Defer a twice-asked widget's local redraw to its parent

A span asks a share child twice in one draw: in the room, whose answer
its slots rest on, and in the decided slot, whose cross-axis answer it
reads. The record keeps only the second question, so a local redraw that
found that answer unchanged never told the row that the first had --
seed 946 at depth 6, where emptying a fixed-height column turns it from
a share into a fixed width as wide as the row. A widget its parent asked
more than once in one draw now defers to that parent, like one whose
declared length changed. Pinned as
unsettled::emptying_a_column_the_row_asked_twice_asks_the_row_again.

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

core/src/layout_diagnostics.rs

@@ -32,7 +32,6 @@ pub(crate) enum Counter {
     SizeReads,
     HintHits,
     HintMisses,
-    RetainedSizeHits,
     ReuseAttempts,
     ReuseExact,
     ReuseMoved,
@@ -42,7 +41,6 @@ pub(crate) enum Counter {
     ReuseOutside,
     ReuseWrongLayer,
     ReuseWrongNode,
-    PlaceRedraws,
     QueuePops,
     DepthReads,
     LocalRedraws,
@@ -70,7 +68,6 @@ impl Counter {
         "draw-result size reads",
         "hint hits",
         "hint misses",
-        "retained size hits",
         "reuse attempts",
         "reuse exact",
         "reuse moved",
@@ -80,7 +77,6 @@ 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, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive, Size,
+    LayerId, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive,
-    TextureHandle, UiRegion, WidgetId,
+    Size, TextureHandle, UiRegion, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -20,21 +20,31 @@ 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,
-    /// What of its parent's extent the drawing was given, and what it was
+    /// The length its frame was narrowed to on each axis, as a length of
-    /// given at the parent's first ask of it -- the question a cold layout
+    /// its parent's frame: a declared rule, or a box its parent decided for
-    /// asks. A part is a length from the extent's start, so an extent that
+    /// it. `None` forwards the parent's frame whole. Kept as a length rather
-    /// moved re-places every child by re-adding that start.
+    /// than a position so that every placement puts the frame back in the
+    /// 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 that ask gave it, in its frame's coordinates. Kept rather
+    /// The box it was asked in, in its frame's coordinates: the box its
-    /// than worked out again from where its parent's own box is now: a
+    /// drawing was made in and the one its contract is about. Its drawing
-    /// parent drawn again in the box its own answer chose gives its children
+    /// is placed elsewhere by re-expression, never by asking again there.
-    /// 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, RenderedText,
+    Axis, Holds, LayoutHolds, LayoutLen, Len, Part, Place, Px, PxVec2, RegionAlign, Rel,
-    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
+    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
-    UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    TextureHandle, UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
         TexturePrimitive,
     },
-    ui::render_state::DrawInfo,
+    ui::render_state::{DrawInfo, Placing},
 };
 const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 
@@ -32,10 +32,6 @@ 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`]
@@ -48,13 +44,6 @@ 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
@@ -62,11 +51,8 @@ pub struct Painter<'a> {
     pub(super) frame_own: [Holds; 2],
     /// The same for its own box.
     pub(super) extent_own: [Holds; 2],
-    /// Pixel-box dependencies used only to compute the answer. These do not
+    /// What each child's drawing depends on. Asking a child again replaces
-    /// constrain a retained drawing placed inside that answer.
+    /// its drawing, so it replaces this too rather than narrowing it.
-    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.
@@ -159,37 +145,40 @@ 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, UiRegion::FULL, [Place::Within(Part::All); 2])
+        self.widget_at(id, [None; 2], [Place::Within(Part::All); 2])
     }
 
