Keep the step 3/4 experiment as evidence

The worker's uncommitted attempt at evaluating children in parent-decided
boxes, preserved as it stood when it stopped: separate answer-only pixel
reads, per-child drawing contracts, provisional Fill asks in Span, and an
assertion that a placed drawing hold for its answer box. The suite passes
and every generated case stops on that assertion at Text. Not the
protocol; wip/one-ask is.

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

core/src/layout_diagnostics.rs

@@ -32,6 +32,7 @@ pub(crate) enum Counter {
     SizeReads,
     HintHits,
     HintMisses,
+    RetainedSizeHits,
     ReuseAttempts,
     ReuseExact,
     ReuseMoved,
@@ -41,6 +42,7 @@ pub(crate) enum Counter {
     ReuseOutside,
     ReuseWrongLayer,
     ReuseWrongNode,
+    PlaceRedraws,
     QueuePops,
     DepthReads,
     LocalRedraws,
@@ -68,6 +70,7 @@ impl Counter {
         "draw-result size reads",
         "hint hits",
         "hint misses",
+        "retained size hits",
         "reuse attempts",
         "reuse exact",
         "reuse moved",
@@ -77,6 +80,7 @@ impl Counter {
         "reuse: outside what it holds for",
         "reuse: another layer",
         "reuse: region-node choice changed",
+        "placed by redrawing",
         "redraw queue pops",
         "depth reads",
         "local redraws",

core/src/ui/active.rs

@@ -1,6 +1,6 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive,
+    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive, Size,
-    Size, TextureHandle, UiRegion, WidgetId,
+    TextureHandle, UiRegion, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -20,31 +20,21 @@ pub struct ActiveData {
     /// length. Its length is the same on every ask, which is what
     /// a local redraw relies on to ask its parent's own question again.
     pub frame: UiRegion,
-    /// The length its frame was narrowed to on each axis, as a length of
+    /// What of its parent's extent the drawing was given, and what it was
-    /// its parent's frame: a declared rule, or a box its parent decided for
+    /// given at the parent's first ask of it -- the question a cold layout
-    /// it. `None` forwards the parent's frame whole. Kept as a length rather
+    /// asks. A part is a length from the extent's start, so an extent that
-    /// than a position so that every placement puts the frame back in the
+    /// moved re-places every child by re-adding that start.
-    /// part it is given.
-    pub narrow: [Option<Len>; 2],
-    /// Where its drawing was put, as a part of its parent's box, and where
-    /// it was asked. The two differ where a container asks in one place and
-    /// places the answer in another -- a row measures from its cursor and
-    /// puts the child in its slot. A part is a length from the box's start,
-    /// so a box that moved re-places every child by re-adding that start.
     pub place: [Place; 2],
     pub offer_place: [Place; 2],
-    /// The box it was asked in, in its frame's coordinates: the box its
+    /// The box that ask gave it, in its frame's coordinates. Kept rather
-    /// drawing was made in and the one its contract is about. Its drawing
+    /// than worked out again from where its parent's own box is now: a
-    /// is placed elsewhere by re-expression, never by asking again there.
+    /// parent drawn again in the box its own answer chose gives its children
+    /// boxes it never measured anything in, and the measurement this widget
+    /// answered is the one its parent's layout was built on.
     pub offer_part: UiRegion,
     /// The measured answer and its dependencies. A hint-only dependency or
     /// a widget first encountered during placement has no measurement yet.
     pub answer: Option<(Size, LayoutHolds)>,
-    /// Asked more than once in its parent's last draw -- measured in one box
-    /// and then asked in the one the parent decided. The parent's layout
-    /// rests on the first answer and its drawing on the last, so only the
-    /// parent can ask either again.
-    pub re_asked: bool,
     /// What the widget said it used of its frame, the last time it drew.
     pub size: Size,
     /// The frame and extent reads that this drawing holds for.

core/src/ui/painter.rs

@@ -1,14 +1,14 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Holds, LayoutHolds, LayoutLen, Len, Part, Place, Px, PxVec2, RegionAlign, Rel,
+    Axis, Holds, LayoutHolds, LayoutLen, Len, Part, Place, Px, PxVec2, RegionAlign, RenderedText,
-    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
+    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
-    TextureHandle, UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
         TexturePrimitive,
     },
-    ui::render_state::{DrawInfo, Placing},
+    ui::render_state::DrawInfo,
 };
 const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 
@@ -32,6 +32,10 @@ pub struct Painter<'a> {
     /// drawing one that holds for that length alone -- the way reading a
     /// length in pixels makes it hold for that number of pixels.
     pub(super) extent_len: [Option<Len>; 2],
+    /// Symbolic box lengths read only to compute the answer. A container can
+    /// replace the provisional drawings used for that answer with drawings
+    /// in decided boxes, so this contract is independent of the final one.
+    pub(super) answer_extent_len: [Option<Len>; 2],
     /// The frame in pixels, which its children's frames are a length of:
     /// threaded down rather than composed back up the chain, so every length
     /// in layout is one multiply from its parent's and [`Holds::through`]
@@ -44,6 +48,13 @@ pub struct Painter<'a> {
     /// Only children whose answers were read constrain this widget's answer.
     pub(super) answer_under: LayoutHolds,
     pub(super) children: Vec<WidgetId>,
+    /// The children asked about so far, so the first place each was asked in
+    /// is the one recorded as its offer.
+    pub(super) offered: Vec<WidgetId>,
+    /// Whether this draw is at the place its parent first asked about, which
+    /// makes the questions it asks the ones a cold layout asks and their
+    /// answers the ones to keep.
+    pub(super) at_offer: bool,
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
     /// What this draw itself read of its frame in pixels, per axis: every
@@ -51,8 +62,11 @@ pub struct Painter<'a> {
     pub(super) frame_own: [Holds; 2],
     /// The same for its own box.
     pub(super) extent_own: [Holds; 2],
-    /// What each child's drawing depends on. Asking a child again replaces
+    /// Pixel-box dependencies used only to compute the answer. These do not
-    /// its drawing, so it replaces this too rather than narrowing it.
+    /// constrain a retained drawing placed inside that answer.
+    pub(super) answer_extent_own: [Holds; 2],
+    /// The final drawing kept for each child. Asking one child again replaces
+    /// its provisional drawing and therefore replaces this contract too.
     pub(super) under: Vec<(WidgetId, LayoutHolds)>,
     /// The movable region this widget's primitives are positioned through:
     /// its own when opted in, otherwise the nearest ancestor's.
@@ -145,40 +159,37 @@ impl<'a> Painter<'a> {
     /// one child wants, and what every transparent container passes for the
     /// frame.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, [None; 2], [Place::Within(Part::All); 2])
+        self.widget_at(id, UiRegion::FULL, [Place::Within(Part::All); 2])
     }
 
