WIP: a widget's region stays put and its placement moves in it

The protocol split: `region` is the box a parent gives a widget -- what a
fraction it declares or reports is a fraction of, and the coordinates
every region it writes composes within -- and it is the same box on the
ask that measures and the ask that places. `placement` is what of that
region the drawing takes, chosen by the parent per axis or by the
widget's own answer and alignment.

That is what stops a fraction being resolved twice: the placing ask no
longer hands the widget its own answer as its box, so nothing under it
re-resolves against a box that came from its own report. `reports_of`
and `decided` are gone, folded into the two regions; `box_of` is gone;
`declared_box` becomes `ask_box`, which gives a rule the region's length
and takes the position from the placement.

84 of 92 suite tests pass. Five text and region-node cases still diverge
warm against cold, and three count a second widget draw where a span's
measuring ask and its placing ask give different placements.

core/src/orientation/axis.rs

@@ -7,6 +7,17 @@ pub enum Axis {
     Y,
 }
 
+impl Axis {
+    /// A per-axis pair with `aligned` on this axis and `ortho` on the other,
+    /// which is what `from_axis` does for a vector.
+    pub fn pair<T>(self, aligned: T, ortho: T) -> [T; 2] {
+        match self {
+            Self::X => [aligned, ortho],
+            Self::Y => [ortho, aligned],
+        }
+    }
+}
+
 impl std::ops::Not for Axis {
     type Output = Self;
 

core/src/ui/active.rs

@@ -9,12 +9,16 @@ use crate::{
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
-    /// The box its drawing is in, in `parent_move`'s coordinates.
+    /// The box its parent gave it, in `parent_move`'s coordinates: what it
+    /// was asked about, and what a fraction under it is a fraction of. A
+    /// local redraw asks here.
     pub region: UiRegion,
-    /// The box its parent gave it, in the same coordinates: what it was
-    /// asked about, before its own answer placed its drawing inside it.
-    /// `region` is that placement, and a local redraw asks here.
-    pub given: UiRegion,
+    /// Where its drawing sits inside that box, in the box's own coordinates.
+    pub placement: UiRegion,
+    /// Whether its drawing read that placement, which is what says whether
+    /// moving it within the region is a redraw or only a different claim on
+    /// the same drawing.
+    pub reads_placement: bool,
     /// The same box as lengths of its parent's box, which is the one route
     /// to a box in pixels: a draw threads these down a level at a time, and
     /// [`crate::UiRenderState::redraw`] takes the same steps back up.

core/src/ui/painter.rs

@@ -2,8 +2,8 @@
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
     Axis, Holds, LayoutLen, Len, Px, PxVec2, RegionAlign, RenderedText, Size, StrongWidget,
-    TextAttrs, TextBuffer, TextData, TextureHandle, UiRegion, UiRenderState, UiRsc, UiVec2, Weight,
-    WidgetId, Widgets,
+    TextAttrs, TextBuffer, TextData, TextureHandle, UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2,
+    Weight, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveHandle, PrimitiveInst,
         PrimitiveKind, TexturePrimitive,
@@ -17,8 +17,20 @@ pub struct Painter<'a> {
     pub(super) state: &'a mut UiRenderState,
     pub(super) rsc: &'a mut dyn UiRsc,
 
-    /// This widget's box, in the coordinates of `move_idx`.
+    /// The box its parent gave it, in the coordinates of `move_idx`: what a
+    /// fraction of this widget's area is a fraction of, and what every region
+    /// it writes composes within. The same box on the ask that measures and
+    /// the ask that places, which is what keeps a fraction under it from
+    /// being resolved twice.
     pub(super) region: UiRegion,
+    /// Where this widget's drawing sits inside that box, in the box's own
+    /// coordinates: `FULL` while its answer is not yet known, and the box
+    /// its answer or its parent chose once one of them has.
+    pub(super) placement: UiRegion,
+    /// Whether this draw read its placement, which makes the drawing one
+    /// that holds for that placement alone -- the way reading a length in
+    /// pixels makes it hold for that length.
+    pub(super) reads_placement: bool,
     /// That box in pixels, which its children's are a length of: threaded
     /// down from the box this widget was given rather than composed back up
     /// the chain, so every length in layout is one multiply from its
@@ -88,28 +100,37 @@ impl<'a> Painter<'a> {
         self.primitives.push(h);
     }
 
-    /// Writes a primitive to be rendered
+    /// Writes a primitive over the whole of this widget's own box.
     pub fn primitive(&mut self, primitive: impl PrimitiveLike) {
+        let at = self.placed();
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, self.region)
+        self.primitive_at(primitive, at)
     }
 
+    /// Writes one somewhere in this widget's region. A widget that wants its
+    /// own box rather than the region it was given composes through
+    /// [`Self::placement`] first.
     pub fn primitive_within(&mut self, primitive: impl PrimitiveLike, region: UiRegion) {
         let primitive = primitive.into_primitive(self);
         self.primitive_at(primitive, region.within(&self.region));
     }
 
+    /// `region` is in this widget's own region, as every region it writes is.
     pub fn set_mask(&mut self, region: UiRegion) {
         assert!(self.mask == MaskIdx::NONE);
         self.mask = self.rsc.ui_mut().masks.push(Mask {
-            region,
+            region: region.within(&self.region),
             move_idx: self.move_idx,
         });
     }
 