-    /// Draws a child, saying what its fractions are of and where its drawing
+    /// Asks a child, saying what its fractions are of and where it is asked.
-    /// goes.
     ///
-    /// `frame` is that reference, in this widget's own frame coordinates:
+    /// `narrow` is a length this widget decided for the child's frame, per
-    /// [`UiRegion::FULL`] forwards this widget's frame, which is what a
+    /// axis, as a length of this widget's own frame: a resolved share, or a
-    /// container that only divides room passes, so a fraction under it means
+    /// box a sibling's answer decided. `None` forwards this widget's frame,
-    /// the same wherever it sits and however deeply it is nested. Narrowing
+    /// which is what a container that only divides room passes, so a
-    /// it is for what is decided from above, and a declared length narrows
+    /// fraction under it means the same wherever it sits and however deeply
-    /// it here.
+    /// it is nested. A declared length narrows the frame here whatever the
+    /// 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 drawing goes, per axis, as a part of this
+    /// `place` is where the child is asked, per axis, as a part of this
-    /// widget's extent: see [`Place`]. A narrowed frame is its own extent,
+    /// widget's box: see [`Place`]. The child draws once, in that box, and
-    /// since the narrowing is what said where the drawing goes.
+    /// its answer is placed inside it by re-expressing the drawing. Nothing
+    /// 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>,
-        frame: UiRegion,
+        narrow: [Option<Len>; 2],
         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 (local, extent) = frame_and_extent(
+        let declared_narrow = narrowed_by(declared);
-            frame,
+        let narrow = [
-            part_of(self.extent, place),
+            declared_narrow[0].or(narrow[0]),
-            narrowed_by(declared, frame),
+            declared_narrow[1].or(narrow[1]),
-            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),
@@ -200,22 +189,11 @@ impl<'a> Painter<'a> {
             diag::region_node(id.id(), self.id, within);
         }
         // A child listed twice would be moved twice.
-        if !self.children.contains(&id.id()) {
+        let re_asked = 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 {
@@ -229,13 +207,16 @@ impl<'a> Painter<'a> {
                 frame_abs: within,
                 part: extent,
                 place,
-                offer_place,
+                offer_place: place,
+                narrow,
+                re_asked,
                 px,
             },
             None,
             self.rsc,
         );
-        let compose = |holds| in_parent(holds, local, extent, place, declared);
+        let own = self.extent;
+        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,
@@ -258,6 +239,33 @@ 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
@@ -294,19 +302,6 @@ 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());
@@ -369,31 +364,9 @@ 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.
@@ -425,14 +398,6 @@ 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 {
@@ -445,17 +410,6 @@ 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
@@ -591,37 +545,48 @@ 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],
-    declared: [Option<LayoutLen>; 2],
+    narrow: [Option<Len>; 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(), declared[n]) {
+        match (place[n].part(), narrow[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) if *frame.axis(axis) == UiSpan::FULL => {
+            (Part::All, None) => {
                 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 range on this widget's box. A length it pinned is this
+            // 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) => {
-                result.extent[n] = holds.extent[n].through(span.len());
+                let part_len = 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: a length of the
+            // Its box is a part of this widget's frame, or a length of it
-            // frame is all that reaches it, so what it holds for is a range
+            // decided here: a length of the frame is all that reaches it,
-            // on the frame and none of it on this widget's own box.
+            // so what it holds for is a range on the frame and none of it
+            // on this widget's own box.
             _ => {
                 result.frame[n] = result.frame[n].and(
                     holds.extent[n]
@@ -724,44 +689,34 @@ 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], frame: UiRegion) -> [Option<Len>; 2] {
+pub(crate) fn narrowed_by(declared: [Option<LayoutLen>; 2]) -> [Option<Len>; 2] {
-    AXES.map(|axis| {
+    declared.map(|declared| declared.map(|len| Len::from_parts(len.rel, len.px)))
-        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 its drawing goes in, both in
+/// The frame a child is asked in and the box it is asked in, both in the
-/// the coordinates of the widget asking.
+/// coordinates of the widget asking.
 ///
-/// `frame` is what the caller said the child's fractions are of, and `part`
+/// `part` is what of the caller's own box the child is given. `narrow` is a
-/// what of the caller's own box the drawing takes. `narrow` is the length a
+/// length decided for the child's frame -- a rule, a share, a box a sibling
-/// declared rule gives the frame, which makes the frame the box the drawing
+/// decided -- which makes the frame the box the child is asked in: that
-/// goes in -- a rule is what decided where it goes, and there is nothing
+/// length is what decided where it goes, placed in the part by the child's
-/// left to place inside it. A caller that narrowed the frame itself said the
+/// alignment. Where nothing narrowed it, the frame is the caller's own and
-/// same thing.
+/// the part is the box.
 ///
-/// The length is the caller's to supply so that a widget asked again gets
+/// A length rather than a position, so that a child placed again is put back
-/// the frame it already has rather than a second resolution of its rule.
+/// in whatever part it is given rather than where it first was.
 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) {
-        let span = frame.axis_mut(axis);
+        if let Some(len) = narrow {
-        let narrowed = match narrow {
-            Some(len) => {
             let slot = part.axis(axis);
             let start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
-                *span = UiSpan::new(start, start + len);
+            *frame.axis_mut(axis) = 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, MaskIdx, MoveIdx, Moves,
+    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx,
-    Painter, Part, PixelRegion, Place, PxVec2, Size, StrongWidget, UiRegion, UiRsc, Weight,
+    Moves, Painter, Part, PixelRegion, Place, PxVec2, Size, StrongWidget, UiRegion, UiRsc, Weight,
     WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
@@ -26,14 +26,20 @@ 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 drawing is given, in the frame's own coordinates: the
+    /// The box the widget is asked in, 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,
-    /// What of the parent's extent the drawing was given, and what it was
+    /// Where the widget is put, and where it was asked, as parts of the
-    /// given at the parent's first ask of it. See [`Place`].
+    /// parent's box. See [`Place`]. The two are one ask's place until the
+    /// 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,
@@ -45,34 +51,20 @@ 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.
-struct Placing {
+pub(super) struct Placing {
-    id: WidgetId,
+    pub id: WidgetId,
-    extent: UiRegion,
+    pub 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.
-    local: UiRegion,
+    pub local: UiRegion,
-    px: PxVec2,
+    pub px: PxVec2,
-    depth: usize,
+    pub depth: usize,
-    move_idx: MoveIdx,
+    pub move_idx: MoveIdx,
-    mask: MaskIdx,
+    pub mask: MaskIdx,
 }
 