-    /// Asks a child, saying what its fractions are of and where it is asked.
+    /// Draws a child, saying what its fractions are of and where its drawing
+    /// goes.
     ///
-    /// `narrow` is a length this widget decided for the child's frame, per
+    /// `frame` is that reference, in this widget's own frame coordinates:
-    /// axis, as a length of this widget's own frame: a resolved share, or a
+    /// [`UiRegion::FULL`] forwards this widget's frame, which is what a
-    /// box a sibling's answer decided. `None` forwards this widget's frame,
+    /// container that only divides room passes, so a fraction under it means
-    /// which is what a container that only divides room passes, so a
+    /// the same wherever it sits and however deeply it is nested. Narrowing
-    /// fraction under it means the same wherever it sits and however deeply
+    /// it is for what is decided from above, and a declared length narrows
-    /// it is nested. A declared length narrows the frame here whatever the
+    /// it here.
-    /// caller says. A narrowed frame is placed in the part by the child's
-    /// alignment and is the box the child is asked in.
     ///
-    /// `place` is where the child is asked, per axis, as a part of this
+    /// `place` is where the drawing goes, per axis, as a part of this
-    /// widget's box: see [`Place`]. The child draws once, in that box, and
+    /// widget's extent: see [`Place`]. A narrowed frame is its own extent,
-    /// its answer is placed inside it by re-expressing the drawing. Nothing
+    /// since the narrowing is what said where the drawing goes.
-    /// is drawn again in a box an answer chose; a container that puts the
-    /// answer somewhere else says so with [`Self::place_at`].
     pub fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
-        narrow: [Option<Len>; 2],
+        frame: UiRegion,
         place: [Place; 2],
     ) -> DrawResult<'s, 'a, W> {
         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());
-        let declared_narrow = narrowed_by(declared);
+        let (local, extent) = frame_and_extent(
-        let narrow = [
+            frame,
-            declared_narrow[0].or(narrow[0]),
+            part_of(self.extent, place),
-            declared_narrow[1].or(narrow[1]),
+            narrowed_by(declared, frame),
-        ];
+            align,
-        let (local, extent) = frame_and_extent(part_of(self.extent, place), narrow, align);
+        );
         let within = match local == UiRegion::FULL {
             true => self.frame,
             false => local.within(&self.frame),
@@ -189,11 +200,22 @@ impl<'a> Painter<'a> {
             diag::region_node(id.id(), self.id, within);
         }
         // A child listed twice would be moved twice.
-        let re_asked = self.children.contains(&id.id());
+        if !self.children.contains(&id.id()) {
-        if !re_asked {
             self.children.push(id.id());
         }
+        let first_ask = self.offer(id.id());
+        let offer_place = if first_ask {
+            place
+        } else {
+            self.state
+                .active
+                .get(&id.id())
+                .map_or(place, |a| a.offer_place)
+        };
         let px = local.size().to_px(self.px);
+        // The answer and what it holds for, both about the place asked in.
+        // The child's record may say something else once its drawing has been
+        // placed: a drawing made again in its placed box holds for that box.
         let (size, answer_holds, holds) = self.state.draw_inner(
             id.id(),
             DrawInfo {
@@ -207,16 +229,13 @@ impl<'a> Painter<'a> {
                 frame_abs: within,
                 part: extent,
                 place,
-                offer_place: place,
+                offer_place,
-                narrow,
-                re_asked,
                 px,
             },
             None,
             self.rsc,
         );
-        let own = self.extent;
+        let compose = |holds| in_parent(holds, local, extent, place, declared);
-        let compose = |holds| in_parent(holds, local, extent, place, narrow, own);
         let holds = compose(holds);
         match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
             Some((_, kept)) => *kept = holds,
@@ -239,33 +258,6 @@ impl<'a> Painter<'a> {
         self.state.undraw_rec(id.id(), self.rsc);
     }
 
-    /// Puts a child asked about in this draw somewhere else in this
-    /// widget's box: its answer, placed in this part instead. The drawing
-    /// is re-expressed there rather than made again -- what a row does once
-    /// it knows every slot, having measured each child from its cursor.
-    pub fn place_at<W: ?Sized>(&mut self, id: &StrongWidget<W>, place: [Place; 2]) {
-        debug_assert!(
-            self.children.contains(&id.id()),
-            "'{}' placed a child it did not ask about in this draw",
-            self.label()
-        );
-        let at = self.placing();
-        self.state.place_in(id.id(), &at, place, self.rsc);
-    }
-
-    /// This widget as the thing its children are placed within.
-    fn placing(&self) -> Placing {
-        Placing {
-            id: self.id,
-            extent: self.extent,
-            local: self.frame,
-            px: self.px,
-            depth: self.depth,
-            move_idx: self.move_idx,
-            mask: self.mask,
-        }
-    }
-
     /// What a widget's rules declare its lengths to be, which whoever draws
     /// it resolves into its frame. Reading them depends on nothing -- the box
     /// that comes of them is kept on the child, and `redraw` compares it
@@ -302,6 +294,19 @@ impl<'a> Painter<'a> {
         }
     }
 
+    /// Whether this is the first box a child is asked about in during a draw
+    /// that is itself the one its parent measured -- the question a cold
+    /// layout asks, whose answer is the one to keep. A drawing made again in
+    /// a box chosen from an answer asks about that box instead, and what it
+    /// hears back is not a measurement of anything.
+    fn offer(&mut self, child: WidgetId) -> bool {
+        if !self.at_offer || self.offered.contains(&child) {
+            return false;
+        }
+        self.offered.push(child);
+        true
+    }
+
     fn depend_on<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
         if !self.size_deps.contains(&child.id()) {
             self.size_deps.push(child.id());
@@ -364,9 +369,31 @@ impl<'a> Painter<'a> {
     pub fn extent_len(&mut self, axis: Axis) -> Len {
         let len = self.extent.axis(axis).len();
         self.extent_len[axis as usize] = Some(len);
+        self.answer_extent_len[axis as usize] = Some(len);
         len
     }
 
+    /// The symbolic length used to compute this widget's answer, where the
+    /// final drawing itself is rebuilt without depending on that length.
+    pub fn answer_extent_len(&mut self, axis: Axis) -> Len {
+        let len = self.extent.axis(axis).len();
+        self.answer_extent_len[axis as usize] = Some(len);
+        len
+    }
+
+    /// Says that the final drawing uses a symbolic length already read for
+    /// the answer.
+    pub fn drawing_uses_extent_len(&mut self, axis: Axis, len: Len) {
+        debug_assert_eq!(self.extent.axis(axis).len(), len);
+        self.extent_len[axis as usize] = Some(len);
+    }
+
+    /// A part of this widget's box, expressed in its frame coordinates so it
+    /// can be used as a child frame decided here.
+    pub fn extent_part(&self, axis: Axis, part: Part) -> UiSpan {
+        part.of(*self.extent.axis(axis))
+    }
+
     /// Where this widget sits in a box longer than the length it takes. A
     /// widget that positions its own content reads it to place that content
     /// the way the box around it would have placed the widget.
@@ -398,6 +425,14 @@ impl<'a> Painter<'a> {
         PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
     }
 
+    /// This widget's own box in pixels, used only to compute its answer.
+    pub fn answer_px_size(&mut self) -> PxVec2 {
+        PxVec2::new(
+            self.answer_px_len(Axis::X),
+            self.answer_px_len(Axis::Y),
+        )
+    }
+
     /// One axis of this widget's own box in pixels. Prefer this to
     /// [`Self::px_size`] when the other axis cannot affect the drawing.
     pub fn px_len(&mut self, axis: Axis) -> Px {
@@ -410,6 +445,17 @@ impl<'a> Painter<'a> {
         len
     }
 