-    /// Draws a widget within this widget's region.
+    /// Draws a widget in the whole of this widget's own box: it gets the
+    /// same region -- the same area for its fractions to be of -- and is put
+    /// where this widget was put. What a container that is only a wrapper
+    /// around one child wants, since its box is the child's.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_within(id, UiRegion::FULL)
+        let own = self.placement();
+        self.widget_at(id, UiRegion::FULL, [Some(own.x), Some(own.y)])
     }
 
     /// What a widget's rules declare its lengths to be, which whoever draws
@@ -129,51 +150,41 @@ impl<'a> Painter<'a> {
     }
 
     /// Draws a widget somewhere within this one. `region` is in this widget's
-    /// own coordinates, and the child's declared lengths are still to be
-    /// taken from it. Where the child's drawing sits inside what it is given
-    /// is the child's alignment, applied where the child is drawn, so a
-    /// container positions a child either by handing it a box of exactly its
-    /// length or by leaving it room and letting its alignment decide.
+    /// own region, and is the child's own region: what its declared lengths
+    /// and its report are fractions of. Where its drawing sits inside that is
+    /// its own answer placed by its alignment.
     pub fn widget_within<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
     ) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, region, region.size(), [false; 2])
+        self.widget_at(id, region, [None; 2])
     }
 
-    /// Draws a widget in `region`, saying what the answer means.
+    /// Draws a widget in `region`, saying where in it the drawing goes.
     ///
-    /// `reports_of` is what a fraction the child reports is a fraction of, as
-    /// lengths of this widget's own box. It is the box the child was given
-    /// wherever that box is the child's whole area -- a pad's inset, a stack
-    /// child, a scroll's content -- and a span passes its own extent along
-    /// the row instead: it offers each child the room left from its cursor,
-    /// because a text has to wrap at the width actually there, while
-    /// `rel(0.5)` still means half the span wherever the child sits in it.
+    /// `region` is the child's own area: what a fraction it declares or
+    /// reports is a fraction of, and the coordinates the regions it writes
+    /// compose within. It is the same box on the ask that measures and the
+    /// ask that places, which is what stops a fraction under it being
+    /// resolved twice.
     ///
-    /// A `decided` axis is one where this box was chosen from the widget's
-    /// own answer. On those the answer is not placed inside the box again: it
-    /// already is the box, and a fraction taken of it a second time would
-    /// shrink it twice. A container uses that where it hands back exactly
-    /// what a child asked for -- a span placing a child at the length it
-    /// reported, a scroll giving its content the content's own length.
+    /// `placement` is what of that region the child's drawing takes, per
+    /// axis, wherever this widget is choosing. `None` leaves the axis to the
+    /// child's own answer and alignment, which is what
+    /// [`Self::widget_within`] passes. A span passes the whole row as the
+    /// region, so `rel(0.5)` is half the row wherever the child sits in it,
+    /// and places the child by passing the slot along its axis.
     pub fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
-        reports_of: UiVec2,
-        decided: [bool; 2],
+        placement: [Option<UiSpan>; 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());
-        // Composing `FULL` through a box is not quite the identity in f32,
-        // so a child with nothing declared keeps the box it would have had.
-        let local = match declared.iter().any(Option::is_some) {
-            true => declared_box(region, declared, align),
-            false => region,
-        };
+        let (local, placement) = ask_box(region, declared, align, placement);
         let within = match local == UiRegion::FULL {
             true => self.region,
             false => local.within(&self.region),
@@ -219,7 +230,7 @@ impl<'a> Painter<'a> {
                 offer_len,
                 px,
                 offered_px,
-                decided,
+                placement,
             },
             None,
             self.rsc,
@@ -235,7 +246,7 @@ impl<'a> Painter<'a> {
         DrawResult {
             child: id,
             painter: self,
-            size: in_parent_frame(size, reports_of, declared),
+            size: in_parent_frame(size, local.size(), declared),
         }
     }
 
@@ -267,20 +278,18 @@ impl<'a> Painter<'a> {
         }
     }
 
-    /// A child's length in the box it is about to be offered, if it can be
-    /// had without drawing it: from its hint, or from a drawing it already
-    /// has that holds for that box. `reports_of` is what a fraction in the
-    /// answer is a fraction of, as it is for [`Self::widget_at`].
+    /// A child's length in the region it is about to be offered, if it can
+    /// be had without drawing it: from its hint, or from a drawing it already
+    /// has that holds for that box.
     pub fn known_len<W: ?Sized>(
         &mut self,
         child: &StrongWidget<W>,
         axis: Axis,
         region: UiRegion,
-        reports_of: UiVec2,
     ) -> Option<LayoutLen> {
         let declared = self.declared_lens(child);
         let align = self.rsc.widgets().alignment(child.id());
-        let local = declared_box(region, declared, align);
+        let (local, _) = ask_box(region, declared, align, [None; 2]);
         let first_ask = self.offer(child.id());
         if first_ask && let Some(active) = self.state.active.get_mut(&child.id()) {
             active.offer_len = local.size();
@@ -302,7 +311,7 @@ impl<'a> Painter<'a> {
         for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
             *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
         }
-        Some(in_parent_frame(size, reports_of, declared).axis(axis))
+        Some(in_parent_frame(size, local.size(), declared).axis(axis))
     }
 