 pub struct UiRenderState {
@@ -137,8 +129,7 @@ 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.
-            let offer = part_of(UiRegion::FULL, active.offer_place);
+            active.answers_at(px, active.offer_part) && active.holds.contains(px, active.offer_part)
-            active.answers_at(px, offer) && active.holds.contains(px, active.extent)
         });
         if !stands {
             widgets.needs_redraw.insert(root);
@@ -164,6 +155,8 @@ 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,
         }
     }
@@ -219,14 +212,8 @@ 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), UiRegion::FULL);
+        let narrow = narrowed_by(declared_lens(widgets, id));
-        frame_and_extent(
+        frame_and_extent(UiRegion::FULL, narrow, widgets.alignment(id)).0
-            UiRegion::FULL,
-            UiRegion::FULL,
-            narrow,
-            widgets.alignment(id),
-        )
-        .0
     }
 
     pub(super) fn draw_inner(
@@ -243,44 +230,48 @@ 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);
-        let retained = match stale {
+        // The widget draws once, in the box it is asked in, and its answer
-            true => None,
+        // is placed inside that box by re-expressing the drawing. The box the
-            false => self
+        // answer chose is never a question: nothing is drawn again in it, so
-                .retained_answer(id, part, info)
+        // an answer is kept only with the drawing that gave it, and both
-                .and_then(|answer| self.try_reuse(id, frame, part, info, rsc).map(|_| answer)),
+        // have to hold for the box asked about.
-        };
+        let reused = (!stale)
-        let answer = retained.unwrap_or_else(|| {
+            .then(|| self.retained_answer(id, part, info))
+            .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);
             }
-            self.draw_at(id, part, info, old.take(), rsc)
+            let answer = 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 gave the drawing. A local redraw asks
+        // and what of its own box it asked in. A local redraw asks the same
-        // the same question again from these.
+        // 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;
@@ -301,29 +292,6 @@ 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,
@@ -346,22 +314,15 @@ 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_answer, old_offer_part) = match old {
+        let old_children = old.map_or_else(Vec::new, |old| old.children);
-            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,
@@ -371,13 +332,10 @@ 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,
@@ -406,13 +364,9 @@ 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,
@@ -454,23 +408,15 @@ impl UiRenderState {
         if let Some(idx) = retired_move {
             self.moves.remove(idx);
         }
-        let drawing_own = LayoutHolds {
+        let own_holds = LayoutHolds {
             frame: frame_own,
             extent: extent_own,
             extent_len,
         };
-        let answer_own = LayoutHolds {
+        let answer_holds = own_holds.and(answer_under);
-            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(drawing_own, |holds, (_, child)| holds.and(child));
+            .fold(answer_holds, |holds, (_, child)| holds.and(child));
         debug_assert!(
             holds.contains(px, extent),
             "'{}' ({id:?}) drew in {px:?}, outside the ranges it reported: {holds:?}",
@@ -495,6 +441,8 @@ 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,
@@ -508,11 +456,13 @@ impl UiRenderState {
             frame_abs: frame,
             extent,
             frame: info.frame,
+            narrow: info.narrow,
             place: info.place,
             offer_place: info.offer_place,
-            offer_part: old_offer_part.unwrap_or(extent),
+            offer_part: extent,
-            // Whoever asked writes the answer, if this was the asking.
+            // Whoever asked writes the answer.
-            answer: old_answer,
+            answer: None,
+            re_asked: info.re_asked,
             size,
             holds,
             drawn: true,
@@ -597,16 +547,17 @@ impl UiRenderState {
         active.frame.size().to_px(parent_px)
     }
 
