WIP: a stack does not take its sizing child's fraction twice

Every child gets the whole of the stack's box rather than `box_of(size)`,
and `widget_at` does not resolve a rule into a box already chosen from it.
Fixes half a row becoming a quarter, which no oracle can see because warm
and cold shrink alike. Pinned by
`a_stack_sized_by_a_child_does_not_take_that_childs_fraction_twice`.

Not landed. Seed 1091 at depth 4, `shuffle-swap-for-three`, disagrees by
three steps of the grid -- warm 1053.9971 against cold 1054 -- where the
oracle tolerates two. Not a structural divergence: `box_of` was also making
a child's placement exact, by handing it a box of exactly the length it
asked for, and giving it the whole box instead puts a rounding back at each
nesting level. Two nested stacks is three steps. Widening AGREE_STEPS is
not the answer; finding the composition that went from exact to rounded is.

Everything else is green: suite, shrinker at 400 seeds of depth 5, oracle at
1000 seeds of depth 6, and the rest of 2000 seeds at depth 4.

Also carries examples/text.rs's top panel taking the full width.

core/src/ui/painter.rs

@@ -168,10 +168,20 @@ impl<'a> Painter<'a> {
         let region_node = self.rsc.widgets().is_region_node(id.id());
         let declared = self.declared_lens(id);
         let align = self.rsc.widgets().alignment(id.id());
+        // A rule this box was already chosen from is not resolved into it a
+        // second time. The box is that rule's length already, so resolving
+        // it again takes the fraction twice -- a widget declaring half of a
+        // stack, in the stack its own answer made half a row, is a quarter
+        // of the row. Pixels survive it, being the same length wherever they
+        // are taken from, which is why only a share ever shrank.
+        let resolve = AXES.map(|axis| match decided[axis as usize] {
+            true => None,
+            false => declared[axis as usize],
+        });
         // Composing `FULL` through a box is not quite the identity in f32,
         // so a child with nothing declared keeps the box it would have had.
-        let local = match declared.iter().any(Option::is_some) {
-            true => declared_box(region, declared, align),
+        let local = match resolve.iter().any(Option::is_some) {
+            true => declared_box(region, resolve, align),
             false => region,
         };
         let within = match local == UiRegion::FULL {
@@ -393,14 +403,6 @@ impl<'a> Painter<'a> {
             .is_some()
     }
 
-    /// The part of this widget's box that something of `size` takes, at the
-    /// near edge. A container that reports one child's size gives every child
-    /// this, so what it draws is inside what it says it occupies.
-    pub fn box_of(&self, size: Size) -> UiRegion {
-        let lens = placed_lens(size, [None; 2], [false; 2]);
-        placed_box(UiRegion::FULL, lens, RegionAlign::NEAR)
-    }
-
     /// This widget's box in pixels. Reading it makes the drawing one that
     /// holds for this box only, until `holds` says how far it goes.
     pub fn px_size(&mut self) -> PxVec2 {

examples/text.rs

@@ -26,6 +26,7 @@ impl DefaultAppState for State {
             .wrap(true)
             .text_align(Align::LEFT)
             .pad(16)
+            .width(rel(1.0))
             .background(panel());
 
         // Each one takes the whole width, because `text_align` puts the

src/widget/position/stack.rs

@@ -13,31 +13,42 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
-        // Whichever child sizes the stack decides the box every child gets.
-        // The stack reports that size, so a child given a longer box would
-        // draw outside what the stack says it occupies.
+        // Every child gets the whole of this stack's box, the sizing one
+        // included, and the stack is then handed a box of the length that
+        // child asked for. Not the part of the box that length takes: the
+        // stack's own box becomes that length, and taking the fraction of it
+        // again is the fraction twice -- a child asking for half of a stack
+        // that is already half a row would have a quarter of the row.
+        //
+        // It cannot be told apart by asking whether this box is the answer
+        // yet, either. A drawing has to be a function of the box alone, since
+        // moving the stack into the box it asked for reuses the drawing by
+        // scaling it, and a drawing made a fraction of one box is right in
+        // any other. So: fractions of this box throughout, and the move is
+        // the whole of the difference.
+        let region = UiRegion::FULL;
+        // Whichever child sizes the stack is asked here and not again below,
+        // on the layer it ends up on: a retained drawing belongs to the layer
+        // it was made on, so measuring it anywhere else costs a second
+        // drawing of it. Its box is its own answer, so the answer is not
+        // placed inside it again.
         let size = match sizing.and_then(|i| self.children.get(i).map(|c| (i, c))) {
-            // On the layer that child ends up on, so the ask below is a reuse
-            // rather than a second drawing of it somewhere else: a retained
-            // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter.widget(child).size()
+                painter
+                    .widget_at(child, region, region.size(), [true; 2])
+                    .size()
             }
             None => Size::LEFTOVER,
         };
-        let region = painter.box_of(size);
         for (i, child) in self.children.iter().enumerate() {
+            if sizing == Some(i) {
+                continue;
+            }
             painter.child_layer_at(i);
-            // The sizing child placed its own content in the box its answer
-            // decided, and this box was derived from that answer, so applying
-            // its alignment again here would place it twice. Every other
-            // child is handed a box that owes nothing to its own answer, and
-            // where it sits in one bigger than itself is its own business.
-            match sizing == Some(i) {
-                true => painter.widget_at(child, region, region.size(), [true; 2]),
-                false => painter.widget_within(child, region),
-            };
+            // A box that owes nothing to this child's own answer: where it
+            // sits in one bigger than itself is its own business.
+            painter.widget_within(child, region);
         }
         size
     }

tests/cases/layout.rs

@@ -82,6 +82,31 @@ fn a_text_in_a_span_wraps_at_the_room_left_rather_than_the_whole_row() {
     assert!(crowded > whole_row, "{crowded} against {whole_row}");
 }
 
+/// A stack takes its size from one child and gives every child that size, so
+/// a child asking for half of it is asking for half of what it is itself the
+/// size of. Once the stack has been placed at the length it reported that
+/// length is the box, and taking the fraction of it again takes it twice:
+/// half a row became a quarter, and a further stack around it a further half.
+/// Nothing pinned it because a pixel is the same length wherever it is taken
+/// from, so only a share ever shrank -- and warm and cold shrink alike, so no
+/// oracle saw it either.
+#[test]
+fn a_stack_sized_by_a_child_does_not_take_that_childs_fraction_twice() {
+    let mut h = Harness::new((400, 200));
+    let half = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
+    let behind = rect(Color::BLUE).add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![behind.add_strong(&mut h.rsc), half.add_strong(&mut h.rsc)],
+        size: StackSize::Child(1),
+    }
+    .add(&mut h.rsc);
+    h.set_root((stack,).span(Dir::RIGHT).width(rel(1.0)));
+
+    assert_corners!(h, stack, (0, 0), (200, 200));
+    assert_corners!(h, half, (0, 0), (200, 200));
+    assert_corners!(h, behind, (0, 0), (200, 200));
+}
+
 /// The same reading through a pad: its inset is the whole box less the
 /// padding, so half of the inset plus the padding is half the box plus one
 /// padding, not two.