+    /// One pixel length used only to compute this widget's answer. The final
+    /// drawing may be retained when that answer is placed in another box.
+    pub fn answer_px_len(&mut self, axis: Axis) -> Px {
+        let len = self.extent.axis(axis).len().to_px(self.px.axis(axis));
+        let own = &mut self.answer_extent_own[axis as usize];
+        if *own == Holds::ANY {
+            *own = Holds::at(len);
+        }
+        len
+    }
+
     /// The lengths of this widget's own box on `axis` that what it is drawing
     /// holds for -- the same primitives, in the same fractions and offsets
     /// of the box, and the same reported size. A widget that read its length
@@ -545,48 +591,37 @@ impl PrimitiveLike for &TextureHandle {
 /// reach it as one length, so what it holds for maps back through that
 /// length exactly -- and where the box it was given is this widget's own,
 /// what it says about that box is what this widget can say about its own.
-/// `own` is this widget's own box, for a pin that cannot be said exactly.
 pub(crate) fn in_parent(
     holds: LayoutHolds,
     frame: UiRegion,
     extent: UiRegion,
     place: [Place; 2],
-    narrow: [Option<Len>; 2],
+    declared: [Option<LayoutLen>; 2],
-    own: UiRegion,
 ) -> LayoutHolds {
     let mut result = LayoutHolds::ANY;
     for axis in AXES {
         let n = axis as usize;
         let frame_len = frame.axis(axis).len();
         result.frame[n] = holds.frame[n].through(frame_len);
-        match (place[n].part(), narrow[n]) {
+        match (place[n].part(), declared[n]) {
             // Its box is this widget's own, or a part of it in that box's
             // own lengths: so what it holds for is a range on this widget's
             // own box, which is what lets that box move without a redraw. A
             // length it pinned is this widget's length wherever the part is
             // the whole of it, and pins the same way.
-            (Part::All, None) => {
+            (Part::All, None) if *frame.axis(axis) == UiSpan::FULL => {
                 result.extent[n] = holds.extent[n];
                 result.extent_len[n] = holds.extent_len[n];
             }
             // Its box is a part of this widget's own box, in that box's own
             // lengths, so what it holds for maps back through that part into
-            // a range on this widget's box. A length it pinned is this
+            // a range on this widget's box.
-            // widget's length less the part's pixels where the part is the
-            // whole of the box less pixels, which is the one shape that
-            // inverts exactly; any other part pins this widget's own length.
             (Part::Of(span), None) => {
-                let part_len = span.len();
+                result.extent[n] = holds.extent[n].through(span.len());
-                result.extent[n] = holds.extent[n].through(part_len);
-                result.extent_len[n] = holds.extent_len[n].map(|pinned| match part_len.rel {
-                    Rel::ONE => pinned - Len::from_parts(Rel::ZERO, part_len.px),
-                    _ => own.axis(axis).len(),
-                });
             }
-            // Its box is a part of this widget's frame, or a length of it
+            // Its box is a part of this widget's frame: a length of the
-            // decided here: a length of the frame is all that reaches it,
+            // frame is all that reaches it, so what it holds for is a range
-            // so what it holds for is a range on the frame and none of it
+            // on the frame and none of it on this widget's own box.
-            // on this widget's own box.
             _ => {
                 result.frame[n] = result.frame[n].and(
                     holds.extent[n]
@@ -689,34 +724,44 @@ pub(crate) fn part_of(extent: UiRegion, place: [Place; 2]) -> UiRegion {
 /// The length a rule gives a child's frame, per axis: a fraction in it is a
 /// fraction of the frame the child was given, which is the one length the
 /// rule can mean.
-pub(crate) fn narrowed_by(declared: [Option<LayoutLen>; 2]) -> [Option<Len>; 2] {
+pub(crate) fn narrowed_by(declared: [Option<LayoutLen>; 2], frame: UiRegion) -> [Option<Len>; 2] {
-    declared.map(|declared| declared.map(|len| Len::from_parts(len.rel, len.px)))
+    AXES.map(|axis| {
+        declared[axis as usize]
+            .map(|len| Len::from_parts(len.rel, len.px).within_len(frame.axis(axis).len()))
+    })
 }
 
-/// The frame a child is asked in and the box it is asked in, both in the
+/// The frame a child is asked in and the box its drawing goes in, both in
-/// coordinates of the widget asking.
+/// the coordinates of the widget asking.
 ///
-/// `part` is what of the caller's own box the child is given. `narrow` is a
+/// `frame` is what the caller said the child's fractions are of, and `part`
-/// length decided for the child's frame -- a rule, a share, a box a sibling
+/// what of the caller's own box the drawing takes. `narrow` is the length a
-/// decided -- which makes the frame the box the child is asked in: that
+/// declared rule gives the frame, which makes the frame the box the drawing
-/// length is what decided where it goes, placed in the part by the child's
+/// goes in -- a rule is what decided where it goes, and there is nothing
-/// alignment. Where nothing narrowed it, the frame is the caller's own and
+/// left to place inside it. A caller that narrowed the frame itself said the
-/// the part is the box.
+/// same thing.
 ///
-/// A length rather than a position, so that a child placed again is put back
+/// The length is the caller's to supply so that a widget asked again gets
-/// in whatever part it is given rather than where it first was.
+/// the frame it already has rather than a second resolution of its rule.
 pub(crate) fn frame_and_extent(
+    mut frame: UiRegion,
     part: UiRegion,
     narrow: [Option<Len>; 2],
     align: RegionAlign,
 ) -> (UiRegion, UiRegion) {
-    let mut frame = UiRegion::FULL;
     let mut extent = part;
     for (axis, narrow) in AXES.into_iter().zip(narrow) {
-        if let Some(len) = narrow {
+        let span = frame.axis_mut(axis);
+        let narrowed = match narrow {
+            Some(len) => {
                 let slot = part.axis(axis);
                 let start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
-            *frame.axis_mut(axis) = UiSpan::new(start, start + len);
+                *span = UiSpan::new(start, start + len);
+                true
+            }
+            None => *span != UiSpan::FULL,
+        };
+        if narrowed {
             *extent.axis_mut(axis) = UiSpan::FULL;
         }
     }

core/src/ui/render_state.rs

@@ -2,8 +2,8 @@
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::ui::painter::{declared_lens, frame_and_extent, narrowed_by, part_of, placed_extent};
 use crate::{
-    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx,
+    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Moves,
-    Moves, Painter, Part, PixelRegion, Place, PxVec2, Size, StrongWidget, UiRegion, UiRsc, Weight,
+    Painter, Part, PixelRegion, Place, PxVec2, Size, StrongWidget, UiRegion, UiRsc, Weight,
     WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
@@ -26,20 +26,14 @@ pub(super) struct DrawInfo {
     /// That frame composed into `parent_move`'s coordinates, which is what
     /// the widget's own drawing is written within.
     pub frame_abs: UiRegion,
-    /// The box the widget is asked in, in the frame's own coordinates: the
+    /// The box the drawing is given, in the frame's own coordinates: the
     /// part of the parent's own box that `place` names, before the widget's
     /// answer is placed inside it.
     pub part: UiRegion,
-    /// Where the widget is put, and where it was asked, as parts of the
+    /// What of the parent's extent the drawing was given, and what it was
-    /// parent's box. See [`Place`]. The two are one ask's place until the
+    /// given at the parent's first ask of it. See [`Place`].
-    /// parent places the answer somewhere else.
     pub place: [Place; 2],
     pub offer_place: [Place; 2],
-    /// The length the frame was narrowed to on each axis, as a length of
-    /// the parent's frame, where anything narrowed it.
-    pub narrow: [Option<Len>; 2],
-    /// Whether the parent already asked about this widget in this draw.
-    pub re_asked: bool,
     /// The frame in pixels: one multiply from the parent's own, which is
     /// where every pixel length in layout comes from.
     pub px: PxVec2,
@@ -51,20 +45,34 @@ impl DrawInfo {
     fn fill(&self) -> [bool; 2] {
         self.place.map(Place::fills)
     }
+
+    /// Whether this ask is the one the widget's answer is kept from: the
+    /// same widget in the place its parent measured it by, however this draw
+    /// came about.
+    ///
+    /// **Open.** A place is a length from where the asking widget's own box
+    /// starts, so two drawings of that widget -- one in the box its parent
+    /// measured it in, one in the box its own answer chose -- ask their
+    /// children in the same places and different boxes, and this cannot tell
+    /// them apart. Shrinker seeds 2 (`repaint`) and 108 (`reorder`) at depth
+    /// 5 are where that shows.
+    fn offer(&self) -> bool {
+        self.place == self.offer_place
+    }
 }
 