-    /// Reuses the actual drawing in a new box if its retained contract holds
+    /// Keeps the retained drawing if its contract holds for `part`, the box
-    /// there. Answers retained from a different ask are handled separately.
+    /// asked about, and puts it at `extent`, where the answer places it.
     fn try_reuse(
         &mut self,
         id: WidgetId,
         frame: UiRegion,
+        part: UiRegion,
         extent: UiRegion,
         info: DrawInfo,
         rsc: &mut dyn UiRsc,
-    ) -> Option<(Size, LayoutHolds)> {
+    ) -> Option<()> {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::ReuseAttempts);
         if rsc.widgets().needs_redraw.contains(&id) {
@@ -655,7 +606,7 @@ 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, extent) {
+        if !active.holds.contains(info.px, part) {
             #[cfg(feature = "layout-diagnostics")]
             {
                 // Which of the three said no, so a frame that redraws more
@@ -664,14 +615,13 @@ 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 != extent.axis(axis).len()) {
+                    if holds.extent_len[n].is_some_and(|pinned| pinned != part.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(extent.axis(axis).len().to_px(info.px.axis(axis)))
+                    if !holds.extent[n].contains(part.axis(axis).len().to_px(info.px.axis(axis))) {
-                    {
                         diag::bump(Counter::OutsideExtent);
                     }
                 }
@@ -680,9 +630,31 @@ 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 (answer, slot) = ((active.size, active.holds), active.move_idx);
+        let slot = active.move_idx;
         if moved {
             if has_region_node {
                 self.moves.set(slot, frame);
@@ -718,20 +690,31 @@ impl UiRenderState {
                 },
             );
         }
-        Some(answer)
     }
 
-    /// Places one child of `at.id` in the box that widget's own box gives
+    /// Places one child of `at.id` where that widget's own box now has it.
-    /// 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, active.place);
+        let (frame, part) = Self::re_ask(active, at.extent, place);
         let extent = placed_extent(
             part,
             active.measured().unwrap_or(active.size),
             active.declared,
-            active.place.map(Place::fills),
+            place.map(Place::fills),
             active.own_align,
         );
         let info = DrawInfo {
@@ -744,34 +727,25 @@ impl UiRenderState {
             frame,
             frame_abs: frame.within(&at.local),
             part,
-            place: active.place,
+            place,
             offer_place: active.offer_place,
+            narrow: active.narrow,
+            re_asked: active.re_asked,
             px: frame.size().to_px(at.px),
         };
-        self.place(child, extent, info, rsc);
+        self.relocate(child, info.frame_abs, extent, info, rsc);
     }
 
