Experiment: say the room from the cursor as an inset, and read the row's length only where a slot depends on it

Measured identical to its parent at every phase of the cost rig; kept as
evidence, not proposed for landing.

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,
@@ -54,7 +52,7 @@ pub(crate) enum Counter {
     TextShapes,
     TextBreaks,
     GlyphPlacements,
-    OutsidePlacement,
+    OutsidePinnedLen,
     OutsideFrame,
     OutsideExtent,
 }
@@ -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",
@@ -92,7 +88,7 @@ impl Counter {
         "text shapes",
         "text line breaks",
         "glyph placements",
-        "reuse outside: the placement it was pinned to",
+        "reuse outside: the length it was pinned to",
         "reuse outside: a frame length",
         "reuse outside: an extent length",
     ];

core/src/ui/active.rs

@@ -1,6 +1,6 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive, Size,
-    TextureHandle, UiRegion, WidgetId,
+    LayerId, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive,
+    Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -9,35 +9,39 @@ use crate::{
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
-    /// Its frame in `parent_move`'s coordinates: what a fraction it declares
-    /// or reports is a fraction of, composed. Everything it draws sits inside
-    /// this by way of `extent`.
-    pub frame_abs: UiRegion,
-    /// Where its drawing goes, in the frame's own coordinates.
+    /// Where its drawing goes, in its region node's coordinates.
     pub extent: UiRegion,
-    /// That frame in its parent's frame coordinates, before composition:
-    /// forwarded whole by a transparent container, narrowed by a declared
-    /// length or an inset. 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
-    /// given at the parent's first ask of it -- the question a cold layout
-    /// asks. A part is a length from the extent's start, so an extent 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
-    /// than worked out again from where its parent's own box is now: a
-    /// parent drawn again in the box its own answer chose gives its children
-    /// boxes it never measured anything in, and the measurement this widget
-    /// answered is the one its parent's layout was built on.
-    pub offer_part: UiRegion,
+    /// What a fraction declared or reported under this widget is a fraction
+    /// of, as a length of the window.
+    pub frame: UiVec2,
+    /// A frame its parent decided for it on each axis -- a row's slot, or
+    /// padding's frame less its pixels -- as a length of the window. `None`
+    /// forwards the parent's frame. What it declared is kept separately in
+    /// `declared` and is a fraction of whichever of the two reached it.
+    pub narrow: [Option<Len>; 2],
+    /// Where its drawing was put, and where it was asked, each as a part of
+    /// its parent's box. The two differ where a container asks in one place
+    /// and puts 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 placed: [Place; 2],
+    pub asked: [Place; 2],
+    /// The box it was asked in, in the parent's region-node coordinates: the
+    /// box its drawing was made in and the one its contract is about. Its
+    /// drawing is placed elsewhere by re-expression, never by asking again.
+    pub 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)>,
-    /// What the widget said it used of its frame, the last time it drew.
+    /// 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 reported, in window-unit lengths.
     pub size: Size,
-    /// The frame and extent reads that this drawing holds for.
+    /// The window and extent reads that this drawing holds for, and the
+    /// frame and box it pinned.
     pub holds: LayoutHolds,
     pub drawn: bool,
     pub parent: Option<WidgetId>,
@@ -63,7 +67,8 @@ pub struct ActiveData {
     /// Its alignment when it was last drawn, which a change to the property
     /// is found against.
     pub own_align: RegionAlign,
-    /// The movable region whose coordinates `frame_abs` uses.
+    /// The movable region whose coordinates `extent` uses when this widget
+    /// does not own a region node.
     pub parent_move: MoveIdx,
     /// The mask its drawing is clipped to: one it set itself, or the one it
     /// inherited from whoever drew it.
@@ -77,12 +82,10 @@ pub struct ActiveData {
 }
 
 impl ActiveData {
-    /// Whether what it answered still stands for a frame of these pixel
-    /// lengths. The answer was given in the box its parent first asked
-    /// about, which is what it is checked against -- `holds` on the record
-    /// is about the box the answer then chose.
-    pub fn answers_at(&self, px: crate::PxVec2, part: UiRegion) -> bool {
-        self.answer
-            .is_some_and(|(_, holds)| holds.contains(px, part))
+    /// What it answered when its parent asked, where it has been asked at
+    /// all. Not `size`, which is what its last drawing reported: a drawing
+    /// re-expressed in the box that answer chose is not a second answer.
+    pub fn measured(&self) -> Option<Size> {
+        self.answer.map(|(size, _)| size)
     }
 }

core/src/ui/holds.rs

@@ -73,6 +73,21 @@ impl Holds {
         }
     }
 
+    /// What a box has to be for a part of it, this many pixels shorter, to
+    /// stay in this range: the range moved by that much, an end that was
+    /// unbounded staying so.
+    pub const fn longer_by(self, px: Px) -> Self {
+        let lo = match self.lo.raw() == Px::MIN.raw() {
+            true => self.lo,
+            false => self.lo.add(px),
+        };
+        let hi = match self.hi.raw() == Px::MAX.raw() {
+            true => self.hi,
+            false => self.hi.add(px),
+        };
+        Self { lo, hi }
+    }
+
     const fn raws(lo: i64, hi: i64) -> Self {
         Self {
             lo: Px::from_raw(narrow(lo)),

core/src/ui/layout_holds.rs

@@ -1,26 +1,35 @@
-use crate::{Axis, Holds, Len, PxVec2, UiRegion};
+use crate::{Axis, Holds, Len, PxVec2, UiRegion, UiVec2};
 
 const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 
-/// What one evaluation of a widget depends on: the pixel lengths of its
-/// frame and of its own box that its drawing and its answer hold for, and
-/// the symbolic length of its own box where it read one.
+/// What one evaluation of a widget depends on: the window lengths its reads
+/// hold for, the pixel lengths of its own box, and the symbolic lengths of
+/// that box and of its frame where either one is what it was expressed in.
 ///
-/// The symbolic length is a pin rather than a range: a container places its
+/// The symbolic lengths are pins rather than ranges: a container places its
 /// children as lengths of its frame measured from where its own box starts,
 /// so what it draws turns on that box's length and on nothing about where it
-/// is. It does not compose into the parent -- a widget pinned this way is
-/// checked when it is re-placed.
+/// is. A box pin reaches the parent only where the box it pinned is the
+/// parent's own; anywhere else the parent chose that length itself, and a
+/// widget pinned this way is checked when it is re-placed.
+///
+/// A frame pin says the answer or the drawing is a fraction of the frame,
+/// which is a different length wherever the frame is a different one -- at
+/// the same window size, so no range of window pixels can say it. A length
+/// of the frame that is only pixels is not one: it is that many pixels
+/// whatever the frame turns out to be.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct LayoutHolds {
-    pub frame: [Holds; 2],
+    pub window: [Holds; 2],
+    pub frame_len: [Option<Len>; 2],
     pub extent: [Holds; 2],
     pub extent_len: [Option<Len>; 2],
 }
 
 impl LayoutHolds {
     pub const ANY: Self = Self {
-        frame: [Holds::ANY; 2],
+        window: [Holds::ANY; 2],
+        frame_len: [None; 2],
         extent: [Holds::ANY; 2],
         extent_len: [None; 2],
     };
@@ -28,34 +37,42 @@ impl LayoutHolds {
     pub fn and(self, other: Self) -> Self {
         let mut result = Self::ANY;
         for n in 0..2 {
-            result.frame[n] = self.frame[n].and(other.frame[n]);
+            result.window[n] = self.window[n].and(other.window[n]);
             result.extent[n] = self.extent[n].and(other.extent[n]);
             debug_assert!(
                 self.extent_len[n].is_none()
                     || other.extent_len[n].is_none()
                     || self.extent_len[n] == other.extent_len[n]
             );
+            debug_assert!(
+                self.frame_len[n].is_none()
+                    || other.frame_len[n].is_none()
+                    || self.frame_len[n] == other.frame_len[n]
+            );
             result.extent_len[n] = self.extent_len[n].or(other.extent_len[n]);
+            result.frame_len[n] = self.frame_len[n].or(other.frame_len[n]);
         }
         result
     }
 
     pub fn covers(self, other: Self) -> bool {
         (0..2).all(|n| {
-            self.frame[n].lo <= other.frame[n].lo
-                && self.frame[n].hi >= other.frame[n].hi
+            self.window[n].lo <= other.window[n].lo
+                && self.window[n].hi >= other.window[n].hi
                 && self.extent[n].lo <= other.extent[n].lo
                 && self.extent[n].hi >= other.extent[n].hi
                 && self.extent_len[n].is_none_or(|len| other.extent_len[n] == Some(len))
+                && self.frame_len[n].is_none_or(|len| other.frame_len[n] == Some(len))
         })
     }
 
