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/offset.rs

@@ -15,6 +15,6 @@ impl Widget for Offset {
             moved(painter.extent_len(Axis::X), self.amt.x),
             moved(painter.extent_len(Axis::Y), self.amt.y),
         ];
-        painter.widget_at(&self.inner, UiRegion::FULL, place).size()
+        painter.widget_at(&self.inner, [None; 2], place).size()
     }
 }

src/widget/position/pad.rs

@@ -30,7 +30,7 @@ impl Widget for Pad {
             inset(self.padding.left, self.padding.right),
             inset(self.padding.top, self.padding.bottom),
         ];
-        let inner = painter.widget_at(&self.inner, UiRegion::FULL, place).size();
+        let inner = painter.widget_at(&self.inner, [None; 2], place).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,

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)),
         );

src/widget/position/span.rs

@@ -25,19 +25,23 @@ impl Widget for Span {
         // A length for every child before their final slots are chosen. The
         // frame passes through unchanged, so `rel(0.5)` is half the area this
         // span was given whatever else is in it and wherever this child sits
-        // among them; what it is drawn in is the room left from the cursor,
-        // because a text has to wrap at the width actually there.
+        // among them; what it is asked in is the room left from the cursor,
+        // because a text has to wrap at the width actually there. This is
+        // the one ask a fixed child gets: its slot is its answer, and the
+        // drawing is moved there once the shares are known.
         let mut cursor = Len::rel_min();
-        let mut lens = Vec::with_capacity(self.children.len());
+        let mut sizes = Vec::with_capacity(self.children.len());
         for child in &self.children {
             let room = Place::Within(Part::From(along(cursor, far)));
-            let len = painter
-                .widget_at(child, UiRegion::FULL, axis.pair(room, across))
-                .len(axis);
+            let size = painter
+                .widget_at(child, [None; 2], axis.pair(room, across))
+                .size();
+            let len = size.axis(axis);
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
-            lens.push(len);
+            sizes.push(size);
         }
+        let lens: Vec<LayoutLen> = sizes.iter().map(|size| size.axis(axis)).collect();
 
         let gaps = self
             .gap
@@ -88,7 +92,8 @@ impl Widget for Span {
         let mut taken = Weight::ZERO;
         let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
-        for (child, len) in self.children.iter().zip(&lens) {
+        for (child, size) in self.children.iter().zip(&sizes) {
+            let len = size.axis(axis);
             // A child asking for nothing but a part of what is left over,
             // when nothing is, is not drawn at all. One that also asked for
             // pixels or a fraction keeps those and overflows.
@@ -106,12 +111,26 @@ impl Widget for Span {
             fixed.rel += len.rel;
             start = shared(fixed, taken, total.leftover, room);
             // Along the row the span says where the child goes, and that slot
-            // is the drawing's box outright rather than something to place an
-            // answer inside again.
-            let slot = Place::Fill(Part::From(along(from, start)));
-            let placed = painter.widget_at(child, UiRegion::FULL, axis.pair(slot, across));
+            // is the child's box outright rather than something to place an
+            // answer inside again. A share is decided here and nowhere
+            // else: its slot narrows its frame, and the child is asked in
+            // it, since a text wraps at the width it is actually given. A
+            // fixed child's slot is its own answer, so its drawing is put
+            // there as it is.
+            let slot = along(from, start);
+            let place = axis.pair(Place::Fill(Part::From(slot)), across);
+            let used = match len.leftover > Weight::ZERO && shares {
+                true => {
+                    let mut narrow = [None; 2];
+                    narrow[axis as usize] = Some(slot.len());
+                    painter.widget_at(child, narrow, place).len(!axis)
+                }
+                false => {
+                    painter.place_at(child, place);
+                    size.axis(!axis)
+                }
+            };
             if shrinks {
-                let used = placed.len(!axis);
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.
                 // A scalable child therefore makes Children scalable too;

src/widget/position/stack.rs

@@ -23,19 +23,26 @@ impl Widget for Stack {
             Some((i, child)) => {
                 painter.child_layer_at(i);
                 painter
-                    .widget_at(child, UiRegion::FULL, [Place::Fill(Part::All); 2])
+                    .widget_at(child, [None; 2], [Place::Fill(Part::All); 2])
                     .size()
             }
             None => Size::LEFTOVER,
         };
+        // Every other child gets the box the sizing child decided: the
+        // stack is that length, so that is the box they are asked in, and a
+        // fraction under them is a fraction of it. A share leaves the axis
+        // to whoever gave the stack its box. Where a child sits in a box
+        // bigger than itself is its own business.
+        let narrow = [Axis::X, Axis::Y].map(|axis| {
+            let len = size.axis(axis);
+            (len.leftover == Weight::ZERO).then(|| Len::from_parts(len.rel, len.px))
+        });
         for (i, child) in self.children.iter().enumerate() {
             if sizing == Some(i) {
                 continue;
             }
             painter.child_layer_at(i);
-            // Every other child is drawn in the stack's own box, and where it
-            // sits in one bigger than itself is its own business.
-            painter.widget(child);
+            painter.widget_at(child, narrow, [Place::Within(Part::All); 2]);
         }
         size
     }