-    /// The frame and the box a widget being asked again is given, from what
+    /// The frame and the box a widget is given at `place` of its parent's
-    /// it already has and where its parent's box is now. A frame's length is
+    /// box, from what it already has. A frame's length is the same on every
-    /// the same on every ask, so a declared length is put back where it sits
+    /// ask, so a narrowed frame is put back where it sits in the part rather
-    /// in the part rather than resolved from its rule a second time.
+    /// than resolved a second time.
     fn re_ask(
         active: &ActiveData,
-        parent_extent: UiRegion,
+        parent_box: UiRegion,
         place: [Place; 2],
     ) -> (UiRegion, UiRegion) {
-        let narrow = AXES.map(|axis| {
+        frame_and_extent(part_of(parent_box, place), active.narrow, active.own_align)
-            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
@@ -942,10 +916,12 @@ 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,
@@ -1145,11 +1121,16 @@ 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.
+        // than keeping it. So is a widget the parent asked twice: its
+        // 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 || alignment_changed || !active.drawn || active.answer.is_none())
+            && (declared_changed
+                || 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
@@ -1177,12 +1158,20 @@ impl UiRenderState {
             return true;
         };
         let px = self.asked_px(id);
-        let (was_answer, was_holds) = (active.answer, active.holds);
+        let (was_answer, was_holds, was_place) = (active.answer, active.holds, active.place);
-        let needs_replacement = active.place != active.offer_place;
+        // The question its parent asked, asked again: the same place of the
-        // Re-ask the question its answer came from, not a later placement of
+        // box the parent was asked in, which is the box the parent's own
-        // that answer. A parent may move the retained drawing into a box the
+        // draw ran in and what its children's parts are of. Where the
-        // answer chose without making that box a new question.
+        // parent's answer put its own drawing is not a question anybody
-        let parent_extent = self.active[&parent].extent;
+        // asked, and nothing is asked in it here either.
+        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,
@@ -1190,11 +1179,13 @@ impl UiRenderState {
             parent_move: active.parent_move,
             region_node: rsc.widgets().is_region_node(id),
             mask: active.parent_mask,
-            frame: active.frame,
+            frame,
-            frame_abs: active.frame_abs,
+            frame_abs: frame.within(&asked.local),
-            part: Self::re_ask(active, parent_extent, active.offer_place).1,
+            part,
             place: active.offer_place,
             offer_place: active.offer_place,
+            narrow: active.narrow,
+            re_asked: false,
             px,
         };
         #[cfg(feature = "layout-diagnostics")]
@@ -1214,7 +1205,7 @@ impl UiRenderState {
         if active.holds.covers(was_holds) && was_holds.contains(px, active.extent) {
             active.holds = was_holds;
         }
-        if needs_replacement || active.answer != was_answer || active.holds != was_holds {
+        if 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")]
@@ -1223,9 +1214,34 @@ 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, 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::Of(UiSpan::new(cut, Len::FULL)));
+        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),
+            true => painter.widget_at(&self.wide, [None; 2], place),
-            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
+            false => painter.widget_at(&self.narrow, [None; 2], 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, 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.
+        // Asked in the whole viewport, then put at the scrolled offset.
-        let whole = UiRegion::FULL;
         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 its own answer decided. Where it goes is the content
-        // content box, scrolled.
+        // box, scrolled: its drawing moved there, not made again there.
-        painter.widget_at(
+        painter.place_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.answer_extent_len(axis);
+        let far = painter.extent_len(axis);
-        let measure_along = |from: Len, to: Len| match self.dir.sign {
+        let along = |from: Len, to: Len| match self.dir.sign {
             Sign::Pos => UiSpan::new(from, to),
             Sign::Neg => UiSpan::new(far - to, far - from),
         };
@@ -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,
+        // 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.
+        // 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::Fill(Part::From(measure_along(cursor, far)));
+            let room = Place::Within(Part::From(along(cursor, far)));
-            let len = painter
+            let size = painter
-                .widget_at(child, UiRegion::FULL, axis.pair(room, across))
+                .widget_at(child, [None; 2], axis.pair(room, across))
-                .len(axis);
+                .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
@@ -50,10 +54,9 @@ 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 - fixed_total;
+        let room = far - Len::from_parts(total.rel, total.px);
         // 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
@@ -73,18 +76,6 @@ 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
@@ -101,7 +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.
@@ -119,23 +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
+            // is the child's box outright rather than something to place an
-            // answer inside again.
+            // answer inside again. A share is decided here and nowhere
-            let span = along(from, start);
+            // else: its slot narrows its frame, and the child is asked in
-            let (frame, slot) = match len.leftover > Weight::ZERO && shares {
+            // it, since a text wraps at the width it is actually given. A
-                true => (
+            // fixed child's slot is its own answer, so its drawing is put
-                    UiRegion::from_axis(
+            // there as it is.
-                        axis,
+            let slot = along(from, start);
-                        painter.extent_part(axis, Part::From(span)),
+            let place = axis.pair(Place::Fill(Part::From(slot)), across);
-                        UiSpan::FULL,
+            let used = match len.leftover > Weight::ZERO && shares {
-                    ),
+                true => {
-                    Place::Fill(Part::All),
+                    let mut narrow = [None; 2];
-                ),
+                    narrow[axis as usize] = Some(slot.len());
-                false => (UiRegion::FULL, Place::Fill(Part::From(span))),
+                    painter.widget_at(child, narrow, place).len(!axis)
+                }
+                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