     /// Whether this is the first box a child is asked about in during a draw
@@ -334,8 +343,10 @@ impl<'a> Painter<'a> {
         ui.text.render(buffer, attrs, width)
     }
 
+    /// `origin` is in this widget's own region, as every region it writes is.
     // TODO: merge the text methods into the primitive ones.
     pub fn glyphs(&mut self, text: &RenderedText, origin: UiRegion) {
+        let origin = origin.within(&self.region);
         let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {
             let mut region = origin;
@@ -362,12 +373,35 @@ impl<'a> Painter<'a> {
         }
     }
 
-    /// This widget's box, in the coordinates its own primitives are written
-    /// in -- so a region composed `within` it may be drawn directly.
+    /// The box this widget's parent gave it, in the coordinates its own
+    /// primitives are written in -- so a region composed `within` it may be
+    /// drawn directly. Its own box is [`Self::placement`] of this one.
     pub fn region(&self) -> UiRegion {
         self.region
     }
 
+    /// Where this widget's drawing goes inside the box it was given, in that
+    /// box's coordinates: what its own answer took of it, or what its parent
+    /// chose for it. `FULL` on the ask that measures, since nothing has been
+    /// placed yet.
+    ///
+    /// Reading it is what says the drawing depends on it, so a widget that
+    /// positions its own content reads it and is drawn again once its box is
+    /// known, and one that fills whatever it is given never is.
+    pub fn placement(&mut self) -> UiRegion {
+        self.reads_placement = true;
+        self.placement
+    }
+
+    /// This widget's own box in the coordinates its primitives are written
+    /// in: its placement composed through the region it was given.
+    fn placed(&mut self) -> UiRegion {
+        match self.placement() == UiRegion::FULL {
+            true => self.region,
+            false => self.placement.within(&self.region),
+        }
+    }
+
     /// 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.
@@ -401,20 +435,47 @@ impl<'a> Painter<'a> {
         placed_box(UiRegion::FULL, lens, RegionAlign::NEAR)
     }
 
-    /// This widget's box in pixels. Reading it makes the drawing one that
-    /// holds for this box only, until `holds` says how far it goes.
+    /// This widget's own box in pixels. Reading it makes the drawing one
+    /// that holds for this box only, until `holds` says how far it goes.
     pub fn px_size(&mut self) -> PxVec2 {
-        for (own, len) in self.own.iter_mut().zip([self.px.x, self.px.y]) {
-            if *own == Holds::ANY {
-                *own = Holds::at(len);
-            }
-        }
-        self.px
+        PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
     }
 
-    /// One axis of this widget's box in pixels. Prefer this to
+    /// 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 {
+        let part = self.placement().axis(axis).len();
+        let len = part.to_px(self.px.axis(axis));
+        let own = &mut self.own[axis as usize];
+        if *own == Holds::ANY {
+            *own = Holds::at(len).through(part);
+        }
+        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
+    /// in pixels holds for that one alone until it says otherwise.
+    pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+        let part = self.placement().axis(axis).len();
+        let holds = holds.into();
+        debug_assert!(
+            holds.contains(part.to_px(self.px.axis(axis))),
+            "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
+            self.label(),
+            self.id
+        );
+        // Kept as a range of the region's lengths, which is the one variable
+        // every range here is about: its own box is a part of that box, and
+        // `through` is the exact preimage of taking the part.
+        self.own[axis as usize] = holds.through(part);
+    }
+
+    /// One axis of the box this widget's parent gave it, in pixels -- what a
+    /// fraction of its area resolves against, and so what a container divides
+    /// among its children. Its own box is a part of this one.
+    pub fn region_px_len(&mut self, axis: Axis) -> Px {
         let len = self.px.axis(axis);
         let own = &mut self.own[axis as usize];
         if *own == Holds::ANY {
@@ -423,15 +484,14 @@ impl<'a> Painter<'a> {
         len
     }
 