-    pub fn contains(self, px: PxVec2, extent: UiRegion) -> bool {
+    pub fn contains(self, window: PxVec2, frame: UiVec2, extent: UiRegion) -> bool {
         AXES.into_iter().all(|axis| {
             let n = axis as usize;
             let len = extent.axis(axis).len();
-            self.frame[n].contains(px.axis(axis))
-                && self.extent[n].contains(len.to_px(px.axis(axis)))
+            self.window[n].contains(window.axis(axis))
+                && self.frame_len[n].is_none_or(|pinned| pinned == frame.axis(axis))
+                && self.extent[n].contains(len.to_px(window.axis(axis)))
                 && self.extent_len[n].is_none_or(|pinned| pinned == len)
         })
     }

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,
-    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
-    UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    Axis, Holds, LayoutHolds, LayoutLen, Len, Part, Place, Px, PxVec2, RegionAlign, Rel,
+    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
+    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];
 
@@ -17,26 +17,20 @@ pub struct Painter<'a> {
     pub(super) state: &'a mut UiRenderState,
     pub(super) rsc: &'a mut dyn UiRsc,
 
-    /// What a fraction this widget declares or reports is a fraction of, in
-    /// the coordinates of `move_idx`: forwarded from its parent unchanged
-    /// through a span, a stack or a scroll, and narrowed only by what was
-    /// decided above it -- a declared length, an inset, the root. Its length
-    /// is the same on every ask of the widget, which is what keeps a fraction
-    /// under it from being resolved twice.
-    pub(super) frame: UiRegion,
-    /// Where this widget's drawing goes, in the frame's own coordinates.
-    /// Everything it writes is in these coordinates, and its children are
-    /// placed as parts of it.
+    /// This widget's frame, per axis: a length of the window, and what a
+    /// fraction it or anything under it declares or reports is a fraction
+    /// of. A length rather than a box, so padding can take from both the
+    /// frame and the box without either becoming the other.
+    pub(super) frame: UiVec2,
+    /// Where this widget's drawing goes, in its region node's coordinates.
     pub(super) extent: UiRegion,
     /// The extent's symbolic length where this draw read it, which makes the
     /// 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],
-    /// 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`]
-    /// inverts exactly that.
-    pub(super) px: PxVec2,
+    /// The window in pixels. Frames and boxes become pixels against this one
+    /// unit, regardless of region-node boundaries.
+    pub(super) window: PxVec2,
     pub(super) mask: MaskIdx,
     pub(super) textures: Vec<TextureHandle>,
     pub(super) primitives: Vec<RetainedPrimitive>,
@@ -44,22 +38,19 @@ 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
-    /// length until it reads one, then that one, unless it says otherwise.
-    pub(super) own: [Holds; 2],
-    /// The same for its extent.
+    /// What this draw itself read of the window in pixels, per axis: every
+    /// window until it reads one, then that one, unless it says otherwise.
+    pub(super) window_own: [Holds; 2],
+    /// Its frame's symbolic length where this draw read it, which makes the
+    /// drawing one that holds for that frame alone.
+    pub(super) frame_own_len: [Option<Len>; 2],
+    /// The window reads' equivalent for its own box.
     pub(super) extent_own: [Holds; 2],
-    /// Dependencies of every child drawing, including unmeasured overlays.
-    pub(super) under: LayoutHolds,
+    /// What each child's drawing depends on. Asking a child again replaces
+    /// its drawing, so it replaces this too rather than narrowing it.
+    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.
     pub(super) move_idx: MoveIdx,
@@ -82,12 +73,10 @@ impl<'a> Painter<'a> {
         self.write_resolved(kind, primitive, region, self.resolve(region));
     }
 
-    /// A box in this widget's extent coordinates, composed into the
-    /// coordinates its move slot is in: through the extent, then through the
-    /// frame the extent is a part of. The same two steps a recomposition
-    /// replays, so a moved drawing lands where a cold one does.
+    /// A box in this widget's extent coordinates, composed into its region
+    /// node's coordinates.
     fn resolve(&self, region: UiRegion) -> UiRegion {
-        region.within(&self.extent).within(&self.frame)
+        region.within(&self.extent)
     }
 
     fn write_resolved<P: Primitive>(
@@ -151,65 +140,47 @@ 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
-    /// goes.
+    /// Asks a child, saying what its fractions are of and where it is asked.
     ///
-    /// `frame` is that reference, in this widget's own frame coordinates:
-    /// [`UiRegion::FULL`] forwards this widget's frame, which is what a
-    /// container that only divides room passes, so a fraction under it means
-    /// the same wherever it sits and however deeply it is nested. Narrowing
-    /// it is for what is decided from above -- an inset's margins -- and a
-    /// declared length narrows it here.
+    /// `narrow` is a length this widget decided for the child's frame, per
+    /// axis, as a length of this widget's own frame: a resolved share, or a
+    /// box a sibling's answer decided. `None` forwards this widget's frame,
+    /// which is what a container that only divides room passes, so a
+    /// fraction under it means the same wherever it sits and however deeply
+    /// 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
-    /// widget's extent: see [`Place`]. A narrowed frame is its own extent,
-    /// since the narrowing is what said where the drawing goes.
+    /// `place` is where the child is asked, per axis, as a part of this
+    /// widget's box: see [`Place`]. The child draws once, in that box, and
+    /// 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 narrow = AXES.map(|axis| {
-            let n = axis as usize;
-            // A rule's fraction is a fraction of the frame the child was
-            // given, which is the one length the rule can mean.
-            declared[n]
-                .map(|len| Len::from_parts(len.rel, len.px).within_len(frame.axis(axis).len()))
-        });
-        let (local, extent) = frame_and_extent(frame, part_of(self.extent, place), narrow, align);
-        let within = match local == UiRegion::FULL {
-            true => self.frame,
-            false => local.within(&self.frame),
-        };
+        let (frame, extent) =
+            frame_and_extent(self.extent, self.frame, place, narrow, declared, align);
         #[cfg(feature = "layout-diagnostics")]
         if region_node {
             diag::bump(Counter::RegionNodeDraws);
-            diag::region_node(id.id(), self.id, within);
+            diag::region_node(id.id(), self.id, extent);
         }
         // 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 px = frame.to_px(self.window);
         let (size, answer_holds, holds) = self.state.draw_inner(
             id.id(),
             DrawInfo {
@@ -219,27 +190,28 @@ impl<'a> Painter<'a> {
                 parent_move: self.move_idx,
                 region_node,
                 mask: self.mask,
-                frame: local,
-                frame_abs: within,
+                frame,
                 part: extent,
-                place,
-                offer_place,
-                // The question its parent measured it by, asked again: the
-                // same widget in the same place, however this draw came
-                // about.
-                offer: place == offer_place,
+                placed: place,
+                asked: place,
+                narrow,
+                re_asked,
                 px,
             },
             None,
             self.rsc,
         );
-        let compose = |holds| in_parent(holds, local, extent, place, declared);
-        self.under = self.under.and(compose(holds));
+        let holds = self.in_parent(holds, extent, place, narrow, declared);
+        let answer_holds = self.in_parent(answer_holds, extent, place, narrow, declared);
+        match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
+            Some((_, kept)) => *kept = holds,
+            None => self.under.push((id.id(), holds)),
+        }
         DrawResult {
             child: id,
             painter: self,
-            size: in_parent_frame(size, local.size(), declared),
-            answer_holds: compose(answer_holds),
+            size,
+            answer_holds,
         }
     }
 
@@ -248,9 +220,37 @@ impl<'a> Painter<'a> {
     /// this frame; what it answered is still something this widget asked.
     pub fn undraw<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
         self.children.retain(|child| *child != id.id());
+        self.under.retain(|(child, _)| *child != id.id());
         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,
+            frame: self.frame,
+            window: self.window,
+            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
@@ -259,17 +259,24 @@ impl<'a> Painter<'a> {
         declared_lens(self.rsc.widgets(), id.id())
     }
 
-    /// What a child says its length is without being drawn, if it can say.
-    /// Asking counts as reading its size.
+    /// What a child says its length is without being drawn, if it can say,
+    /// as the length its draw would report: a fraction in it is resolved
+    /// against this widget's frame, which is the frame a child asked with
+    /// nothing narrowed gets. Asking counts as reading its size.
     pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
         let widgets = self.rsc.widgets();
         // A rule is the answer where there is one: it wins over whatever the
         // widget would draw, so it has to win over what the widget says too.
-        let hint = widgets.size_rules(id.id()).axis(axis).exact().or_else(|| {
+        let hint = widgets
+            .size_rules(id.id())
+            .axis(axis)
+            .exact()
+            .or_else(|| {
                 widgets
                     .get_dyn(id.id())
                     .and_then(|widget| widget.size_hint(axis))
-        });
+            })
+            .map(|hint| hint.within_len(self.frame.axis(axis)));
         #[cfg(feature = "layout-diagnostics")]
         diag::hint_read(id.id(), self.id, axis, hint);
         match hint {
@@ -277,6 +284,11 @@ impl<'a> Painter<'a> {
                 #[cfg(feature = "layout-diagnostics")]
                 diag::bump(Counter::HintHits);
                 self.depend_on(id);
+                // A fraction was just resolved against this frame, so what
+                // this draw does with it is a function of the frame's length.
+                if hint.rel != Rel::ZERO {
+                    self.frame_own_len[axis as usize] = Some(self.frame.axis(axis));
+                }
                 Some(hint)
             }
             None => {
@@ -287,19 +299,6 @@ 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());
@@ -365,6 +364,17 @@ impl<'a> Painter<'a> {
         len
     }
 