 /// What a widget's children are placed in: its own box, the coordinates its
 /// drawing is in, and what else one ask of a child is decided from.
-pub(super) struct Placing {
+struct Placing {
-    pub id: WidgetId,
+    id: WidgetId,
-    pub extent: UiRegion,
+    extent: UiRegion,
     /// The widget's frame in the coordinates its children compose within:
     /// `FULL` where it is a region node, since its box is that node.
-    pub local: UiRegion,
+    local: UiRegion,
-    pub px: PxVec2,
+    px: PxVec2,
-    pub depth: usize,
+    depth: usize,
-    pub move_idx: MoveIdx,
+    move_idx: MoveIdx,
-    pub mask: MaskIdx,
+    mask: MaskIdx,
 }
 
 pub struct UiRenderState {
@@ -129,7 +137,8 @@ impl UiRenderState {
             let px = active.frame.size().to_px(size);
             // Nothing above the root chose anything, so the box it was first
             // asked about is the whole of its frame.
-            active.answers_at(px, active.offer_part) && active.holds.contains(px, active.offer_part)
+            let offer = part_of(UiRegion::FULL, active.offer_place);
+            active.answers_at(px, offer) && active.holds.contains(px, active.extent)
         });
         if !stands {
             widgets.needs_redraw.insert(root);
@@ -155,8 +164,6 @@ impl UiRenderState {
             part: UiRegion::FULL,
             place: [Place::Within(Part::All); 2],
             offer_place: [Place::Within(Part::All); 2],
-            narrow: [None; 2],
-            re_asked: false,
             px,
         }
     }
@@ -212,8 +219,14 @@ impl UiRenderState {
     /// The root's frame: the window, narrowed by the root's own rules. Its
     /// extent is that frame, since nothing above it chose anything else.
     fn root_region(id: WidgetId, widgets: &Widgets) -> UiRegion {
-        let narrow = narrowed_by(declared_lens(widgets, id));
+        let narrow = narrowed_by(declared_lens(widgets, id), UiRegion::FULL);
-        frame_and_extent(UiRegion::FULL, narrow, widgets.alignment(id)).0
+        frame_and_extent(
+            UiRegion::FULL,
+            UiRegion::FULL,
+            narrow,
+            widgets.alignment(id),
+        )
+        .0
     }
 
     pub(super) fn draw_inner(
@@ -230,48 +243,44 @@ impl UiRenderState {
             diag::draw_request(id, info.parent, frame, info.px, info.region_node);
         }
         let align = rsc.widgets().alignment(id);
-        let declared = declared_lens(rsc.widgets(), id);
         // Nothing this widget measured can be dirty while it draws: layout is
         // one bottom-up walk, so anything deeper has settled or deferred to
         // its own parent, and a deferred one leaves that parent marked.
         let stale = rsc.widgets().needs_redraw.contains(&id);
-        // The widget draws once, in the box it is asked in, and its answer
+        let retained = match stale {
-        // is placed inside that box by re-expressing the drawing. The box the
+            true => None,
-        // answer chose is never a question: nothing is drawn again in it, so
+            false => self
-        // an answer is kept only with the drawing that gave it, and both
+                .retained_answer(id, part, info)
-        // have to hold for the box asked about.
+                .and_then(|answer| self.try_reuse(id, frame, part, info, rsc).map(|_| answer)),
-        let reused = (!stale)
+        };
-            .then(|| self.retained_answer(id, part, info))
+        let answer = retained.unwrap_or_else(|| {
-            .flatten()
-            .and_then(|answer| {
-                let extent = placed_extent(part, answer.0, declared, info.fill(), align);
-                self.try_reuse(id, frame, part, extent, info, rsc)
-                    .map(|()| answer)
-            });
-        let answer = reused.unwrap_or_else(|| {
             if old.is_none() {
                 old = self.remove(id, false, rsc);
             }
-            let answer = self.draw_at(id, part, info, old.take(), rsc);
+            self.draw_at(id, part, info, old.take(), rsc)
-            // Where the drawing goes: the part its parent gave it, with the
-            // answer placed inside that part on any axis the parent left
-            // open.
-            let extent = placed_extent(part, answer.0, declared, info.fill(), align);
-            if extent != part {
-                self.reposition(id, frame, extent, info, rsc);
-            }
-            answer
         });
 
+        // Where the drawing goes, in the frame's own coordinates: the part
+        // its parent gave it, with the answer placed inside that part on any
+        // axis the parent left open. The frame itself does not change, so
+        // nothing under it resolves a fraction a second time.
+        //
+        let extent = placed_extent(
+            part,
+            answer.0,
+            declared_lens(rsc.widgets(), id),
+            info.fill(),
+            align,
+        );
+        self.place(id, extent, info, rsc);
+
         let drawing_holds = self.active[&id].holds;
         let active = self.active.get_mut(&id).unwrap();
         // Whoever asked owns how the boxes were reached: the frame it stated,
-        // and what of its own box it asked in. A local redraw asks the same
+        // and what of its own box it gave the drawing. A local redraw asks
-        // question again from these.
+        // the same question again from these.
         active.frame_abs = frame;
         active.frame = info.frame;
-        active.narrow = info.narrow;
-        active.re_asked = info.re_asked;
         active.answer = Some(answer);
         active.offer_place = info.offer_place;
         active.offer_part = part;
@@ -292,6 +301,29 @@ impl UiRenderState {
         (answer.0, answer.1, drawing_holds)
     }
 
+    /// Recompose retained geometry when the evaluation still holds at this extent.
+    fn place(&mut self, id: WidgetId, extent: UiRegion, info: DrawInfo, rsc: &mut dyn UiRsc) {
+        if self
+            .try_reuse(id, info.frame_abs, extent, info, rsc)
+            .is_some()
+        {
+            return;
+        }
+        assert_eq!(
+            extent,
+            info.part,
+            "'{}' ({id:?}) does not hold for the box its answer chose: part {:?}, wanted {extent:?}, retained {:?} with {:?}",
+            rsc.widgets().label(id),
+            info.part,
+            self.active.get(&id).map(|active| active.extent),
+            self.active.get(&id).map(|active| active.holds),
+        );
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::PlaceRedraws);
+        let old = self.remove(id, false, rsc);
+        self.draw_at(id, extent, info, old, rsc);
+    }
+
     /// Calls a widget's `draw` and keeps what it drew in `extent` of `frame`.
     fn draw_at(
         &mut self,
@@ -314,15 +346,22 @@ impl UiRenderState {
             // Reusing its index sooner could make an old parent look current.
             false => (info.parent_move, frame, self.slots.remove(&id)),
         };
-        let old_children = old.map_or_else(Vec::new, |old| old.children);
+        let (old_children, old_answer, old_offer_part) = match old {
+            Some(old) => (old.children, old.answer, Some(old.offer_part)),
+            None => (Vec::new(), None, None),
+        };
         rsc.widgets_mut().needs_redraw.remove(&id);
+        // Only evaluation at the original offer establishes the children's
+        // offers. A placing evaluation must not overwrite that question.
         let px = info.px;
+        let at_offer = info.offer();
 