-    /// The lengths of this widget's 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 in pixels holds for that one alone until it says otherwise.
-    pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+    /// [`Self::holds`] stated about the region rather than about this
+    /// widget's own box, for a container whose drawing turns on the box it
+    /// was given rather than on the part of it it took.
+    pub fn region_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
         let holds = holds.into();
         debug_assert!(
             holds.contains(self.px.axis(axis)),
-            "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
+            "'{}' ({:?}) says its drawing holds for lengths that leave out its region",
             self.label(),
             self.id
         );
@@ -521,19 +581,16 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
-/// A child's answer as lengths of the parent's own box. A widget reports a
-/// fraction, and `reports_of` is the length that fraction is of: the box the
-/// child was given wherever that is the child's whole area, and the parent's
-/// own extent wherever the box is a positional remainder, as a span's is
-/// after an earlier child. Pixels come through untouched either way, being
-/// that many pixels wherever they end up. A declared axis is already the
-/// parent's: it resolved the rule in its own box, and the rule is what the
-/// report says.
-fn in_parent_frame(size: Size, reports_of: UiVec2, declared: [Option<LayoutLen>; 2]) -> Size {
+/// A child's answer as lengths of the parent's own region. A widget reports
+/// a fraction of its own region, and `of` is that region as a length of this
+/// one. Pixels come through untouched, being that many pixels wherever they
+/// end up. A declared axis is already the parent's: it resolved the rule in
+/// its own region, and the rule is what the report says.
+fn in_parent_frame(size: Size, of: UiVec2, declared: [Option<LayoutLen>; 2]) -> Size {
     let mut size = size;
     for (axis, declared) in AXES.into_iter().zip(declared) {
         if declared.is_none() {
-            *size.axis_mut(axis) = size.axis(axis).within_len(reports_of.axis(axis));
+            *size.axis_mut(axis) = size.axis(axis).within_len(of.axis(axis));
         }
     }
     size
@@ -562,9 +619,9 @@ pub(crate) fn declared_lens(widgets: &Widgets, id: WidgetId) -> [Option<LayoutLe
 /// Whether what a widget reported along an axis is the whole of the box it
 /// is in rather than a part to be placed inside it. A share fills, because a
 /// share is a length only to whoever divides one, and whoever did is the one
-/// that handed down this box. A declared axis does too: `declared_box`
-/// already placed it, in the parent's box, and the rule's length is what the
-/// widget reports there. And an axis the parent decided from the answer is
+/// that handed down this box. A declared axis does too: the rule already gave
+/// the region its length, and the rule's length is what the widget reports
+/// there. And an axis the parent decided from the answer is
 /// the answer already.
 pub(crate) fn fills(reported: LayoutLen, declared: Option<LayoutLen>, decided: bool) -> bool {
     reported.leftover != Weight::ZERO || declared.is_some() || decided
@@ -612,21 +669,35 @@ pub(crate) fn placed_box(region: UiRegion, lens: UiVec2, align: RegionAlign) ->
     placed
 }
 
-/// Takes a widget's declared lengths in the box `region` is given in, since a
-/// fraction of a length means a fraction of that one, and puts what is left
-/// over on the side its alignment says. A caller that already reserved the
-/// space hands back the same length, so this is the identity for it.
-pub(crate) fn declared_box(
+/// A child's own region and where in it its drawing goes, from the box it is
+/// offered, the lengths its rules declare, and what its parent chose.
+///
+/// A rule gives the region its length outright -- that is what makes a rule
+/// win, and it is why a widget under one never learns of it -- and the region
+/// then sits where its parent placed it, or where its alignment says if its
+/// parent left the axis open. With no rule the region is the whole of what
+/// was offered, since that is the area a fraction under it is a fraction of,
+/// and what the parent chose is where in it the drawing goes. So the two
+/// coordinate spaces are the same one wherever a placement survives.
+pub(crate) fn ask_box(
     mut region: UiRegion,
     declared: [Option<LayoutLen>; 2],
     align: RegionAlign,
-) -> UiRegion {
-    for (axis, len) in AXES.into_iter().zip(declared) {
-        let Some(len) = len else { continue };
+    placement: [Option<UiSpan>; 2],
+) -> (UiRegion, [Option<UiSpan>; 2]) {
+    let mut placed = [None; 2];
+    for (axis, (len, chosen)) in AXES.into_iter().zip(declared.into_iter().zip(placement)) {
+        let Some(len) = len else {
+            placed[axis as usize] = chosen;
+            continue;
+        };
         let span = region.axis_mut(axis);
         let len = Len::from_parts(len.rel, len.px);
-        span.start += (span.len() - len).scale(align.axis(axis).rel());
+        span.start = match chosen {
+            Some(chosen) => chosen.start,
+            None => span.start + (span.len() - len).scale(align.axis(axis).rel()),
+        };
         span.end = span.start + len;
     }
-    region
+    (region, placed)
 }

core/src/ui/render_state.rs

@@ -1,6 +1,6 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
-use crate::ui::painter::{declared_box, declared_lens, placed_box, placed_lens};
+use crate::ui::painter::{ask_box, declared_lens, placed_box, placed_lens};
 use crate::{
     ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutLen, Len, MaskIdx, MoveIdx, Moves, Painter,
     PixelRegion, Px, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc, UiSpan, UiVec2, Weight,
@@ -30,10 +30,27 @@ pub(super) struct DrawInfo {
     /// parent's own, which is where every pixel length in layout comes from.
     pub px: PxVec2,
     pub offered_px: PxVec2,
-    /// The axes along which the parent chose this box from the widget's own
-    /// answer, so the answer is not placed inside it again. See
-    /// [`Painter::widget_at`].
-    pub decided: [bool; 2],
+    /// What of that region the parent chose to put the drawing in, per axis.
+    /// `None` leaves the axis to the widget's own answer and its alignment.
+    /// See [`Painter::widget_at`].
+    pub placement: [Option<UiSpan>; 2],
+}
+
+impl DrawInfo {
+    /// The axes the parent chose the placement on, which are the axes the
+    /// answer is not placed inside its region again.
+    fn decided(&self) -> [bool; 2] {
+        self.placement.map(|span| span.is_some())
+    }
+
+    /// The placement to draw in before the answer is known: what the parent
+    /// chose, and the whole region on any axis it left open.
+    fn offered_placement(&self) -> UiRegion {
+        UiRegion {
+            x: self.placement[0].unwrap_or(UiSpan::FULL),
+            y: self.placement[1].unwrap_or(UiSpan::FULL),
+        }
+    }
 }
 
 pub struct UiRenderState {
@@ -127,7 +144,7 @@ impl UiRenderState {
             offer_len: UiVec2::FULL_SIZE,
             px,
             offered_px: px,
-            decided: [false; 2],
+            placement: [None; 2],
         }
     }
 