+    /// The symbolic length of this widget's frame along one axis: what a
+    /// fraction it or anything under it declares is a fraction of. A
+    /// container reads it to hand a length of it down -- padding, which
+    /// takes its pixels off. Reading it pins the drawing to that frame, the
+    /// way [`Self::extent_len`] pins it to the box.
+    pub fn frame_len(&mut self, axis: Axis) -> Len {
+        let len = self.frame.axis(axis);
+        self.frame_own_len[axis as usize] = Some(len);
+        len
+    }
+
     /// 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.
@@ -400,7 +410,7 @@ impl<'a> Painter<'a> {
     /// [`Self::px_size`] when the other axis cannot affect the drawing.
     pub fn px_len(&mut self, axis: Axis) -> Px {
         let part = self.extent.axis(axis).len();
-        let len = part.to_px(self.px.axis(axis));
+        let len = part.to_px(self.window.axis(axis));
         let own = &mut self.extent_own[axis as usize];
         if *own == Holds::ANY {
             *own = Holds::at(len);
@@ -416,7 +426,7 @@ impl<'a> Painter<'a> {
         let part = self.extent.axis(axis).len();
         let holds = holds.into();
         debug_assert!(
-            holds.contains(part.to_px(self.px.axis(axis))),
+            holds.contains(part.to_px(self.window.axis(axis))),
             "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
             self.label(),
             self.id
@@ -424,30 +434,34 @@ impl<'a> Painter<'a> {
         self.extent_own[axis as usize] = holds;
     }
 
-    /// One axis of this widget's frame 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 frame_px_len(&mut self, axis: Axis) -> Px {
-        let len = self.px.axis(axis);
-        let own = &mut self.own[axis as usize];
+    /// A window length in pixels, which is what every length in layout is
+    /// measured in. Reading one pins the drawing to this window wherever the
+    /// length is a fraction of it; one that is only pixels is that many
+    /// pixels in any window and pins nothing.
+    pub fn to_px(&mut self, len: Len, axis: Axis) -> Px {
+        let window = self.window.axis(axis);
+        if len.rel != Rel::ZERO {
+            let own = &mut self.window_own[axis as usize];
             if *own == Holds::ANY {
-            *own = Holds::at(len);
+                *own = Holds::at(window);
             }
-        len
+        }
+        len.to_px(window)
     }
 
-    /// [`Self::holds`] stated about the frame rather than about this
-    /// widget's own box, for a container whose drawing turns on what its
-    /// fractions are of rather than on the part of it it took.
-    pub fn frame_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+    /// The windows this drawing holds for, stated rather than taken: a
+    /// container that branched on a length in pixels says which side of the
+    /// boundary it was on, which is wider than the one window reading that
+    /// length pins, and replaces it.
+    pub fn window_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 frame",
+            holds.contains(self.window.axis(axis)),
+            "'{}' ({:?}) says its drawing holds for windows that leave out this one",
             self.label(),
             self.id
         );
-        self.own[axis as usize] = holds;
+        self.window_own[axis as usize] = holds;
     }
 
     pub fn text_data(&mut self) -> &mut TextData {
@@ -535,70 +549,86 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
-/// What a child depends on, said about the boxes the widget that drew it
-/// has rather than the ones the child was given.
-///
-/// `frame` is the child's frame in this widget's frame coordinates and
-/// `extent` the box it was given, in the child's own frame coordinates. Both
-/// 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.
-pub(crate) fn in_parent(
+/// Moves what a child depends on into this widget's own terms: this
+/// method's `impl` block is where a `Painter`'s own boxes are, so it takes
+/// only what the child was asked with.
+impl Painter<'_> {
+    /// Window ranges are already about the one unit and combine directly.
+    /// A frame pin becomes this widget's own frame wherever a length of it
+    /// is what reached the child; where only pixels did, no length of this
+    /// frame can change the child's and the pin stops here.
+    ///
+    /// Extent validity maps back through the part of this widget's box,
+    /// where the box the child was asked in is that part; a declared length
+    /// places the box inside the part instead, and then only that length
+    /// reaches the child. A narrowed frame is not one of these: it decides
+    /// what fractions under the child mean and leaves the box the part it
+    /// was given.
+    fn in_parent(
+        &self,
         holds: LayoutHolds,
-    frame: UiRegion,
         extent: UiRegion,
         place: [Place; 2],
+        narrow: [Option<Len>; 2],
         declared: [Option<LayoutLen>; 2],
-) -> LayoutHolds {
+    ) -> 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]) {
-            // 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) => {
+            // Every read became pixels against the window, so a range on
+            // it is already in this widget's terms.
+            result.window[n] = holds.window[n];
+            let reaches = narrow[n].is_none()
+                && !matches!(place[n].part(), Part::Sized(_))
+                && declared[n].is_none_or(|len| len.rel != Rel::ZERO);
+            result.frame_len[n] = holds.frame_len[n].and(reaches.then(|| self.frame.axis(axis)));
+            match (place[n].part(), declared[n].is_some()) {
+                // 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, false) => {
                     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.
-            (Part::Of(span), None) => {
-                result.extent[n] = holds.extent[n].through(span.len());
+                // Its box is this widget's own less the inset. Where that is
+                // pixels, its box is exactly that many shorter in any window,
+                // so what it holds for is a range on this widget's box moved
+                // by them, and a length it pinned is this widget's length
+                // less them. An inset with a fraction in it is a different
+                // number of pixels in each window, and taking it off a length
+                // rounds once more than taking it off pixels does: there the
+                // child's box is a fixed expression of this one, so this
+                // widget's length is pinned and the range goes on the window
+                // through the child's box, the way a slot's does.
+                (Part::Inset { lead, trail }, false) => {
+                    let inset = lead + trail;
+                    match inset.rel == Rel::ZERO {
+                        true => {
+                            result.extent[n] = holds.extent[n].longer_by(inset.px);
+                            result.extent_len[n] = holds.extent_len[n].map(|pinned| pinned + inset);
                         }
-            // Its box is a part of this widget's frame: a length of the
-            // frame is all that reaches it, so what it holds for is a range
-            // on the frame and none of it on this widget's own box.
+                        false => {
+                            result.window[n] = result.window[n]
+                                .and(holds.extent[n].through(extent.axis(axis).len()));
+                            result.extent_len[n] = Some(self.extent.axis(axis).len());
+                        }
+                    }
+                }
+                // Its box is a length this widget decided, from its own
+                // frame or from a sibling's answer: no length of this
+                // widget's box reaches it, so what it holds for is a range
+                // on the window and none of it on that box.
                 _ => {
-                result.frame[n] = result.frame[n].and(
-                    holds.extent[n]
-                        .through(extent.axis(axis).len())
-                        .through(frame_len),
-                );
+                    result.window[n] =
+                        result.window[n].and(holds.extent[n].through(extent.axis(axis).len()));
                 }
             }
         }
         result
-}
-
-/// 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(of.axis(axis));
     }
-    }
-    size
 }
 
 /// What a widget declares a length of its box to be. `leftover` is not one: a
@@ -663,51 +693,58 @@ pub(crate) fn placed_extent(
     placed
 }
 