         let mut painter = Painter {
             state: self,
             frame: local,
             extent,
             extent_len: [None; 2],
+            answer_extent_len: [None; 2],
             px,
             mask: info.mask,
             layer: info.layer,
@@ -332,10 +371,13 @@ impl UiRenderState {
             primitives: Vec::new(),
             mask_region: None,
             children: Vec::new(),
+            offered: Vec::new(),
+            at_offer,
             size_deps: Vec::new(),
             frame_own: [Holds::ANY; 2],
             under: Vec::new(),
             extent_own: [Holds::ANY; 2],
+            answer_extent_own: [Holds::ANY; 2],
             answer_under: LayoutHolds::ANY,
             depth: info.depth,
             move_idx,
@@ -364,9 +406,13 @@ impl UiRenderState {
             primitives,
             mask_region,
             extent_own,
+            answer_extent_own,
             extent_len,
+            answer_extent_len,
             answer_under,
             children,
+            offered: _,
+            at_offer: _,
             size_deps,
             frame_own,
             under,
@@ -408,15 +454,23 @@ impl UiRenderState {
         if let Some(idx) = retired_move {
             self.moves.remove(idx);
         }
-        let own_holds = LayoutHolds {
+        let drawing_own = LayoutHolds {
             frame: frame_own,
             extent: extent_own,
             extent_len,
         };
-        let answer_holds = own_holds.and(answer_under);
+        let answer_own = LayoutHolds {
+            extent: [
+                drawing_own.extent[0].and(answer_extent_own[0]),
+                drawing_own.extent[1].and(answer_extent_own[1]),
+            ],
+            extent_len: answer_extent_len,
+            ..drawing_own
+        };
+        let answer_holds = answer_own.and(answer_under);
         let holds = under
             .into_iter()
-            .fold(answer_holds, |holds, (_, child)| holds.and(child));
+            .fold(drawing_own, |holds, (_, child)| holds.and(child));
         debug_assert!(
             holds.contains(px, extent),
             "'{}' ({id:?}) drew in {px:?}, outside the ranges it reported: {holds:?}",
@@ -441,8 +495,6 @@ impl UiRenderState {
                         part: UiRegion::FULL,
                         place: [Place::Within(Part::All); 2],
                         offer_place: [Place::Within(Part::All); 2],
-                        narrow: [None; 2],
-                        re_asked: false,
                         px,
                     },
                     rsc,
@@ -456,13 +508,11 @@ impl UiRenderState {
             frame_abs: frame,
             extent,
             frame: info.frame,
-            narrow: info.narrow,
             place: info.place,
             offer_place: info.offer_place,
-            offer_part: extent,
+            offer_part: old_offer_part.unwrap_or(extent),
-            // Whoever asked writes the answer.
+            // Whoever asked writes the answer, if this was the asking.
-            answer: None,
+            answer: old_answer,
-            re_asked: info.re_asked,
             size,
             holds,
             drawn: true,
@@ -547,17 +597,16 @@ impl UiRenderState {
         active.frame.size().to_px(parent_px)
     }
 
-    /// Keeps the retained drawing if its contract holds for `part`, the box
+    /// Reuses the actual drawing in a new box if its retained contract holds
-    /// asked about, and puts it at `extent`, where the answer places it.
+    /// there. Answers retained from a different ask are handled separately.
     fn try_reuse(
         &mut self,
         id: WidgetId,
         frame: UiRegion,
-        part: UiRegion,
         extent: UiRegion,
         info: DrawInfo,
         rsc: &mut dyn UiRsc,
-    ) -> Option<()> {
+    ) -> Option<(Size, LayoutHolds)> {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::ReuseAttempts);
         if rsc.widgets().needs_redraw.contains(&id) {
@@ -606,7 +655,7 @@ impl UiRenderState {
         // In pixels, because the frame is a fraction of its parent's and
         // that may be what changed -- an unchanged fraction of a box half the
         // size is half the widget.
-        if !active.holds.contains(info.px, part) {
+        if !active.holds.contains(info.px, extent) {
             #[cfg(feature = "layout-diagnostics")]
             {
                 // Which of the three said no, so a frame that redraws more
@@ -615,13 +664,14 @@ impl UiRenderState {
                 let holds = active.holds;
                 for axis in AXES {
                     let n = axis as usize;
-                    if holds.extent_len[n].is_some_and(|pinned| pinned != part.axis(axis).len()) {
+                    if holds.extent_len[n].is_some_and(|pinned| pinned != extent.axis(axis).len()) {
                         diag::bump(Counter::OutsidePinnedLen);
                     }
                     if !holds.frame[n].contains(info.px.axis(axis)) {
                         diag::bump(Counter::OutsideFrame);
                     }
-                    if !holds.extent[n].contains(part.axis(axis).len().to_px(info.px.axis(axis))) {
+                    if !holds.extent[n].contains(extent.axis(axis).len().to_px(info.px.axis(axis)))
+                    {
                         diag::bump(Counter::OutsideExtent);
                     }
                 }
@@ -630,31 +680,9 @@ impl UiRenderState {
             }
             return None;
         }
-        self.relocate(id, frame, extent, info, rsc);
-        Some(())
-    }
-
-    /// Puts a retained drawing where its parent now has it, without drawing:
-    /// a moved frame recomposes the subtree or rewrites its node, and a box
-    /// of another length re-expresses everything inside it.
-    fn relocate(
-        &mut self,
-        id: WidgetId,
-        frame: UiRegion,
-        extent: UiRegion,
-        info: DrawInfo,
-        rsc: &mut dyn UiRsc,
-    ) {
-        let active = &self.active[&id];
-        debug_assert!(
-            !rsc.widgets().needs_redraw.contains(&id),
-            "'{}' ({id:?}) placed while marked to draw",
-            rsc.widgets().label(id)
-        );
-        let has_region_node = active.move_idx != active.parent_move;
         let extent_moved = active.extent != extent;
         let moved = active.frame_abs != frame;
-        let slot = active.move_idx;
+        let (answer, slot) = ((active.size, active.holds), active.move_idx);
         if moved {
             if has_region_node {
                 self.moves.set(slot, frame);
@@ -690,31 +718,20 @@ impl UiRenderState {
                 },
             );
         }
+        Some(answer)
     }
 
-    /// Places one child of `at.id` where that widget's own box now has it.
+    /// Places one child of `at.id` in the box that widget's own box gives
+    /// it: its part of the extent, with its answer placed inside that part
+    /// where the ask left the axis open.
     fn place_child(&mut self, child: WidgetId, at: &Placing, rsc: &mut dyn UiRsc) {
-        let place = self.active[&child].place;
-        self.place_in(child, at, place, rsc);
-    }
-
-    /// Puts a child of `at.id` in `place` of that widget's box: its answer
-    /// placed inside that part where the place leaves the axis open, the
-    /// drawing re-expressed there.
-    pub(super) fn place_in(
-        &mut self,
-        child: WidgetId,
-        at: &Placing,
-        place: [Place; 2],
-        rsc: &mut dyn UiRsc,
-    ) {
         let active = &self.active[&child];
-        let (frame, part) = Self::re_ask(active, at.extent, place);
+        let (frame, part) = Self::re_ask(active, at.extent, active.place);
         let extent = placed_extent(
             part,
             active.measured().unwrap_or(active.size),
             active.declared,
-            place.map(Place::fills),
+            active.place.map(Place::fills),
             active.own_align,
         );
         let info = DrawInfo {
@@ -727,25 +744,34 @@ impl UiRenderState {
             frame,
             frame_abs: frame.within(&at.local),
             part,
-            place,
+            place: active.place,
             offer_place: active.offer_place,
-            narrow: active.narrow,
-            re_asked: active.re_asked,
             px: frame.size().to_px(at.px),
         };
-        self.relocate(child, info.frame_abs, extent, info, rsc);
+        self.place(child, extent, info, rsc);
     }
 