@@ -181,11 +198,13 @@ impl UiRenderState {
     }
 
     fn root_region(id: WidgetId, widgets: &Widgets) -> UiRegion {
-        declared_box(
+        ask_box(
             UiRegion::FULL,
             declared_lens(widgets, id),
             widgets.alignment(id),
+            [None; 2],
         )
+        .0
     }
 
     pub(super) fn draw_inner(
@@ -217,41 +236,41 @@ impl UiRenderState {
             if old.is_none() {
                 old = self.remove(id, false, rsc);
             }
-            self.draw_at(id, region, info, old.take(), rsc)
+            self.draw_at(id, region, info.offered_placement(), info, old.take(), rsc)
         });
 
         let declared = declared_lens(rsc.widgets(), id);
-        // The second, final ask is in a box chosen from the answer on both
-        // axes, which is also what makes it terminate.
-        let lens = placed_lens(answer.0, declared, info.decided);
-        let placed = placed_box(region, lens, align);
-        let placed_info = DrawInfo {
-            px: lens.to_px(info.px),
-            decided: [true; 2],
-            ..info
+        // Where the drawing goes, in the region's own coordinates: what the
+        // parent chose, and on any axis it left open, what the answer took of
+        // the region placed by the widget's alignment. The region itself does
+        // not change, so nothing under it resolves a fraction a second time.
+        let lens = placed_lens(answer.0, declared, info.decided());
+        let own = placed_box(UiRegion::FULL, lens, align);
+        let placement = UiRegion {
+            x: info.placement[0].unwrap_or(own.x),
+            y: info.placement[1].unwrap_or(own.y),
         };
-        self.place(id, placed, placed_info, rsc);
+        self.place(id, region, placement, info, rsc);
 
         // The answer is only reusable while both parts of the operation are:
-        // what the widget reported in the box it was asked in, and what it
-        // drew in the box its report selected. Express the latter's contract
-        // back in terms of the box asked in before handing it to the parent.
+        // what the widget reported, and what it drew once placed. Both are
+        // ranges of the region's own lengths, since that is the box neither
+        // ask changes.
         let drawing_holds = self.active[&id].holds;
         let mut settled = answer;
         for axis in AXES {
-            settled.1[axis as usize] =
-                settled.1[axis as usize].and(drawing_holds[axis as usize].through(lens.axis(axis)));
+            settled.1[axis as usize] = settled.1[axis as usize].and(drawing_holds[axis as usize]);
         }
 
         let active = self.active.get_mut(&id).unwrap();
         // Whoever asked owns how the box was reached: the box it stated, and
         // what of that box the answer then took. A local redraw asks the
         // same question again from these.
-        active.given = region;
+        active.region = region;
         active.given_len = info.given_len;
         active.offer_len = info.offer_len;
         active.answer = settled;
-        active.decided = info.decided;
+        active.decided = info.decided();
         active.own_align = align;
         // A subtree can be reused whole under a different parent -- same box,
         // same layer, same region node -- and nothing in the drawing says it
@@ -268,17 +287,33 @@ impl UiRenderState {
         settled
     }
 
-    /// Draws a widget in the final box its answer chose, reusing the drawing
-    /// already there where its retained contract holds for that box. The
-    /// symbolic box can be unchanged while the box it sits in changed pixel
-    /// length, so what reuse checks is the box in pixels.
-    fn place(&mut self, id: WidgetId, placed: UiRegion, info: DrawInfo, rsc: &mut dyn UiRsc) {
-        if self.try_reuse(id, placed, info, rsc).is_none() {
-            #[cfg(feature = "layout-diagnostics")]
-            diag::bump(Counter::PlaceRedraws);
-            let old = self.remove(id, false, rsc);
-            self.draw_at(id, placed, info, old, rsc);
+    /// Puts the drawing where the answer says it goes. A drawing that never
+    /// read its placement is the same drawing wherever it is put, so all that
+    /// changes is what box the widget is recorded as occupying; one that read
+    /// it is drawn again, and only where the placement it read has moved.
+    fn place(
+        &mut self,
+        id: WidgetId,
+        region: UiRegion,
+        placement: UiRegion,
+        info: DrawInfo,
+        rsc: &mut dyn UiRsc,
+    ) {
+        let active = &self.active[&id];
+        // A drawing that never read its placement is the same drawing
+        // wherever it is put, so only what the widget is recorded as
+        // occupying changes; one that read it stands only for the placement
+        // it read. Either way the drawing still has to be where the region
+        // now is, which is what a retained answer on its own does not do.
+        let stands = !active.reads_placement || active.placement == placement;
+        if stands && self.try_reuse(id, region, info, rsc).is_some() {
+            self.active.get_mut(&id).unwrap().placement = placement;
+            return;
         }
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::PlaceRedraws);
+        let old = self.remove(id, false, rsc);
+        self.draw_at(id, region, placement, info, old, rsc);
     }
 