-/// The part of a widget's extent a `place` names, in the coordinates its
-/// extent is in: a span is measured in frame lengths from where the extent
-/// starts, so nothing under it depends on where that is, and an extent that
-/// moved re-places every child by re-adding its start.
-pub(crate) fn part_of(extent: UiRegion, place: [Place; 2]) -> UiRegion {
-    let mut part = extent;
-    for axis in AXES {
-        *part.axis_mut(axis) = place[axis as usize].part().of(*extent.axis(axis));
-    }
-    part
-}
-
-/// The frame a child is asked in and the box its drawing goes in, both in
-/// the coordinates of the widget asking.
+/// The frame length and the box a child is asked in, in the coordinates the
+/// widget asking draws in.
 ///
-/// `frame` is what the caller said the child's fractions are of, and `part`
-/// what of the caller's own box the drawing takes. `narrow` is the length a
-/// declared rule gives the frame, which makes the frame the box the drawing
-/// goes in -- a rule is what decided where it goes, and there is nothing
-/// left to place inside it. A caller that narrowed the frame itself said the
-/// same thing.
-///
-/// The length is the caller's to supply so that a widget asked again gets
-/// the frame it already has rather than a second resolution of its rule.
+/// `own` is that widget's own box, and `place` what of it the child is
+/// given. `narrow` is a frame the container decided for the child -- a row's
+/// slot, or padding's frame less its pixels -- and [`Part::Sized`] one a
+/// sibling's answer decided; both are window lengths, like every other
+/// length here, since a slot of a row is not a fraction of anything the row
+/// can name. The child's declaration is a fraction of whichever reached it,
+/// and is the only one of the three that also places the box: a box the
+/// caller decided is what `place` names.
 pub(crate) fn frame_and_extent(
-    mut frame: UiRegion,
-    part: UiRegion,
+    own: UiRegion,
+    parent_frame: UiVec2,
+    place: [Place; 2],
     narrow: [Option<Len>; 2],
+    declared: [Option<LayoutLen>; 2],
     align: RegionAlign,
-) -> (UiRegion, UiRegion) {
+) -> (UiVec2, UiRegion) {
+    let part = part_of(own, place, align);
+    let mut frame = parent_frame;
     let mut extent = part;
-    for (axis, narrow) in AXES.into_iter().zip(narrow) {
-        let span = frame.axis_mut(axis);
-        let narrowed = match narrow {
-            Some(len) => {
+    for axis in AXES {
+        let n = axis as usize;
+        let sized = match place[n].part() {
+            Part::Sized(len) => Some(len),
+            _ => None,
+        };
+        let base = sized
+            .or(narrow[n])
+            .unwrap_or_else(|| parent_frame.axis(axis));
+        let len = declared[n]
+            .map(|len| Len::from_parts(len.rel, len.px).within_len(base))
+            .unwrap_or(base);
+        *frame.axis_mut(axis) = len;
+        if declared[n].is_some() {
             let slot = part.axis(axis);
             let start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
-                *span = UiSpan::new(start, start + len);
-                true
-            }
-            None => *span != UiSpan::FULL,
-        };
-        if narrowed {
-            *extent.axis_mut(axis) = UiSpan::FULL;
+            *extent.axis_mut(axis) = UiSpan::new(start, start + len);
         }
     }
     (frame, extent)
 }
+
+/// The part of a widget's own box a `place` names, in the coordinates that
+/// box is in.
+fn part_of(extent: UiRegion, place: [Place; 2], align: RegionAlign) -> UiRegion {
+    let mut part = extent;
+    for axis in AXES {
+        *part.axis_mut(axis) = place[axis as usize]
+            .part()
+            .of(*extent.axis(axis), align.axis(axis));
+    }
+    part
+}

core/src/ui/place.rs

@@ -1,30 +1,43 @@
-use crate::{PrimitiveHandle, UiRegion, UiSpan};
+use crate::{AxisAlign, Len, PrimitiveHandle, UiRegion, UiSpan};
 
 /// What of a widget's own box a child is given, along one axis.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum Part {
     /// The whole of it.
     All,
-    /// Frame lengths from where the box starts, which is what a container
-    /// dividing room among its children speaks: a child's report is a length
-    /// of the frame, so the cursor that sums those reports is one too. A
-    /// moved box re-places every child by re-adding its start, exactly.
+    /// Window lengths from where the box starts, which is what a container
+    /// dividing room among its children speaks: a child's report is a window
+    /// length, so the cursor that sums those reports is one too. A moved box
+    /// re-places every child by re-adding its start, exactly. A fraction
+    /// here is a fraction of the window and not of the box -- the whole of a
+    /// box is [`Self::All`], not a `rel(1.0)` span.
     From(UiSpan),
-    /// A part of the box in its own coordinates, which is what a container
-    /// that insets one speaks: taking eleven pixels off the end needs no
-    /// length, where saying the same thing in frame lengths would make the
-    /// container read its own box -- and a box chosen from its own answer
-    /// then feeds back into the answer.
-    Of(UiSpan),
+    /// The box less a window length at each end, which is what a container
+    /// that insets one speaks -- padding, or a row asking a child in the
+    /// room left from its cursor. Neither end names the box's length, so a
+    /// container can say "from here to my end" without reading how long it
+    /// is, and a box chosen from its own answer does not feed back into the
+    /// answer.
+    Inset { lead: Len, trail: Len },
+    /// A box of this length, wherever in the parent's box the child's own
+    /// alignment puts it, and that same length as its frame. Unlike `From`,
+    /// it is a length decided from above rather than a place along a
+    /// container's cursor -- what a stack's sizing child decides for the
+    /// rest.
+    Sized(Len),
 }
 
 impl Part {
     /// Where it lands in the coordinates `extent` is in.
-    pub(crate) fn of(self, extent: UiSpan) -> UiSpan {
+    pub(crate) fn of(self, extent: UiSpan, align: AxisAlign) -> UiSpan {
         match self {
             Self::All => extent,
             Self::From(span) => UiSpan::new(extent.start + span.start, extent.start + span.end),
-            Self::Of(span) => span.within(&extent),
+            Self::Inset { lead, trail } => UiSpan::new(extent.start + lead, extent.end - trail),
+            Self::Sized(len) => {
+                let start = extent.start + (extent.len() - len).scale(align.rel());
+                UiSpan::new(start, start + len)
+            }
         }
     }
 }

src/random.rs

@@ -120,18 +120,32 @@ 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 len = measured.apply_leftover();
+        let px = painter.to_px(len, Axis::X);
+        // The range it actually branched on, said the way a container says
+        // one: pinning the window instead would redraw this widget on every
+        // resize, which is a fixture that never exercises reuse.
+        let threshold = Px::from_f32(self.threshold);
+        let holds = match px > threshold {
+            true => Holds::from(threshold + Px::STEP..=Px::MAX),
+            false => Holds::from(Px::MIN..=threshold),
+        };
+        painter.window_holds(Axis::X, holds.through(len));
 
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
         let place = [Place::Within(Part::All), below];
-        match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
-            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
+        match px > threshold {
+            true => painter.widget_at(&self.wide, [None; 2], place),
+            false => painter.widget_at(&self.narrow, [None; 2], place),
         };
         Size::LEFTOVER
     }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
 }
 
 pub struct Spanned {

src/widget/mask.rs

@@ -15,4 +15,8 @@ impl Widget for Masked {
         // draw, and the framework would place the drawing it clipped away.
         Size::LEFTOVER
     }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::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

@@ -14,25 +14,31 @@ impl Widget for Pad {
         // 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.
         //
-        // The padding goes around what it pads: the frame passes through, so
-        // the inner's fractions mean what they would without it, and only
-        // the box it draws in is moved in by the pixels.
-        // A part of this widget's own box, in that box's own lengths: the
-        // padding is pixels, which are the same pixels wherever the box
-        // lands, so nothing here reads how long the box is -- and a box
-        // chosen from this widget's own answer therefore does not feed back
-        // into that answer.
+        // Padding is an inset of both: it comes off the frame, so `rel(1)`
+        // under it fills this widget rather than overflowing it by the
+        // padding, and it comes off the box, so what is drawn sits inside.
+        // The two stay distinct -- the box can be narrower still, where a row
+        // asked this widget in the room left, and a text wraps at that.
         let inset = |lead: Px, trail: Px| {
-            Place::Within(Part::Of(UiSpan::new(
-                Len::from_parts(Rel::ZERO, lead),
-                Len::from_parts(Rel::ONE, -trail),
-            )))
+            Place::Within(Part::Inset {
+                lead: Len::from_parts(Rel::ZERO, lead),
+                trail: Len::from_parts(Rel::ZERO, trail),
+            })
         };
         let place = [
             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();
+        // Read from this widget's own frame rather than written as a
+        // fraction of it: a frame is a length of the window like everything
+        // else here, and taking the padding off is the whole of what this
+        // widget does to it.
+        let narrow = [
+            (Axis::X, self.padding.left + self.padding.right),
+            (Axis::Y, self.padding.top + self.padding.bottom),
+        ]
+        .map(|(axis, pixels)| Some(painter.frame_len(axis) - Len::from_parts(Rel::ZERO, pixels)));
+        let inner = painter.widget_at(&self.inner, narrow, place).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,

