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>

tests/cases/determinism.rs

@@ -24,15 +24,15 @@ impl Widget for BranchesOnMeasurement {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
         let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
         let measured = painter
-            .widget_at(&self.probe, UiRegion::FULL, [Place::Within(Part::All), top])
+            .widget_at(&self.probe, [None; 2], [Place::Within(Part::All), top])
             .len(Axis::X);
         let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
 
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
         let place = [Place::Within(Part::All), below];
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
-            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
+            true => painter.widget_at(&self.wide, [None; 2], place),
+            false => painter.widget_at(&self.narrow, [None; 2], place),
         };
         Size::LEFTOVER
     }

tests/cases/retained.rs

@@ -156,9 +156,9 @@ fn a_span_child_that_declares_its_length_is_drawn_once() {
     h.set_root((hinted, asked).span(Dir::RIGHT));
 
     assert_eq!(told_draws.get(), 1);
-    // Only the available length changes: positioning the final slot does
-    // not invalidate a numeric size read.
-    assert_eq!(asked_draws.get(), 2);
+    // Asked once, from the cursor; its slot is its answer and the drawing is
+    // moved there.
+    assert_eq!(asked_draws.get(), 1);
 }
 
 #[test]
@@ -216,7 +216,7 @@ impl Widget for FromHint {
         let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
         painter.widget_at(
             &self.inner,
-            UiRegion::FULL,
+            [None; 2],
             [Place::Within(Part::All), Place::Within(Part::From(top))],
         );
         Size::LEFTOVER
@@ -255,10 +255,9 @@ impl Widget for ReadsBox {
 /// Reads its box across one axis only, so its drawing holds for a taller
 /// box on its own and only a wider one is worth a draw.
 ///
-/// Both of these report a quarter of what they read, without saying that the
-/// drawing holds there too, so each length they are asked at costs two draws:
-/// one to answer, and one in the quarter-sized box that answer places them
-/// in. The counts below are in those pairs.
+/// Both of these report a quarter of what they read. The quarter-sized box
+/// the answer places them in is not a question: the drawing is moved there,
+/// so each length they are asked at costs one draw.
 struct ReadsWidth {
     draws: Rc<Cell<usize>>,
 }
@@ -384,7 +383,7 @@ fn a_resize_redraws_what_read_its_box() {
     h.resize((800, 100));
     h.frame();
 
-    assert_eq!(draws.get(), settled + 2);
+    assert_eq!(draws.get(), settled + 1);
 }
 
 #[test]
@@ -404,7 +403,7 @@ fn a_resize_only_redraws_read_axes() {
 
     h.resize((800, 300));
     h.frame();
-    assert_eq!(draws.get(), settled + 2, "width changes its answer");
+    assert_eq!(draws.get(), settled + 1, "width changes its answer");
 }
 
 /// A window is measured onto the grid like everything else, so a resize too
@@ -431,7 +430,7 @@ fn a_resize_within_one_step_is_not_a_resize() {
 
     h.resize((400.0 + step, 200.0));
     h.frame();
-    assert_eq!(draws.get(), settled + 2);
+    assert_eq!(draws.get(), settled + 1);
 }
 
 /// The same for a box that changes because a sibling did: what is compared
@@ -878,7 +877,7 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
+                [None; 2],
                 [
                     Place::Within(Part::From(self.region.x)),
                     Place::Within(Part::From(self.region.y)),
@@ -967,7 +966,7 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                self.region,
+                [Some(self.region.x.len()), None],
                 [
                     Place::Fill(Part::From(self.extent.x)),
                     Place::Fill(Part::From(self.extent.y)),
@@ -1137,7 +1136,7 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
+                [None; 2],
                 [
                     Place::Fill(Part::From(self.extent.x)),
                     Place::Fill(Part::From(self.extent.y)),
@@ -1225,7 +1224,7 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
+                [None; 2],
                 [
                     Place::Fill(Part::From(UiSpan::new(
                         Len::px(self.start),
@@ -1267,7 +1266,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    UiRegion::FULL,
+                    [None; 2],
                     [
                         Place::Within(Part::From(self.region.x)),
                         Place::Within(Part::From(self.region.y)),
@@ -1287,7 +1286,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 painter
                     .widget_at(
                         &self.child,
-                        UiRegion::FULL,
+                        [None; 2],
                         [
                             Place::Fill(Part::From(self.extent.x)),
                             Place::Fill(Part::From(self.extent.y)),