@@ -23,25 +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);
-            let place = [Axis::X, Axis::Y].map(|axis| {
+            painter.widget_at(child, narrow, [Place::Within(Part::All); 2]);
-                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, 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),
+            true => painter.widget_at(&self.wide, [None; 2], place),
-            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
+            false => painter.widget_at(&self.narrow, [None; 2], place),
         };
         Size::LEFTOVER
     }

tests/cases/retained.rs

@@ -12,7 +12,6 @@ struct Counted {
     draws: Rc<Cell<usize>>,
     size: Size,
     reads_box: bool,
-    reads_answer_box: bool,
 }
 
 impl Widget for Counted {
@@ -20,8 +19,6 @@ 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
     }
@@ -41,18 +38,11 @@ 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,
@@ -166,9 +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);
-    // Its final slot is a parent decision, so it is evaluated there after
+    // Asked once, from the cursor; its slot is its answer and the drawing is
-    // the provisional ask established its length.
+    // moved there.
-    assert_eq!(asked_draws.get(), 2);
+    assert_eq!(asked_draws.get(), 1);
 }
 
 #[test]
@@ -226,7 +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
@@ -250,7 +240,7 @@ 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 to compute its answer.
+/// Reads its box's size, which nothing but its own draw can put right.
 struct ReadsBox {
     draws: Rc<Cell<usize>>,
 }
@@ -258,36 +248,25 @@ 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.answer_px_size().div_int(4))
+        Size::from_px(painter.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 report a quarter of what they read. Their empty drawings are
+/// Both of these report a quarter of what they read. The quarter-sized box
-/// independent of that read, so a changed question costs one draw.
+/// 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>>,
 }
 
-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.answer_px_len(Axis::X).div_int(4),
+            painter.px_len(Axis::X).div_int(4),
             Px::from_int(20),
         ))
     }
@@ -343,7 +322,6 @@ 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),
@@ -521,7 +499,7 @@ 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()), false);
+    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
     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));
@@ -899,7 +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)),
@@ -988,7 +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)),
@@ -1087,7 +1065,7 @@ 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 = ReadsDrawingWidth {
+    let leaf = ReadsWidth {
         draws: draws.clone(),
     }
     .add(&mut h.rsc);
@@ -1098,7 +1076,7 @@ fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     h.frame();
 
     assert!(draws.get() > settled);
-    assert_corners!(h, leaf, (0, 0), (800, 200));
+    assert_corners!(h, leaf, (300, 90), (500, 110));
 }
 
 #[test]
@@ -1158,7 +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)),
@@ -1233,7 +1211,7 @@ 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.answer_px_len(Axis::X);
+            let width = painter.px_len(Axis::X);
             painter.primitive(RectPrimitive::color(Color::BLUE));
             Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
         }
@@ -1246,7 +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),
@@ -1288,7 +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)),
@@ -1308,7 +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)),
@@ -1333,10 +1311,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 } else {
                     Size::from((80, 27))
                 };
-                let (leaf, _) = match fractional {
+                let (leaf, _) = counted(h, size, !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

@@ -846,3 +846,84 @@ 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);
+}