src/widget/position/scroll.rs

@@ -12,14 +12,13 @@ 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.
-        let whole = UiRegion::FULL;
+        // Asked in the whole viewport, then put at the scrolled offset.
         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 content = answer_len.apply_leftover();
+        let fixed = painter.to_px(Len::from_parts(answer_len.rel, answer_len.px), self.axis);
         self.container_len = container_len;
-        self.content_len = content.to_px(container_len);
+        self.content_len = fixed.max(container_len);
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
@@ -32,9 +31,9 @@ impl Widget for Scroll {
         // the drawing holds for that length alone. One scrolled part way sits
         // where it is until the box shrinks past what is left of it. Kept to
         // the end, it moves with every length.
-        let fixed_len = content.rel == Rel::ZERO;
+        let fixed_len = answer_len.rel == Rel::ZERO && answer_len.leftover == Weight::ZERO;
         if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
-            painter.holds(self.axis, self.content_len..=Px::MAX);
+            painter.holds(self.axis, fixed..=Px::MAX);
         } else if fixed_len && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, Px::MIN..=left);
@@ -45,7 +44,6 @@ impl Widget for Scroll {
         // have placed the whole scroll in a box longer than it.
         let slack = (self.container_len - self.content_len).max(Px::ZERO);
         let anchor = slack.mul(align.rel());
-        let mut content = UiSpan::FULL;
         // Content that fills the viewport and has not been scrolled is the
         // viewport, and is handed back as it came. Writing the same box as
         // its own length in pixels is the same box in another form, and the
@@ -53,19 +51,20 @@ impl Widget for Scroll {
         // one centred in `px 900`, since halving a difference is not halving
         // each part of it.
         let moved = anchor != Px::ZERO || self.amt != Px::ZERO;
-        if moved || self.content_len != self.container_len {
+        let content = match moved || self.content_len != self.container_len {
+            true => {
                 let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
-            content = UiSpan::new(start, start.offset(self.content_len));
+                Part::From(UiSpan::new(start, start.offset(self.content_len)))
             }
+            false => Part::All,
+        };
         // 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, scrolled.
-        painter.widget_at(
+        // content box its own answer decided. Where it goes is the content
+        // box, scrolled: its drawing moved there, not made again there.
+        painter.place_at(
             &self.inner,
-            whole,
-            self.axis
-                .pair(Place::Fill(Part::From(content)), Place::Fill(Part::All)),
+            self.axis.pair(Place::Fill(content), Place::Fill(Part::All)),
         );
         // 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
@@ -73,6 +72,10 @@ impl Widget for Scroll {
         // is.
         Size::LEFTOVER
     }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
 }
 
 impl Scroll {

src/widget/position/span.rs

@@ -10,30 +10,51 @@ 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 length of the frame its children
-        // are laid out against. Its start is nothing's business -- a slot is
-        // a length from it -- so what this reads is the length alone.
-        let far = painter.extent_len(axis);
-        let along = |from: Len, to: Len| match self.dir.sign {
-            Sign::Pos => UiSpan::new(from, to),
-            Sign::Neg => UiSpan::new(far - to, far - from),
+        // The room left from the cursor to the row's end, said without the
+        // row's length: a child measured in it does not make this drawing
+        // depend on how long the row is.
+        let room_from = |cursor: Len| match self.dir.sign {
+            Sign::Pos => Part::Inset {
+                lead: cursor,
+                trail: Len::ZERO,
+            },
+            Sign::Neg => Part::Inset {
+                lead: Len::ZERO,
+                trail: cursor,
+            },
         };
         // Across itself the child sits where its own alignment says, in the
         // whole of the row: a span is what contains its children there, and
         // nothing divides that axis.
         let across = Place::Within(Part::All);
-        // 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,
-        // because a text has to wrap at the width actually there.
+        // A length for every child before their final slots are chosen: from
+        // a hint where one says, and from drawing otherwise. 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 a drawn child is asked in is the room left from the
+        // cursor, because a text has to wrap at the width actually there.
+        // This is the one ask a drawn fixed child gets: its slot is its
+        // answer, and the drawing is moved there once the shares are known.
+        // A hinted child is asked once, in its slot.
         let mut cursor = Len::rel_min();
         let mut lens = Vec::with_capacity(self.children.len());
+        let mut measured = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let room = Place::Within(Part::From(along(cursor, far)));
-            let len = painter
-                .widget_at(child, UiRegion::FULL, axis.pair(room, across))
-                .len(axis);
+            let size = match painter.size_hint(child, axis) {
+                Some(len) => {
+                    measured.push(None);
+                    len
+                }
+                None => {
+                    let room = Place::Within(room_from(cursor));
+                    let size = painter
+                        .widget_at(child, [None; 2], axis.pair(room, across))
+                        .size();
+                    measured.push(Some(size));
+                    size.axis(axis)
+                }
+            };
+            let len = size;
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
             lens.push(len);
@@ -50,9 +71,25 @@ impl Widget for Span {
             |sum, len| sum + *len,
         );
 
+        // The row: this span's own box as a length of the window, read only
+        // where a slot depends on it -- shares divide what is left of it,
+        // and a negative row counts from its end. Reading it pins the
+        // drawing to this length; a positive row of fixed children is not
+        // pinned and holds for any length its children do. Its start is
+        // nothing's business: a slot is a length from it.
+        let far = (total.leftover > Weight::ZERO || self.dir.sign == Sign::Neg)
+            .then(|| painter.extent_len(axis));
+        let along = |from: Len, to: Len| match self.dir.sign {
+            Sign::Pos => UiSpan::new(from, to),
+            Sign::Neg => {
+                let far = far.expect("a negative row reads its length");
+                UiSpan::new(far - to, far - from)
+            }
+        };
         // What is left for the shares to divide: the row less everything
         // fixed, as a length of the frame rather than a number of pixels.
-        let room = far - Len::from_parts(total.rel, total.px);
+        // Nothing where there are no shares, and nothing reads it there.
+        let room = far.map_or(Len::ZERO, |far| 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
@@ -64,12 +101,12 @@ impl Widget for Span {
         // exist at all turns on this.
         let mut shares = false;
         if total.leftover > Weight::ZERO {
-            shares = room.to_px(painter.frame_px_len(axis)) > Px::ZERO;
+            shares = painter.to_px(room, axis) > Px::ZERO;
             let holds = match shares {
                 true => Holds::from(Px::STEP..=Px::MAX),
                 false => Holds::from(Px::MIN..=Px::ZERO),
             };
-            painter.frame_holds(axis, holds.through(room));
+            painter.window_holds(axis, holds.through(room));
         }
 
         // Across itself a span is as long as its longest child -- unless a
@@ -88,7 +125,8 @@ 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, len), measured) in self.children.iter().zip(&lens).zip(&measured) {
+            let len = *len;
             // 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.
@@ -106,12 +144,26 @@ 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
-            // answer inside again.
-            let slot = Place::Fill(Part::From(along(from, start)));
-            let placed = painter.widget_at(child, UiRegion::FULL, axis.pair(slot, across));
+            // is the child's box outright rather than something to place an
+            // answer inside again. A share is decided here and nowhere
+            // else: its slot narrows its frame, and the child is asked in
+            // it, since a text wraps at the width it is actually given. A
+            // fixed child's slot is its own answer, so a drawing made in the
+            // room is put there as it is, and one not made yet is made here.
+            let slot = along(from, start);
+            let place = axis.pair(Place::Fill(Part::From(slot)), across);
+            let mut narrow = [None; 2];
+            if len.leftover > Weight::ZERO && shares {
+                narrow[axis as usize] = Some(slot.len());
+            }
+            let used = match (measured, narrow[axis as usize]) {
+                (Some(size), None) => {
+                    painter.place_at(child, place);
+                    size.axis(!axis)
+                }
+                _ => painter.widget_at(child, narrow, place).len(!axis),
+            };
             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

@@ -23,22 +23,41 @@ 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 place = [Axis::X, Axis::Y].map(|axis| {
+            let len = size.axis(axis);
+            match len.leftover == Weight::ZERO {
+                true => Place::Fill(Part::Sized(Len::from_parts(len.rel, len.px))),
+                false => Place::Within(Part::All),
+            }
+        });
         for (i, child) in self.children.iter().enumerate() {
             if sizing == Some(i) {
                 continue;
             }
             painter.child_layer_at(i);
-            // Every other child is drawn in the stack's own box, and where it
-            // sits in one bigger than itself is its own business.
-            painter.widget(child);
+            painter.widget_at(child, [None; 2], place);
         }
         size
     }
+
+    /// Without a sizing child a stack is whatever box it is given, which it
+    /// can say without drawing anything.
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        match self.size {
+            StackSize::Default => Some(LayoutLen::LEFTOVER),
+            StackSize::Child(_) => None,
+        }
+    }
 }
 
 #[derive(Default, Debug)]

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 px = painter.to_px(measured.apply_leftover(), Axis::X);
 
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
         let place = [Place::Within(Part::All), below];
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
-            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
+            true => painter.widget_at(&self.wide, [None; 2], place),
+            false => painter.widget_at(&self.narrow, [None; 2], place),
         };
         Size::LEFTOVER
     }

tests/cases/layout.rs

@@ -83,10 +83,8 @@ fn a_text_in_a_span_wraps_at_the_room_left_rather_than_the_whole_row() {
     assert!(crowded > whole_row, "{crowded} against {whole_row}");
 }
 
-/// The same reading through a pad: padding goes around what it pads and
-/// does not narrow what a fraction under it is a fraction of, so half of the
-/// window plus the padding is what the pad takes and where the next child
-/// starts.
+/// Padding is an inset: it narrows the frame a fraction resolves against and
+/// adds itself back to the padded widget's reported length.
 #[test]
 fn a_pad_puts_its_padding_around_a_fraction_of_the_whole_box() {
     let mut h = Harness::new((400, 100));
@@ -97,9 +95,83 @@ fn a_pad_puts_its_padding_around_a_fraction_of_the_whole_box() {
     // placed inside it by its own alignment, which is not what is under test.
     h.set_root((padded, tail).span(Dir::RIGHT).width(rel(1.0)));
 