     /// Calls a widget's `draw` and keeps what it drew in `region`.
@@ -286,6 +321,7 @@ impl UiRenderState {
         &mut self,
         id: WidgetId,
         region: UiRegion,
+        placement: UiRegion,
         info: DrawInfo,
         old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
@@ -317,6 +353,8 @@ impl UiRenderState {
         let mut painter = Painter {
             state: self,
             region: local,
+            placement,
+            reads_placement: false,
             px,
             mask: info.mask,
             layer: info.layer,
@@ -351,6 +389,8 @@ impl UiRenderState {
             state: _,
             rsc: _,
             region: _,
+            placement: _,
+            reads_placement,
             px: _,
             mask,
             textures,
@@ -425,7 +465,7 @@ impl UiRenderState {
                         offer_len: UiVec2::FULL_SIZE,
                         px,
                         offered_px: px,
-                        decided: [false; 2],
+                        placement: [None; 2],
                     },
                     rsc,
                 );
@@ -436,10 +476,8 @@ impl UiRenderState {
         let active = ActiveData {
             id,
             region,
-            // The box a placing ask draws in is a part of the one its parent
-            // gave, which `draw_inner` writes back over these once the
-            // placement is done.
-            given: region,
+            placement,
+            reads_placement,
             given_len: info.given_len,
             offer_len: info.offer_len,
             // Whoever asked writes the answer, if this was the asking.
@@ -454,7 +492,7 @@ impl UiRenderState {
             children,
             size_deps,
             declared: declared_lens(rsc.widgets(), id),
-            decided: info.decided,
+            decided: info.decided(),
             own_align: rsc.widgets().alignment(id),
             move_idx,
             parent_move: info.parent_move,
@@ -540,11 +578,7 @@ impl UiRenderState {
         // Nothing above the root: the window is where a fraction becomes
         // pixels, which is also the whole of the box the root is given.
         let (parent_px, parent_offer) = match active.parent.and_then(|p| self.active.get(&p)) {
-            Some(parent) => {
-                let (given, offer) = self.asked_px(parent.id);
-                let lens = placed_lens(parent.answer.0, parent.declared, parent.decided);
-                (lens.to_px(given), offer)
-            }
+            Some(parent) => self.asked_px(parent.id),
             None => (self.output_size, self.output_size),
         };
         let px = active.given_len.to_px(parent_px);
@@ -639,7 +673,6 @@ impl UiRenderState {
         self.redepth(id, info.depth);
         let active = self.active.get_mut(&id).unwrap();
         active.region = region;
-        active.given = region;
         active.given_len = info.given_len;
         active.offer_len = info.offer_len;
         #[cfg(feature = "layout-diagnostics")]
@@ -694,7 +727,6 @@ impl UiRenderState {
         rsc: &mut dyn UiRsc,
     ) {
         let active = self.active.get_mut(&id).unwrap();
-        active.given = remap.apply(active.given);
         if active.move_idx != parent_move {
             let region = remap.apply(active.region);
             active.region = region;
@@ -792,7 +824,8 @@ impl UiRenderState {
             ActiveData {
                 id,
                 region: UiRegion::FULL,
-                given: UiRegion::FULL,
+                placement: UiRegion::FULL,
+                reads_placement: false,
                 given_len: UiVec2::FULL_SIZE,
                 offer_len: UiVec2::FULL_SIZE,
                 answer: (size, [Holds::ANY; 2]),
@@ -951,8 +984,9 @@ impl UiRenderState {
     pub fn window_region(&self, id: &impl IdLike) -> Option<PixelRegion> {
         let active = self.active.get(&id.id())?;
         active.drawn.then(|| {
+            let placed = active.placement.within(&active.region);
             self.moves
-                .resolve(active.parent_move, active.region)
+                .resolve(active.parent_move, placed)
                 .to_px(self.output_size)
         })
     }
@@ -1031,9 +1065,12 @@ impl UiRenderState {
             offer_len: active.offer_len,
             px: given_px,
             offered_px,
-            decided: active.decided,
+            // The same question its parent asked: the axes its parent chose
+            // the placement on, put back where they were.
+            placement: AXES
+                .map(|axis| active.decided[axis as usize].then(|| *active.placement.axis(axis))),
         };
-        let (given, was_answer) = (active.given, active.answer);
+        let (given, was_answer) = (active.region, active.answer);
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::LocalRedraws);
 

src/widget/mask.rs

@@ -6,7 +6,8 @@ pub struct Masked {
 
 impl Widget for Masked {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        painter.set_mask(painter.region());
+        let own = painter.placement();
+        painter.set_mask(own);
         painter.widget(&self.inner);
         // What it occupies is its box, on both axes, for the reason `Scroll`
         // reports the same: it clips what is inside to that box, so it can