-    /// The frame and the box a widget is given at `place` of its parent's
+    /// The frame and the box a widget being asked again is given, from what
-    /// box, from what it already has. A frame's length is the same on every
+    /// it already has and where its parent's box is now. A frame's length is
-    /// ask, so a narrowed frame is put back where it sits in the part rather
+    /// the same on every ask, so a declared length is put back where it sits
-    /// than resolved a second time.
+    /// in the part rather than resolved from its rule a second time.
     fn re_ask(
         active: &ActiveData,
-        parent_box: UiRegion,
+        parent_extent: UiRegion,
         place: [Place; 2],
     ) -> (UiRegion, UiRegion) {
-        frame_and_extent(part_of(parent_box, place), active.narrow, active.own_align)
+        let narrow = AXES.map(|axis| {
+            let n = axis as usize;
+            active.declared[n]
+                .is_some()
+                .then(|| active.frame.axis(axis).len())
+        });
+        frame_and_extent(
+            active.frame,
+            part_of(parent_extent, place),
+            narrow,
+            active.own_align,
+        )
     }
 
     /// Re-places everything inside a widget whose own box moved. Every child
@@ -916,12 +942,10 @@ impl UiRenderState {
                 frame_abs: UiRegion::FULL,
                 extent: UiRegion::FULL,
                 frame: UiRegion::FULL,
-                narrow: [None; 2],
                 place: [Place::Within(Part::All); 2],
                 offer_place: [Place::Within(Part::All); 2],
                 offer_part: UiRegion::FULL,
                 answer: None,
-                re_asked: false,
                 size,
                 holds: LayoutHolds::ANY,
                 drawn: false,
@@ -1121,16 +1145,11 @@ impl UiRenderState {
         // Its parent resolved its declared lengths into its box and decided
         // whether to draw it at all, so a change to either is the parent's
         // to draw -- with the mark left on, so the parent draws it rather
-        // than keeping it. So is a widget the parent asked twice: its
+        // than keeping it.
-        // layout rests on an answer this widget cannot give again alone.
         let declared_changed = declared_lens(rsc.widgets(), id) != active.declared;
         let alignment_changed = rsc.widgets().alignment(id) != active.own_align;
         if let Some(parent) = active.parent
-            && (declared_changed
+            && (declared_changed || alignment_changed || !active.drawn || active.answer.is_none())
-                || alignment_changed
-                || active.re_asked
-                || !active.drawn
-                || active.answer.is_none())
         {
             // Both stay marked: the parent because it has this to draw, and
             // this because the parent must draw it rather than keep what it
@@ -1158,20 +1177,12 @@ impl UiRenderState {
             return true;
         };
         let px = self.asked_px(id);
-        let (was_answer, was_holds, was_place) = (active.answer, active.holds, active.place);
+        let (was_answer, was_holds) = (active.answer, active.holds);
-        // The question its parent asked, asked again: the same place of the
+        let needs_replacement = active.place != active.offer_place;
-        // box the parent was asked in, which is the box the parent's own
+        // Re-ask the question its answer came from, not a later placement of
-        // draw ran in and what its children's parts are of. Where the
+        // that answer. A parent may move the retained drawing into a box the
-        // parent's answer put its own drawing is not a question anybody
+        // answer chose without making that box a new question.
-        // asked, and nothing is asked in it here either.
+        let parent_extent = self.active[&parent].extent;
-        let asked = self.placing_of(parent, self.active[&parent].offer_part);
-        let (frame, part) = Self::re_ask(active, asked.extent, active.offer_place);
-        debug_assert_eq!(
-            frame.size(),
-            active.frame.size(),
-            "'{}' ({id:?}) asked again in a frame of another length",
-            rsc.widgets().label(id)
-        );
         let info = DrawInfo {
             layer: active.layer,
             parent: active.parent,
@@ -1179,13 +1190,11 @@ impl UiRenderState {
             parent_move: active.parent_move,
             region_node: rsc.widgets().is_region_node(id),
             mask: active.parent_mask,
-            frame,
+            frame: active.frame,
-            frame_abs: frame.within(&asked.local),
+            frame_abs: active.frame_abs,
-            part,
+            part: Self::re_ask(active, parent_extent, active.offer_place).1,
             place: active.offer_place,
             offer_place: active.offer_place,
-            narrow: active.narrow,
-            re_asked: false,
             px,
         };
         #[cfg(feature = "layout-diagnostics")]
@@ -1205,7 +1214,7 @@ impl UiRenderState {
         if active.holds.covers(was_holds) && was_holds.contains(px, active.extent) {
             active.holds = was_holds;
         }
-        if active.answer != was_answer || active.holds != was_holds {
+        if needs_replacement || active.answer != was_answer || active.holds != was_holds {
             // The parent retains both the answer and the drawing's validity;
             // even an unchanged size can narrow the range safe for a resize.
             #[cfg(feature = "layout-diagnostics")]
@@ -1214,34 +1223,9 @@ impl UiRenderState {
                 diag::bump(Counter::ReaderEdges);
             }
             self.mark(parent, rsc.widgets_mut());
-        } else {
-            // The answer stands, so where the parent put it stands: the
-            // fresh drawing goes back there -- the same place, of the box
-            // the parent's answer chose rather than the one it was asked in.
-            let at = self.placing_of(parent, self.active[&parent].extent);
-            self.place_in(id, &at, was_place, rsc);
         }
         true
     }
-
-    /// A drawn widget as the thing its children are placed within, with
-    /// `extent` as the box their parts are of: the box it was asked in for
-    /// asking one of them again, the box its answer chose for placing one.
-    fn placing_of(&self, id: WidgetId, extent: UiRegion) -> Placing {
-        let active = &self.active[&id];
-        Placing {
-            id,
-            extent,
-            local: match active.move_idx != active.parent_move {
-                true => UiRegion::FULL,
-                false => active.frame_abs,
-            },
-            px: self.asked_px(id),
-            depth: active.depth,
-            move_idx: active.move_idx,
-            mask: active.mask,
-        }
-    }
 }
 
 /// Whether what a widget reports along `axis` is inside the box it drew in.

src/random.rs

@@ -120,15 +120,15 @@ impl Widget for Branch {
         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, [None; 2], [Place::Within(Part::All), top])
+            .widget_at(&self.probe, UiRegion::FULL, [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 below = Place::Within(Part::Of(UiSpan::new(cut, Len::FULL)));
         let place = [Place::Within(Part::All), below];
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, [None; 2], place),
+            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
-            false => painter.widget_at(&self.narrow, [None; 2], place),
+            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
         };
         Size::LEFTOVER
     }