-    assert_corners!(h, inner, (10, 10), (210, 90));
-    assert_corners!(h, padded, (0, 0), (220, 100));
-    assert_corners!(h, tail, (220, 0), (320, 100));
+    assert_corners!(h, inner, (10, 10), (200, 90));
+    assert_corners!(h, padded, (0, 0), (210, 100));
+    assert_corners!(h, tail, (210, 0), (310, 100));
+}
+
+const PARAGRAPH: &str = "Wrapping shapes one source into as many lines as the box \
+    leaves room for, so a paragraph's height is an answer and not a setting.";
+
+/// The worked example of what padding insets: in a 900 px row after a 24 px
+/// icon, a `rel(1.0)` inside `pad(16)` is 900 - 32 and overflows the row by
+/// the icon's width, while a wrapping text beside it is asked in the room
+/// left, 900 - 24 - 32, and wraps there.
+#[test]
+fn padding_keeps_the_frame_distinct_from_the_room_left_in_a_row() {
+    let mut h = Harness::new((900, 200));
+    let icon = rect(Color::RED).width(24).add(&mut h.rsc);
+    let fill = rect(Color::GREEN).width(rel(1.0)).add(&mut h.rsc);
+    let padded = fill.pad(16).add(&mut h.rsc);
+    h.set_root((icon, padded).span(Dir::RIGHT).width(rel(1.0)));
+    let fill_width = h.region(&fill).unwrap().size().x;
+    assert_eq!(fill_width, Px::from_int(868));
+
+    let mut h = Harness::new((900, 200));
+    let icon = rect(Color::RED).width(24).add(&mut h.rsc);
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let padded = text.pad(16).add(&mut h.rsc);
+    h.set_root((icon, padded).span(Dir::RIGHT).width(rel(1.0)));
+    let active = &h.render.active[&text.id()];
+    let window = h.render.output_size().x;
+    let asked = active.part.x.len().to_px(window);
+    assert_eq!(active.frame.x.to_px(window), Px::from_int(868));
+    assert_eq!(asked, Px::from_int(844));
+}
+
+/// The other way round: a share inside padding. A slot is a length of the
+/// row, which is already the padded width, so what the span decided reaches
+/// the child as it stands -- taking the padding off a second time would make
+/// `rel(1.0)` in the slot shorter than the slot.
+#[test]
+fn a_share_inside_padding_fills_the_slot_it_was_given() {
+    let mut h = Harness::new((900, 200));
+    let fill = rect(Color::GREEN).width(rel(1.0)).add(&mut h.rsc);
+    let first = Span {
+        children: vec![fill.add_strong(&mut h.rsc)],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .width(leftover(1))
+    .add(&mut h.rsc);
+    let second = rect(Color::BLUE).width(leftover(1)).add(&mut h.rsc);
+    let row = (first, second).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root(row.pad(16));
+
+    assert_eq!(h.region(&first).unwrap().size().x, Px::from_int(434));
+    assert_eq!(h.region(&fill).unwrap().size().x, Px::from_int(434));
+}
+
+/// The same padding in a share instead: the slot is 450, so both the
+/// fraction and the wrap are the slot less the padding, and the two agree.
+#[test]
+fn padding_narrows_both_frame_and_box_inside_a_share() {
+    let mut h = Harness::new((900, 200));
+    let fill = rect(Color::GREEN).width(rel(1.0)).add(&mut h.rsc);
+    let padded = fill.pad(16).width(leftover(1)).add(&mut h.rsc);
+    let other = rect(Color::BLUE).width(leftover(1)).add(&mut h.rsc);
+    h.set_root((padded, other).span(Dir::RIGHT).width(rel(1.0)));
+    assert_eq!(h.region(&fill).unwrap().size().x, Px::from_int(418));
+
+    let mut h = Harness::new((900, 200));
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let padded = text.pad(16).width(leftover(1)).add(&mut h.rsc);
+    let other = rect(Color::BLUE).width(leftover(1)).add(&mut h.rsc);
+    h.set_root((padded, other).span(Dir::RIGHT).width(rel(1.0)));
+    let active = &h.render.active[&text.id()];
+    let window = h.render.output_size().x;
+    assert_eq!(active.frame.x.to_px(window), Px::from_int(418));
+    assert_eq!(active.part.x.len().to_px(window), Px::from_int(418));
 }
 
 #[test]
@@ -428,8 +500,7 @@ fn a_row_of_equal_shares_fills_it_exactly() {
 /// a step of. Kept in step with `snap_floor` in `prelude.wgsl`.
 fn drawn_edges(h: &Harness, id: WidgetId, axis: Axis) -> (f32, f32) {
     let active = &h.render.active[&id];
-    let drawn = active.extent.within(&active.frame_abs);
-    let region = h.render.moves.resolve(active.parent_move, drawn);
+    let region = h.render.moves.resolve(active.move_idx, active.extent);
     let dim = h.size().axis(axis);
     let snap = |v: f32| (v + Px::STEP.to_f32() * 0.5).floor();
     let edge = |s: Len| snap(s.rel.to_f32() * dim + s.px.to_f32());
@@ -740,3 +811,13 @@ fn a_fixed_child_is_centered_in_its_wrappers_share() {
     assert_corners!(h, wrapper, (200, 0), (900, 400));
     assert_corners!(h, leaf, (500, 150), (600, 250));
 }
+
+/// The root's frame is the window and its rule is a fraction of that, which
+/// is one resolution and not two: nothing above it narrowed anything.
+#[test]
+fn a_root_with_a_fraction_rule_is_that_fraction_of_the_window() {
+    let mut h = Harness::new((900, 200));
+    let root = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
+    h.set_root(root);
+    assert_eq!(h.region(&root).unwrap().size().x, Px::from_int(450));
+}

tests/cases/retained.rs

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

tests/cases/scroll.rs

@@ -60,6 +60,69 @@ fn a_wheel_scrolls_the_content_and_stops_at_its_end() {
     assert_corners!(h, top, (0, 0), (400, 200));
 }
 
+#[test]
+fn fixed_content_and_a_share_fill_one_viewport() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED)
+        .width(LayoutLen {
+            px: Px::from_int(600),
+            rel: Rel::ZERO,
+            leftover: Weight::ONE,
+        })
+        .add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (0, 0), (900, 100));
+}
+
+#[test]
+fn fixed_content_wider_than_the_viewport_still_scrolls() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED).width(1200).add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (-300, 0), (900, 100));
+}
+
+#[test]
+fn a_lone_share_fills_without_scrolling() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED).width(LayoutLen::LEFTOVER).add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (0, 0), (900, 100));
+}
+
+#[test]
+fn wrapping_content_beside_a_fixed_length_is_stable_warm_and_cold() {
+    fn plant(h: &mut Harness) -> (WidgetId, WidgetId) {
+        let fixed = rect(Color::RED).width(600).add(&mut h.rsc);
+        let text = wtext("Wrapping shapes one source into as many lines as the box leaves room for, so a paragraph's height is an answer and not a setting.")
+            .size(16)
+            .wrap(true)
+            .width(LayoutLen::LEFTOVER)
+            .add(&mut h.rsc);
+        let content = (fixed, text).span(Dir::RIGHT).add(&mut h.rsc);
+        let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+        h.set_root(scroll);
+        (text.id(), content.id())
+    }
+
+    let mut warm = Harness::new((900, 300));
+    let (text, content) = plant(&mut warm);
+    warm.rsc.widgets_mut().get_dyn_mut(text);
+    warm.frame();
+
+    let mut cold = Harness::new((900, 300));
+    let (cold_text, cold_content) = plant(&mut cold);
+
+    assert_eq!(warm.region(&text), cold.region(&cold_text));
+    assert_eq!(warm.region(&content), cold.region(&cold_content));
+}
+
 /// A widget that clips to its box may not report more than the box: its
 /// parent would place the part it cut off, and the framework would put a
 /// drawing longer than its box somewhere. `Masked` is the second of these
@@ -82,3 +145,19 @@ fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
     h.set_root(clipper);
     h.frame();
 }
+
+/// Content that fits sits in the viewport, not in a box of the window's
+/// length anchored at the viewport's start. `Part::From` takes window
+/// lengths, so a `rel(1.0)` span in one is the window, and only a scroll
+/// filling the window would land right.
+#[test]
+fn content_that_fits_is_placed_in_the_viewport_and_not_in_the_window() {
+    let mut h = Harness::new((400, 400));
+    let head = rect(Color::RED).height(100).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).height(50).add(&mut h.rsc);
+    let scroll = Scroll::new(inner.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    h.set_root((head, scroll).span(Dir::DOWN));
+
+    assert_corners!(h, scroll, (0, 100), (400, 400));
+    assert_corners!(h, inner, (0, 225), (400, 275));
+}

tests/cases/unsettled.rs

@@ -14,6 +14,180 @@ use iris::harness::Harness;
 use iris::prelude::*;
 use iris::random::Branch;
 