src/widget/position/pad.rs

@@ -13,9 +13,8 @@ impl Widget for Pad {
         // it; where the box is bigger -- a share of a row, a rule over this
         // widget -- the slack is the inner's to sit in, and forcing the near
         // edge pinned it to a corner it had not asked for.
-        let inner = painter
-            .widget_within(&self.inner, self.padding.region())
-            .size();
+        let inside = self.padding.region_of(painter.placement());
+        let inner = painter.widget_within(&self.inner, inside).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,
@@ -53,14 +52,18 @@ impl Padding {
             bottom: amt,
         }
     }
-    pub fn region(&self) -> UiRegion {
-        let mut region = UiRegion::FULL;
+    /// `region` less this padding on each side.
+    pub fn region_of(&self, mut region: UiRegion) -> UiRegion {
         region.x.start.px += self.left;
         region.y.start.px += self.top;
         region.x.end.px -= self.right;
         region.y.end.px -= self.bottom;
         region
     }
+
+    pub fn region(&self) -> UiRegion {
+        self.region_of(UiRegion::FULL)
+    }
     pub fn x(amt: impl UiNum) -> Self {
         let amt = Px::from_num(amt);
         Self {

src/widget/position/scroll.rs

@@ -15,7 +15,7 @@ impl Widget for Scroll {
         // Draw in the whole container only when its scrolling-axis length is
         // not already known, then draw it at the scrolled offset.
         let whole = UiRegion::FULL;
-        let answer_len = match painter.known_len(&self.inner, self.axis, whole, whole.size()) {
+        let answer_len = match painter.known_len(&self.inner, self.axis, whole) {
             Some(len) => len,
             None => painter.widget(&self.inner).size().axis(self.axis),
         };
@@ -64,7 +64,11 @@ impl Widget for Scroll {
             region = region.offset(offset);
             region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
         }
-        painter.widget_at(&self.inner, region, region.size(), [true; 2]);
+        // The viewport is the inner's region, 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 put is the content
+        // box, scrolled.
+        painter.widget_at(&self.inner, whole, [Some(region.x), Some(region.y)]);
         // What it occupies is its box, on both axes: it clips its content to
         // that box, so it can neither take less of one nor honestly ask for
         // more. The content's length is what it scrolls through, not what it

src/widget/position/span.rs

@@ -10,25 +10,36 @@ pub struct Span {
 impl Widget for Span {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
+        // The row: this span's own box, as a span of the region it was given.
+        // Its children are laid out along it, and what they declare or report
+        // is a fraction of the region -- the area this span was told it has,
+        // which it passes on unchanged.
+        let own = painter.placement();
+        let row = *own.axis(axis);
+        // Across itself the span's own box is the child's region: a span is
+        // what contains its children there, and nothing divides that axis.
+        // Along it the whole region is, so a fraction means the same thing
+        // for every child however much of the row is left when it is asked.
+        let region = UiRegion::from_axis(axis, UiSpan::FULL, *own.axis(!axis));
+        let along = |from: Len, to: Len| match self.dir.sign {
+            Sign::Pos => UiSpan::new(row.start + from, row.start + to),
+            Sign::Neg => UiSpan::new(row.end - to, row.end - from),
+        };
+        let far = row.len();
         // A length for every child before their final boxes are chosen: from
         // a hint where one exists, and from drawing otherwise.
         let mut cursor = Len::rel_min();
         let mut lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let mut span = UiSpan::new(cursor, Len::rel_max());
-            if self.dir.sign == Sign::Neg {
-                span.flip();
-            }
-            let region = UiRegion::from_axis(axis, span, UiSpan::FULL);
-            // Offered the room left from the cursor, because a text has to
-            // wrap at the width actually there, but reporting a fraction of
-            // the whole row: `rel(0.5)` is half the span whatever else is in
-            // it and wherever this child sits among them.
-            let len = match painter.known_len(child, axis, region, UiVec2::FULL_SIZE) {
+            // The whole region is the child's, 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 placed in is the room left
+            // from the cursor, because a text has to wrap at the width
+            // actually there.
+            let room = axis.pair(Some(along(cursor, far)), None);
+            let len = match painter.known_len(child, axis, region) {
                 Some(len) => len,
-                None => painter
-                    .widget_at(child, region, UiVec2::FULL_SIZE, [false; 2])
-                    .len(axis),
+                None => painter.widget_at(child, region, room).len(axis),
             };
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
@@ -46,9 +57,9 @@ impl Widget for Span {
             |sum, len| sum + *len,
         );
 
-        // What is left for the shares to divide: the box less everything
-        // fixed, as a length of the box rather than a number of pixels.
-        let room = Len::rel_max() - 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 region rather than a number of pixels.
+        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
@@ -60,12 +71,12 @@ impl Widget for Span {
         // exist at all turns on this.
         let mut shares = false;
         if total.leftover > Weight::ZERO {
-            shares = room.to_px(painter.px_len(axis)) > Px::ZERO;
+            shares = room.to_px(painter.region_px_len(axis)) > Px::ZERO;
             let holds = match shares {
                 true => Holds::from(Px::STEP..=Px::MAX),
                 false => Holds::from(Px::MIN..=Px::ZERO),
             };
-            painter.holds(axis, holds.through(room));
+            painter.region_holds(axis, holds.through(room));
         }
 