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, [None; 2], place).size()
+        painter.widget_at(&self.inner, UiRegion::FULL, 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, [None; 2], place).size();
+        let inner = painter.widget_at(&self.inner, UiRegion::FULL, place).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,

src/widget/position/scroll.rs

@@ -12,9 +12,10 @@ pub struct Scroll {
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let container_len = painter.px_len(self.axis);
-        // Asked in the whole viewport, then put at the scrolled offset.
+        // Measured in the whole viewport, then drawn at the scrolled offset.
+        let whole = UiRegion::FULL;
         let answer_len = painter
-            .widget_at(&self.inner, [None; 2], [Place::Fill(Part::All); 2])
+            .widget_at(&self.inner, whole, [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;
@@ -58,10 +59,11 @@ 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 goes is the content
+        // content box its own answer decided. Where it is drawn is the
-        // box, scrolled: its drawing moved there, not made again there.
+        // content box, scrolled.
-        painter.place_at(
+        painter.widget_at(
             &self.inner,
+            whole,
             self.axis
                 .pair(Place::Fill(Part::From(content)), Place::Fill(Part::All)),
         );

src/widget/position/span.rs

@@ -13,8 +13,8 @@ impl Widget for Span {
         // The row: this span's own box, as a length of the frame its children
         // are laid out against. Its start is nothing's business -- a slot is
         // a length from it -- so what this reads is the length alone.
-        let far = painter.extent_len(axis);
+        let far = painter.answer_extent_len(axis);
-        let along = |from: Len, to: Len| match self.dir.sign {
+        let measure_along = |from: Len, to: Len| match self.dir.sign {
             Sign::Pos => UiSpan::new(from, to),
             Sign::Neg => UiSpan::new(far - to, far - from),
         };
@@ -25,23 +25,19 @@ 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 asked in is the room left from the cursor,
+        // 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. This is
+        // because a text has to wrap at the width actually there.
-        // 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 sizes = Vec::with_capacity(self.children.len());
+        let mut lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let room = Place::Within(Part::From(along(cursor, far)));
+            let room = Place::Fill(Part::From(measure_along(cursor, far)));
-            let size = painter
+            let len = painter
-                .widget_at(child, [None; 2], axis.pair(room, across))
+                .widget_at(child, UiRegion::FULL, axis.pair(room, across))
-                .size();
+                .len(axis);
-            let len = size.axis(axis);
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
-            sizes.push(size);
+            lens.push(len);
         }
-        let lens: Vec<LayoutLen> = sizes.iter().map(|size| size.axis(axis)).collect();
 
         let gaps = self
             .gap
@@ -54,9 +50,10 @@ impl Widget for Span {
             |sum, len| sum + *len,
         );
 
+        let fixed_total = Len::from_parts(total.rel, total.px);
         // What is left for the shares to divide: the row less everything
         // fixed, as a length of the frame rather than a number of pixels.
-        let room = far - Len::from_parts(total.rel, total.px);
+        let room = far - fixed_total;
         // Whether anything is left over is a question in pixels: `rel(0.5)`
         // beside 300 px is full at 600 and overfull at 400. Asked of `room`
         // itself, and answered back through the same expression, so the
@@ -76,6 +73,18 @@ impl Widget for Span {
             painter.frame_holds(axis, holds.through(room));
         }
 
+        if shares {
+            painter.drawing_uses_extent_len(axis, far);
+        }
+        let drawing_far = match shares {
+            true => far,
+            false => fixed_total,
+        };
+        let along = |from: Len, to: Len| match self.dir.sign {
+            Sign::Pos => UiSpan::new(from, to),
+            Sign::Neg => UiSpan::new(drawing_far - to, drawing_far - from),
+        };
+
         // Across itself a span is as long as its longest child -- unless a
         // rule beside it gives that length outright, and then reading them
         // answers nothing and makes its size depend on theirs for it. A rule
@@ -92,8 +101,7 @@ impl Widget for Span {
         let mut taken = Weight::ZERO;
         let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
-        for (child, size) in self.children.iter().zip(&sizes) {
+        for (child, len) in self.children.iter().zip(&lens) {
-            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.
@@ -111,26 +119,23 @@ 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 child's box outright rather than something to place an
+            // is the drawing's box outright rather than something to place an
-            // answer inside again. A share is decided here and nowhere
+            // answer inside again.
-            // else: its slot narrows its frame, and the child is asked in
+            let span = along(from, start);
-            // it, since a text wraps at the width it is actually given. A
+            let (frame, slot) = match len.leftover > Weight::ZERO && shares {
-            // fixed child's slot is its own answer, so its drawing is put
+                true => (
-            // there as it is.
+                    UiRegion::from_axis(
-            let slot = along(from, start);
+                        axis,
-            let place = axis.pair(Place::Fill(Part::From(slot)), across);
+                        painter.extent_part(axis, Part::From(span)),
-            let used = match len.leftover > Weight::ZERO && shares {
+                        UiSpan::FULL,
-                true => {
+                    ),
-                    let mut narrow = [None; 2];
+                    Place::Fill(Part::All),
-                    narrow[axis as usize] = Some(slot.len());
+                ),
-                    painter.widget_at(child, narrow, place).len(!axis)
+                false => (UiRegion::FULL, Place::Fill(Part::From(span))),
-                }
-                false => {
-                    painter.place_at(child, place);
-                    size.axis(!axis)
-                }
             };
+            let placed = painter.widget_at(child, frame, axis.pair(slot, across));
             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

@@ -23,26 +23,25 @@ impl Widget for Stack {
             Some((i, child)) => {
                 painter.child_layer_at(i);
                 painter
-                    .widget_at(child, [None; 2], [Place::Fill(Part::All); 2])
+                    .widget_at(child, UiRegion::FULL, [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);
-            painter.widget_at(child, narrow, [Place::Within(Part::All); 2]);
+            let place = [Axis::X, Axis::Y].map(|axis| {
+                let len = size.axis(axis);
+                let part = match len.leftover > Weight::ZERO {
+                    true => Part::All,
+                    false => Part::From(UiSpan::new(Len::ZERO, Len::from_parts(len.rel, len.px))),
+                };
+                Place::Within(part)
+            });
+            painter.widget_at(child, UiRegion::FULL, place);
         }
         size
     }

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, [None; 2], [Place::Within(Part::All), top])
+            .widget_at(&self.probe, UiRegion::FULL, [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, [None; 2], place),
+            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
-            false => painter.widget_at(&self.narrow, [None; 2], place),
+            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
         };
         Size::LEFTOVER
     }

tests/cases/retained.rs

@@ -12,6 +12,7 @@ struct Counted {
     draws: Rc<Cell<usize>>,
     size: Size,
     reads_box: bool,
+    reads_answer_box: bool,
 }
 
 impl Widget for Counted {
@@ -19,6 +20,8 @@ impl Widget for Counted {
         self.draws.set(self.draws.get() + 1);
         if self.reads_box {
             painter.px_size();
+        } else if self.reads_answer_box {
+            painter.answer_px_size();
         }
         self.size
     }
@@ -38,11 +41,18 @@ fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>
         draws: draws.clone(),
         size,
         reads_box,
+        reads_answer_box: false,
     }
     .add(&mut h.rsc);
     (id, Counts(draws))
 }
 
+fn answer_counted(h: &mut Harness, size: Size) -> (WeakWidget<Counted>, Counts) {
+    let (id, draws) = counted(h, size, false);
+    h.rsc[id].reads_answer_box = true;
+    (id, draws)
+}
+
 struct Layered {
     children: [StrongWidget<Rect>; 2],
     _revision: usize,
@@ -156,9 +166,9 @@ 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);
-    // Asked once, from the cursor; its slot is its answer and the drawing is
+    // Its final slot is a parent decision, so it is evaluated there after
-    // moved there.
+    // the provisional ask established its length.
-    assert_eq!(asked_draws.get(), 1);
+    assert_eq!(asked_draws.get(), 2);
 }
 
 #[test]
@@ -216,7 +226,7 @@ impl Widget for FromHint {
         let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
         painter.widget_at(
             &self.inner,
-            [None; 2],
+            UiRegion::FULL,
             [Place::Within(Part::All), Place::Within(Part::From(top))],
         );
         Size::LEFTOVER
@@ -240,7 +250,7 @@ fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
     assert_corners!(h, inner, (0, 0), (400, 120));
 }
 
-/// Reads its box's size, which nothing but its own draw can put right.
+/// Reads its box's size to compute its answer.
 struct ReadsBox {
     draws: Rc<Cell<usize>>,
 }
@@ -248,25 +258,36 @@ struct ReadsBox {
 impl Widget for ReadsBox {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
-        Size::from_px(painter.px_size().div_int(4))
+        Size::from_px(painter.answer_px_size().div_int(4))
     }
 }
 
