is drawn is the
-        // content box, scrolled.
-        painter.widget_at(
+        // content box its own answer decided. Where it goes is the content
+        // box, scrolled: its drawing moved there, not made again there.
+        painter.place_at(
             &self.inner,
-            whole,
             self.axis
                 .pair(Place::Fill(Part::From(content)), Place::Fill(Part::All)),
         );