         // Across itself a span is as long as its longest child -- unless a
@@ -94,28 +105,19 @@ impl Widget for Span {
                 fixed.px += self.gap;
                 continue;
             }
-            let mut span = UiSpan::FULL;
-            span.start = start;
+            let from = start;
             if len.leftover > Weight::ZERO && shares {
                 taken += len.leftover;
             }
             fixed.px += len.px;
             fixed.rel += len.rel;
             start = shared(fixed, taken, total.leftover, room);
-            span.end = start;
-            let mut region = UiRegion::from_axis(axis, span, UiSpan::FULL);
-            if self.dir.sign == Sign::Neg {
-                region.flip(axis);
-            }
-            // Along the row this box is the child's own answer, so the answer
-            // is not placed in it again; across it the child sits where its
-            // alignment says.
-            let placed = painter.widget_at(
-                child,
-                region,
-                UiVec2::FULL_SIZE,
-                [axis == Axis::X, axis == Axis::Y],
-            );
+            // Along the row the span says where the child goes; across it the
+            // child sits where its own alignment says. Its region is the
+            // whole of what this span was given either way, which is what its
+            // fractions are of.
+            let placed =
+                painter.widget_at(child, region, axis.pair(Some(along(from, start)), None));
             if shrinks {
                 let used = placed.len(!axis);
                 // Choosing between a fixed and a relative length from the

src/widget/position/stack.rs

@@ -13,31 +13,37 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
-        // Whichever child sizes the stack decides the box every child gets.
-        // The stack reports that size, so a child given a longer box would
-        // draw outside what the stack says it occupies.
+        // This stack's own box, which is `FULL` until its answer is known.
+        let placement = painter.placement();
+        // Whichever child sizes the stack keeps the stack's whole region as
+        // its own -- the stack is the length that child asked for, so taking
+        // the fraction of the stack's box again would take it twice -- and is
+        // put where the stack itself is put.
         let size = match sizing.and_then(|i| self.children.get(i).map(|c| (i, c))) {
             // On the layer that child ends up on, so the ask below is a reuse
             // rather than a second drawing of it somewhere else: a retained
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter.widget(child).size()
+                painter
+                    .widget_at(
+                        child,
+                        UiRegion::FULL,
+                        [Some(placement.x), Some(placement.y)],
+                    )
+                    .size()
             }
             None => Size::LEFTOVER,
         };
-        let region = painter.box_of(size);
         for (i, child) in self.children.iter().enumerate() {
+            if sizing == Some(i) {
+                continue;
+            }
             painter.child_layer_at(i);
-            // The sizing child placed its own content in the box its answer
-            // decided, and this box was derived from that answer, so applying
-            // its alignment again here would place it twice. Every other
-            // child is handed a box that owes nothing to its own answer, and
-            // where it sits in one bigger than itself is its own business.
-            match sizing == Some(i) {
-                true => painter.widget_at(child, region, region.size(), [true; 2]),
-                false => painter.widget_within(child, region),
-            };
+            // Every other child has the stack's own box for its region, since
+            // the stack is what contains it, and where it sits in one bigger
+            // than itself is its own business.
+            painter.widget_within(child, placement);
         }
         size
     }

src/widget/text/mod.rs

@@ -89,7 +89,7 @@ impl TextView {
         // hair under that line, and the break made in it is not the break a
         // cold layout makes there.
         let size = Size::from_px(PxVec2::ceil_from_f32(tex.size));
-        let within = region.within(&painter.region());
+        let within = region.within(&painter.placement());
         painter.glyphs(tex, within);
         (region, size)
     }

tests/cases/layout.rs

@@ -20,11 +20,12 @@ fn a_span_gives_each_child_the_width_it_asked_for() {
     assert_corners!(h, right, (100, 0), (400, 200));
 }
 
-/// A span offers each child the room left after the one before, because a
-/// text has to wrap at the width actually there, but reads what the child
-/// reports as a fraction of the whole row. So two children asking for half
-/// each take the whole row between them, however much of it was left when
-/// each was asked, and a third overflows.
+/// A span places each child in the room left after the one before, because a
+/// text has to wrap at the width actually there, but the child's region is
+/// the whole row. So two children asking for half each take the whole row
+/// between them, however much of it was left when each was asked, and a third
+/// overflows -- and a span passes its own region on unchanged, so a child of
+/// a nested span asking for half asks for half of the same row.
 #[test]
 fn a_span_reads_a_child_report_as_a_fraction_of_the_row() {
     let mut h = Harness::new((400, 100));
@@ -34,10 +35,10 @@ fn a_span_reads_a_child_report_as_a_fraction_of_the_row() {
     let tail = rect(Color::BLUE).width(100).add(&mut h.rsc);
     h.set_root((half, nested, tail).span(Dir::RIGHT).width(rel(1.0)));
 
-    // The nested span is placed at the length it reported and drawn there
-    // once more; half of that final box is what its own child takes.
+    // The nested span is placed at the length it reported, and its own child
+    // asks for half of the row rather than half of that placement.
     assert_corners!(h, nested, (200, 0), (400, 100));
-    assert_corners!(h, inner, (200, 0), (300, 100));
+    assert_corners!(h, inner, (200, 0), (400, 100));
     assert_corners!(h, tail, (400, 0), (500, 100));
 }