 /// 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. The quarter-sized box
+/// Both report a quarter of what they read. Their empty drawings are
-/// the answer places them in is not a question: the drawing is moved there,
+/// independent of that read, so a changed question costs one draw.
-/// so each length they are asked at costs one draw.
 struct ReadsWidth {
     draws: Rc<Cell<usize>>,
 }
 
+struct ReadsDrawingWidth {
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for ReadsDrawingWidth {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.px_len(Axis::X);
+        Size::LEFTOVER
+    }
+}
+
 impl Widget for ReadsWidth {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
         Size::from_px(PxVec2::new(
-            painter.px_len(Axis::X).div_int(4),
+            painter.answer_px_len(Axis::X).div_int(4),
             Px::from_int(20),
         ))
     }
@@ -322,6 +343,7 @@ fn a_row_moves_what_follows_a_child_that_grew_rather_than_drawing_it() {
             draws: ruled.clone(),
             size: Size::LEFTOVER,
             reads_box: false,
+            reads_answer_box: false,
         };
         let second = match declared {
             true => second.width(rel(0.25)).add(&mut h.rsc),
@@ -499,7 +521,7 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     // Every wrapper up to the outer pad read the size below it, so the outer
     // pad is what draws again -- and the span it hands the box to is the same
     // size as before, which is what lets a draw reuse its way past the leaf.
-    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
+    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), false);
     let padded = leaf.pad(10).add(&mut h.rsc);
     let below = rect(Color::RED).add(&mut h.rsc);
     h.set_root((padded, below).span(Dir::DOWN).pad(12));
@@ -877,7 +899,7 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
+                UiRegion::FULL,
                 [
                     Place::Within(Part::From(self.region.x)),
                     Place::Within(Part::From(self.region.y)),
@@ -966,7 +988,7 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [Some(self.region.x.len()), None],
+                self.region,
                 [
                     Place::Fill(Part::From(self.extent.x)),
                     Place::Fill(Part::From(self.extent.y)),
@@ -1065,7 +1087,7 @@ fn a_declared_size_change_stops_at_an_independent_parent() {
 fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     let mut h = Harness::new((400, 200));
     let draws = Rc::new(Cell::new(0));
-    let leaf = ReadsWidth {
+    let leaf = ReadsDrawingWidth {
         draws: draws.clone(),
     }
     .add(&mut h.rsc);
@@ -1076,7 +1098,7 @@ fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     h.frame();
 
     assert!(draws.get() > settled);
-    assert_corners!(h, leaf, (300, 90), (500, 110));
+    assert_corners!(h, leaf, (0, 0), (800, 200));
 }
 
 #[test]
@@ -1136,7 +1158,7 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
+                UiRegion::FULL,
                 [
                     Place::Fill(Part::From(self.extent.x)),
                     Place::Fill(Part::From(self.extent.y)),
@@ -1211,7 +1233,7 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
     struct Measured;
     impl Widget for Measured {
         fn draw(&mut self, painter: &mut Painter) -> Size {
-            let width = painter.px_len(Axis::X);
+            let width = painter.answer_px_len(Axis::X);
             painter.primitive(RectPrimitive::color(Color::BLUE));
             Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
         }
@@ -1224,7 +1246,7 @@ 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,
-                [None; 2],
+                UiRegion::FULL,
                 [
                     Place::Fill(Part::From(UiSpan::new(
                         Len::px(self.start),
@@ -1266,7 +1288,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    [None; 2],
+                    UiRegion::FULL,
                     [
                         Place::Within(Part::From(self.region.x)),
                         Place::Within(Part::From(self.region.y)),
@@ -1286,7 +1308,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 painter
                     .widget_at(
                         &self.child,
-                        [None; 2],
+                        UiRegion::FULL,
                         [
                             Place::Fill(Part::From(self.extent.x)),
                             Place::Fill(Part::From(self.extent.y)),
@@ -1311,7 +1333,10 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 } else {
                     Size::from((80, 27))
                 };
-                let (leaf, _) = counted(h, size, !fractional);
+                let (leaf, _) = match fractional {
+                    true => counted(h, size, false),
+                    false => answer_counted(h, size),
+                };
                 let child = Container {
                     child: leaf.add_strong(&mut h.rsc),
                     region,

tests/cases/unsettled.rs

@@ -846,84 +846,3 @@ fn adding_text_to_a_reverse_row_keeps_its_shared_height() {
     let (_, other, _) = build(&mut cold, true);
     assert_eq!(warm.region(&shared), cold.region(&other));
 }
-
-/// Nine widgets, shrunk from seed 946 at depth 6. The column is a share of
-/// the row while its rect has room to draw and a fixed width once it has
-/// not, so the row asks it twice: in the room, where it answers a share,
-/// and in its slot, where it answers its text's width. Emptying the column
-/// changes only the first answer. A local redraw that asked only the second
-/// question kept the row as it was; the column has to defer to the row.
-fn plant_column_that_is_a_share_only_while_its_rect_fits(
-    h: &mut Harness,
-    emptied: bool,
-) -> (Vec<WidgetId>, WeakWidget<Span>, Vec<StrongWidget>) {
-    let first = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
-    let filler = rect(Color::CYAN.alpha(126)).add(&mut h.rsc);
-    let second = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
-    let mut spare: Vec<StrongWidget> =
-        vec![filler.add_strong(&mut h.rsc), second.add_strong(&mut h.rsc)];
-    let mut children: Vec<StrongWidget> = vec![first.add_strong(&mut h.rsc)];
-    if !emptied {
-        children.append(&mut spare);
-    }
-    let column = Span {
-        children,
-        dir: Dir::DOWN,
-        gap: Px::ZERO,
-    }
-    .height(159)
-    .add(&mut h.rsc);
-    let left = rect(Color::MAGENTA.alpha(189)).add(&mut h.rsc);
-    let right = rect(Color::BLUE.alpha(0)).add(&mut h.rsc);
-    let row = Span {
-        children: vec![
-            left.add_strong(&mut h.rsc),
-            column.add_strong(&mut h.rsc),
-            right.add_strong(&mut h.rsc),
-        ],
-        dir: Dir::RIGHT,
-        gap: Px::ZERO,
-    }
-    .add(&mut h.rsc);
-    let end = rect(Color::MAGENTA.alpha(189)).add(&mut h.rsc);
-    let root = Span {
-        children: vec![end.add_strong(&mut h.rsc), row.add_strong(&mut h.rsc)],
-        dir: Dir::LEFT,
-        gap: Px::ZERO,
-    }
-    .add(&mut h.rsc);
-    h.set_root(root);
-    (
-        vec![
-            first.id(),
-            filler.id(),
-            second.id(),
-            column.id(),
-            left.id(),
-            right.id(),
-            row.id(),
-            end.id(),
-            root.id(),
-        ],
-        column,
-        spare,
-    )
-}
-
-#[test]
-fn emptying_a_column_the_row_asked_twice_asks_the_row_again() {
-    let mut warm = Harness::new((900, 1200));
-    let (ids, column, _spare) =
-        plant_column_that_is_a_share_only_while_its_rect_fits(&mut warm, false);
-    warm.frame();
-    // Kept alive: dropping the last share of a widget frees its id.
-    let _removed: Vec<StrongWidget> = warm.rsc[column].children.drain(1..).collect();
-    warm.frame();
-
-    let mut cold = Harness::new((900, 1200));
-    let (cold_ids, _, _spare) =
-        plant_column_that_is_a_share_only_while_its_rect_fits(&mut cold, true);
-    cold.frame();
-
-    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-}