+fn assert_same_regions(
+    warm: &Harness,
+    warm_ids: &[WidgetId],
+    cold: &Harness,
+    cold_ids: &[WidgetId],
+) {
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in warm_ids.iter().zip(cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Ten widgets, shrunk from seed 2 at depth 5. The stack is as tall as its
+/// first child, so its other children belong in that one-line box. A cold
+/// layout used to keep the span's answer from the larger measuring box while
+/// a repaint asked it in the stack's final box.
+fn plant_stack_in_its_sizing_childs_box(h: &mut Harness) -> Vec<WidgetId> {
+    let sizing = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let filler = rect(Color::CYAN.alpha(252)).add(&mut h.rsc);
+    let plain = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let span = (filler, plain).span(Dir::DOWN).add(&mut h.rsc);
+    let pad = Pad {
+        padding: Padding::ZERO,
+        inner: span.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    let probe = rect(Color::RED).add(&mut h.rsc);
+    let wide = rect(Color::YELLOW.alpha(252)).add(&mut h.rsc);
+    let narrow = rect(Color::RED).add(&mut h.rsc);
+    let branch = Branch {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 55.0,
+    }
+    .add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![
+            sizing.add_strong(&mut h.rsc),
+            pad.add_strong(&mut h.rsc),
+            branch.add_strong(&mut h.rsc),
+        ],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_size_rules(stack.id(), Some(LayoutLen::LEFTOVER), None);
+    h.set_root(stack);
+    vec![
+        sizing.id(),
+        filler.id(),
+        plain.id(),
+        span.id(),
+        pad.id(),
+        probe.id(),
+        wide.id(),
+        narrow.id(),
+        branch.id(),
+        stack.id(),
+    ]
+}
+
+#[test]
+fn repainting_a_stack_uses_the_box_its_sizing_child_decided() {
+    let mut warm = Harness::new((900, 1200));
+    let ids = plant_stack_in_its_sizing_childs_box(&mut warm);
+    for &id in &ids {
+        warm.rsc.widgets_mut().get_dyn_mut(id);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let cold_ids = plant_stack_in_its_sizing_childs_box(&mut cold);
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
+/// Ten widgets, shrunk from seed 108 at depth 5. The nested reverse spans
+/// evaluate the branch in successively narrower boxes. The answer from the
+/// final, decided box must be the one retained after every span is reordered.
+fn plant_branch_in_nested_reverse_spans(
+    h: &mut Harness,
+    reordered: bool,
+) -> (Vec<WidgetId>, [WeakWidget<Span>; 3]) {
+    let pair = |first: StrongWidget, second: StrongWidget| match reordered {
+        true => vec![second, first],
+        false => vec![first, second],
+    };
+    let probe = rect(Color::RED.alpha(63)).add(&mut h.rsc);
+    let wide = rect(Color::RED).add(&mut h.rsc);
+    let narrow = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let branch = Branch {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 483.0,
+    }
+    .add(&mut h.rsc);
+    let wrapped = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let down = Span {
+        children: pair(
+            branch.add_strong(&mut h.rsc),
+            wrapped.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::DOWN,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let inner_filler = rect(Color::CYAN.alpha(63)).add(&mut h.rsc);
+    let inner = Span {
+        children: pair(
+            down.add_strong(&mut h.rsc),
+            inner_filler.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::LEFT,
+        gap: Px::ZERO,
+    }
+    .height(LayoutLen::rel(1.0))
+    .add(&mut h.rsc);
+    let outer_filler = rect(Color::GREEN.alpha(63)).add(&mut h.rsc);
+    let outer = Span {
+        children: pair(
+            inner.add_strong(&mut h.rsc),
+            outer_filler.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::LEFT,
+        gap: Px::ZERO,
+    }
+    .height(LayoutLen::rel(1.0))
+    .add(&mut h.rsc);
+    h.set_root(outer);
+    (
+        vec![
+            probe.id(),
+            wide.id(),
+            narrow.id(),
+            branch.id(),
+            wrapped.id(),
+            down.id(),
+            inner_filler.id(),
+            inner.id(),
+            outer_filler.id(),
+            outer.id(),
+        ],
+        [down, inner, outer],
+    )
+}
+
+#[test]
+fn reordering_nested_spans_keeps_the_answer_from_the_decided_box() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, spans) = plant_branch_in_nested_reverse_spans(&mut warm, false);
+    for span in spans {
+        warm.rsc[span].children.rotate_left(1);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _) = plant_branch_in_nested_reverse_spans(&mut cold, true);
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
 /// Six widgets, shrunk from a 402-widget tree the fuzzer found. Nothing about
 /// the tree changes -- every widget is marked for redraw and the frame is
 /// taken again -- so no box may move, and a warm frame has to land where a
@@ -559,6 +733,51 @@ fn a_widget_under_a_region_node_is_asked_in_the_box_that_node_was_offered() {
 const PARAGRAPH: &str = "Wrapping shapes one source into as many lines as the \
     box leaves room for, so a paragraph's height is an answer and not a setting.";
 
+fn plant_stack_resized_from_free(h: &mut Harness, fixed: bool) -> (Vec<WidgetId>, WidgetId) {
+    let sizing = rect(Color::CYAN.alpha(126)).add(&mut h.rsc);
+    h.rsc.widgets_mut().set_size_rules(sizing.id(), None, None);
+    if fixed {
+        h.rsc.widgets_mut().set_size_rules(
+            sizing.id(),
+            Some(LayoutLen::px(112)),
+            Some(LayoutLen::px(101)),
+        );
+    }
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let pad = Pad {
+        padding: Padding::ZERO,
+        inner: text.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![sizing.add_strong(&mut h.rsc), pad.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    h.set_root(stack);
+    (
+        vec![sizing.id(), text.id(), pad.id(), stack.id()],
+        sizing.id(),
+    )
+}
+
+#[test]
+fn fixing_a_stacks_sizing_child_repositions_its_overlay() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, sizing) = plant_stack_resized_from_free(&mut warm, false);
+    warm.frame();
+    warm.rsc.widgets_mut().set_size_rules(
+        sizing,
+        Some(LayoutLen::px(112)),
+        Some(LayoutLen::px(101)),
+    );
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _) = plant_stack_resized_from_free(&mut cold, true);
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
 /// Eight widgets, shrunk from a 118-widget tree (seed 1121, depth 4,
 /// `shuffle-swap-for-three`). The stack takes its size from the span above,
 /// the span takes its width from the longest line of the texts in it, and
@@ -672,3 +891,179 @@ 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);
+}
+
+/// Six widgets, shrunk from seed 59 at depth 5 (`resize-size`). The column
+/// divides the box it is given between two shares, so its drawing holds for
+/// that box's length alone, and the pads above it pass that dependency up:
+/// each one's box is a part of the box it was asked in. Padding narrowing
+/// the frame it hands down does not change that, and while it was taken to,
+/// changing the rule over the pads relocated the column's drawing into the
+/// new box instead of dividing it again.
+fn plant_two_shares_under_two_pads(h: &mut Harness, height: f32) -> Vec<WidgetId> {
+    let top = rect(Color::CYAN.alpha(126)).add(&mut h.rsc);
+    let bottom = rect(Color::RED).add(&mut h.rsc);
+    let column = (top, bottom).span(Dir::DOWN).add(&mut h.rsc);
+    let inner = Pad {
+        padding: Padding::ZERO,
+        inner: column.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    let outer = Pad {
+        padding: Padding::ZERO,
+        inner: inner.add_strong(&mut h.rsc),
+    }
+    .height(height)
+    .add(&mut h.rsc);
+    let beside = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((outer, beside).span(Dir::RIGHT));
+    vec![
+        top.id(),
+        bottom.id(),
+        column.id(),
+        inner.id(),
+        outer.id(),
+        beside.id(),
+    ]
+}
+
+#[test]
+fn changing_a_rule_over_two_pads_divides_the_column_again() {
+    let mut warm = Harness::new((900, 1200));
+    let ids = plant_two_shares_under_two_pads(&mut warm, 88.0);
+    warm.frame();
+    warm.rsc
+        .widgets_mut()
+        .set_size_rules(ids[4], None, Some(LayoutLen::px(105)));
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let cold_ids = plant_two_shares_under_two_pads(&mut cold, 105.0);
+    cold.frame();
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
+/// Six widgets, shrunk from seed 942 at depth 6 (`resize`). A `Branch` asks
+/// its probe in the top 40 px of its box and forwards the frame, so the
+/// scroll's own box is 40 px tall whatever the window is -- but its content
+/// is as tall as the frame, which is the window, and a scroll kept to its
+/// end has to be told when that changes. Resolving a length against the
+/// window is what reads it, so that is where the dependency is taken.
+fn plant_a_window_tall_column_in_a_short_scroll(h: &mut Harness) -> Vec<WidgetId> {
+    let leaf = rect(Color::RED).add(&mut h.rsc);
+    let column = Span {
+        children: vec![leaf.add_strong(&mut h.rsc)],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .height(rel(1.0))
+    .add(&mut h.rsc);
+    let scroll = Scroll::new(column.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    let wide = rect(Color::BLUE).add(&mut h.rsc);
+    let narrow = rect(Color::GREEN).add(&mut h.rsc);
+    let root = Branch {
+        probe: scroll.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 55.0,
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+    vec![leaf.id(), column.id(), scroll.id(), root.id()]
+}
+
+#[test]
+fn resizing_under_a_short_scroll_snaps_its_window_tall_content_again() {
+    let mut warm = Harness::new((1920, 1200));
+    let ids = plant_a_window_tall_column_in_a_short_scroll(&mut warm);
+    warm.frame();
+    warm.resize((640, 900));
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let cold_ids = plant_a_window_tall_column_in_a_short_scroll(&mut cold);
+    cold.frame();
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}

tests/layout_dump.rs

@@ -0,0 +1,42 @@
+//! Prints where a cold layout puts every widget of many grown trees, so two
+//! commits can be compared on cold layout alone. The warm/cold oracle cannot
+//! see a change that moves cold layout, since both of its sides move; this
+//! can, by diffing its output across the change:
+//!
+//!     IRIS_DUMP_SEEDS=400 IRIS_DUMP_DEPTH=5 cargo test --release \
+//!       --test layout_dump -- --ignored --nocapture > /tmp/before.txt
+//!
+//! then the same after, and `diff` the two. A line is one widget: the seed,
+//! its index in creation order, and its box in window pixels, or `-` where
+//! it is not drawn.
+
+use iris::harness::Harness;
+use iris::random::{Edits, grow};
+
+fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|value| value.parse().ok())
+        .unwrap_or(fallback)
+}
+
+#[test]
+#[ignore = "a dump to diff across commits, not a check"]
+fn every_cold_layout_is_printed() {
+    let seeds = env("IRIS_DUMP_SEEDS", 400_u64);
+    let depth = env("IRIS_DUMP_DEPTH", 5_usize);
+    let mut out = String::new();
+    for seed in 1..=seeds {
+        let mut harness = Harness::new((1920.0, 1200.0));
+        let (root, tree) = grow(&mut harness.rsc, seed, depth, &Edits::default());
+        harness.state.root = Some(root);
+        harness.frame();
+        for (index, id) in tree.ids.iter().enumerate() {
+            match harness.region(id) {
+                Some(region) => out.push_str(&format!("{seed} {index} {region:?}\n")),
+                None => out.push_str(&format!("{seed} {index} -\n")),
+            }
+        }
+    }
+    print!("{out}");
+}

tests/scenario/mod.rs

@@ -103,6 +103,11 @@ pub enum Case {
     /// A resize and then a size change, so a retained answer is asked to
     /// survive two different kinds of invalidation in a row.
     ResizeSize,
+    /// A size change and then a resize, which is the other order and not the
+    /// same test: a length answered as a fraction of one box and kept as a
+    /// fraction of another agrees at the size it was changed at and parts
+    /// from it at every other one.
+    SizeResize,
     /// A few declared sizes.
     Size,
     /// Every declared size at once, so every reader of a size has a changed
@@ -119,12 +124,13 @@ pub enum Case {
     Shuffle(Shuffle),
 }
 
-pub const ALL: [Case; 15] = [
+pub const ALL: [Case; 16] = [
     Case::Repaint,
     Case::RepaintSome,
     Case::Resize,
     Case::ResizeRepaint,
     Case::ResizeSize,
+    Case::SizeResize,
     Case::Size,
     Case::EverySize,
     Case::Align,
@@ -146,6 +152,7 @@ impl Case {
             Self::Resize => "resize",
             Self::ResizeRepaint => "resize-repaint",
             Self::ResizeSize => "resize-size",
+            Self::SizeResize => "size-resize",
             Self::Size => "size",
             Self::EverySize => "every-size",
             Self::Align => "align",
@@ -170,6 +177,15 @@ impl Case {
             _ => (STILL, STILL),
         }
     }
+
+    /// The window the warm tree is taken to after the change, where the case
+    /// is about what the change left behind rather than about the change.
+    fn then_resize(self) -> Option<(f32, f32)> {
+        match self {
+            Self::SizeResize => Some(INNER),
+            _ => None,
+        }
+    }
 }
 
 fn mark(warm: &mut Harness, tree: &Tree, step: usize) {
@@ -284,7 +300,7 @@ fn change(case: Case, warm: &mut Harness, tree: &mut Tree, plan: &Plan, rng: &mu
             warm.frame();
             return out;
         }
-        Case::Size | Case::ResizeSize => Edits {
+        Case::Size | Case::ResizeSize | Case::SizeResize => Edits {
             sizes: some_sizes(warm, tree, rng),
             ..Default::default()
         },
@@ -384,6 +400,17 @@ fn describe_widget(id: WidgetId, h: &Harness) -> String {
     label
 }
 
+/// One widget's layout as it stands: the frame its fractions resolved
+/// against, the box it was asked in, the box its drawing went in, and what
+/// it reported. In window units, which is what both trees are in.
+fn record(id: WidgetId, h: &Harness) -> String {
+    let active = &h.render.active[&id];
+    format!(
+        "frame {} ask {} box {} size {}",
+        active.frame, active.part, active.extent, active.size,
+    )
+}
+
 /// Runs `case` on the tree `plan` describes, warm and cold, and says where
 /// the two disagree. `seed` chooses only the values a case picks at random,
 /// so one plan under one case is one comparison however it was reached.
@@ -400,12 +427,29 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
         warm.frame();
     }
     let cold_plan = change(case, &mut warm, &mut tree, plan, &mut Rng::new(seed));
+    // Whatever the change left, seen at another window: an answer kept as a
+    // fraction of the wrong length is the same number of pixels where it was
+    // made and a different one everywhere else.
+    let end = match case.then_resize() {
+        Some(after) => {
+            warm.resize(after);
+            warm.frame();
+            after
+        }
+        None => end,
+    };
 
     let mut cold = Harness::new(end);
     let (root, cold_tree) = build(&mut cold.rsc, &cold_plan);
     cold.state.root = Some(root);
     cold.frame();
 
+    let places: HashMap<WidgetId, usize> = tree
+        .ids
+        .iter()
+        .enumerate()
+        .map(|(i, &id)| (id, i))
+        .collect();
     let mut drawn = 0;
     for (i, (&w, &c)) in tree.ids.iter().zip(&cold_tree.ids).enumerate() {
         let (got, want) = (warm.region(&w), cold.region(&c));
@@ -416,6 +460,7 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
         // Where two trees disagree is rarely where the cause is, so the
         // ancestry comes with it, marking the widgets that own a region.
         let mut chain = Vec::new();
+        let mut records = Vec::new();
         let mut at = Some(w);
         while let Some(id) = at {
             let active = &warm.render.active[&id];
@@ -424,11 +469,22 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
                 false => "*",
             };
             chain.push(format!("{}{node}", describe(id, &warm)));
+            // What each level was asked in on both sides, since the level
+            // where the two stop agreeing is the one to look at rather than
+            // the leaf that reported the difference.
+            let cold_id = places.get(&id).and_then(|&i| cold_tree.ids.get(i));
+            records.push(format!(
+                "    {}\n      warm {}\n      cold {}",
+                describe(id, &warm),
+                record(id, &warm),
+                cold_id.map_or("-".into(), |&id| record(id, &cold)),
+            ));
             at = active.parent;
         }
         return Some(format!(
-            "widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}",
-            chain.join(" < ")
+            "widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}\n{}",
+            chain.join(" < "),
+            records.join("\n"),
         ));
     }
     match drawn {

tests/shrink.rs

@@ -70,10 +70,14 @@ fn no_grown_tree_lays_out_differently_warm_than_cold() {
     over_seeds(seeds, |seed| {
         let grown = plan(seed, depth, &Edits::default());
         for &case in &cases {
-            let Some(how) = diverges(&grown, case, seed) else {
+            if diverges(&grown, case, seed).is_none() {
                 continue;
-            };
+            }
             let small = shrink(grown.clone(), case, seed);
+            // Described from the shrunk tree: the grown tree's chain names
+            // widgets that are no longer there, and the ancestry of the
+            // failure is what a test is written from.
+            let how = diverges(&small, case, seed).unwrap_or_default();
             println!(
                 "seed {seed} case {}: {how}\ngrown {} widgets, shrank to {}\n{small:#?}",
                 case.name(),