Take a span child's length from its hint, and ask it once in its slot

A share child was drawn in the measuring room and again in its slot, and
one record holding two questions made every local change under it defer
to the span. Where a rule or a hint gives the length along the span, the
first ask answers nothing the rule does not, so the child is asked once,
in its slot; the widgets that always report the whole of their box now
say so. A hint with a fraction resolves against the frame and pins it.

The dump rig prints every cold layout so a change to it shows in a diff.

core/src/layout_diagnostics.rs

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

core/src/ui/active.rs

@@ -1,6 +1,6 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive, Size,
-    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. 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.
@@ -77,20 +82,10 @@ pub struct ActiveData {
 }
 
 impl ActiveData {
-    /// What it answered when its parent measured it, where it has been
-    /// measured at all. Not `size`, which is what its last drawing reported:
-    /// a drawing made in the box that answer chose is answering a different
-    /// question.
+    /// 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)
     }
-
-    /// 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))
-    }
 }

core/src/ui/layout_holds.rs

@@ -1,27 +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 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.
+/// 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],
     };
@@ -29,34 +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];
 
@@ -17,30 +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, or 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],
-    /// Symbolic box lengths read only to compute the answer. A container can
-    /// replace the provisional drawings used for that answer with drawings
-    /// in decided boxes, so this contract is independent of the final one.
-    pub(super) answer_extent_len: [Option<Len>; 2],
-    /// The frame in pixels, which its children's frames are a length of:
-    /// threaded down rather than composed back up the chain, so every length
-    /// in layout is one multiply from its parent's and [`Holds::through`]
-    /// 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>,
@@ -48,25 +38,18 @@ 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) frame_own: [Holds; 2],
-    /// The same for its own box.
+    /// 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],
-    /// Pixel-box dependencies used only to compute the answer. These do not
-    /// constrain a retained drawing placed inside that answer.
-    pub(super) answer_extent_own: [Holds; 2],
-    /// The final drawing kept for each child. Asking one child again replaces
-    /// its provisional drawing and therefore replaces this contract too.
+    /// 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.
@@ -90,12 +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>(
@@ -159,63 +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, 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 (local, extent) = frame_and_extent(
-            frame,
-            part_of(self.extent, place),
-            narrowed_by(declared, frame),
-            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 {
@@ -225,18 +190,19 @@ 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,
+                placed: place,
+                asked: place,
+                narrow,
+                re_asked,
                 px,
             },
             None,
             self.rsc,
         );
-        let compose = |holds| in_parent(holds, local, extent, place, declared);
-        let holds = 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)),
@@ -244,8 +210,8 @@ impl<'a> Painter<'a> {
         DrawResult {
             child: id,
             painter: self,
-            size: in_parent_frame(size, local.size(), declared),
-            answer_holds: compose(answer_holds),
+            size,
+            answer_holds,
         }
     }
 
@@ -258,6 +224,33 @@ impl<'a> Painter<'a> {
         self.state.undraw_rec(id.id(), self.rsc);
     }
 
+    /// Puts a child asked about in this draw somewhere else in this
+    /// widget's box: its answer, placed in this part instead. The drawing
+    /// is re-expressed there rather than made again -- what a row does once
+    /// it knows every slot, having measured each child from its cursor.
+    pub fn place_at<W: ?Sized>(&mut self, id: &StrongWidget<W>, place: [Place; 2]) {
+        debug_assert!(
+            self.children.contains(&id.id()),
+            "'{}' placed a child it did not ask about in this draw",
+            self.label()
+        );
+        let at = self.placing();
+        self.state.place_in(id.id(), &at, place, self.rsc);
+    }
+
+    /// This widget as the thing its children are placed within.
+    fn placing(&self) -> Placing {
+        Placing {
+            id: self.id,
+            extent: self.extent,
+            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
@@ -266,17 +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 {
@@ -284,6 +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 => {
@@ -294,19 +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());
@@ -369,31 +361,20 @@ impl<'a> Painter<'a> {
     pub fn extent_len(&mut self, axis: Axis) -> Len {
         let len = self.extent.axis(axis).len();
         self.extent_len[axis as usize] = Some(len);
-        self.answer_extent_len[axis as usize] = Some(len);
         len
     }
 
-    /// The symbolic length used to compute this widget's answer, where the
-    /// final drawing itself is rebuilt without depending on that length.
-    pub fn answer_extent_len(&mut self, axis: Axis) -> Len {
-        let len = self.extent.axis(axis).len();
-        self.answer_extent_len[axis as usize] = Some(len);
+    /// 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
     }
 
-    /// Says that the final drawing uses a symbolic length already read for
-    /// the answer.
-    pub fn drawing_uses_extent_len(&mut self, axis: Axis, len: Len) {
-        debug_assert_eq!(self.extent.axis(axis).len(), len);
-        self.extent_len[axis as usize] = Some(len);
-    }
-
-    /// A part of this widget's box, expressed in its frame coordinates so it
-    /// can be used as a child frame decided here.
-    pub fn extent_part(&self, axis: Axis, part: Part) -> UiSpan {
-        part.of(*self.extent.axis(axis))
-    }
-
     /// Where this widget sits in a box longer than the length it takes. A
     /// widget that positions its own content reads it to place that content
     /// the way the box around it would have placed the widget.
@@ -425,19 +406,11 @@ impl<'a> Painter<'a> {
         PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
     }
 
-    /// This widget's own box in pixels, used only to compute its answer.
-    pub fn answer_px_size(&mut self) -> PxVec2 {
-        PxVec2::new(
-            self.answer_px_len(Axis::X),
-            self.answer_px_len(Axis::Y),
-        )
-    }
-
     /// One axis of this widget's own box in pixels. Prefer this to
     /// [`Self::px_size`] when the other axis cannot affect the drawing.
     pub fn px_len(&mut self, axis: Axis) -> Px {
         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);
@@ -445,17 +418,6 @@ impl<'a> Painter<'a> {
         len
     }
 
-    /// One pixel length used only to compute this widget's answer. The final
-    /// drawing may be retained when that answer is placed in another box.
-    pub fn answer_px_len(&mut self, axis: Axis) -> Px {
-        let len = self.extent.axis(axis).len().to_px(self.px.axis(axis));
-        let own = &mut self.answer_extent_own[axis as usize];
-        if *own == Holds::ANY {
-            *own = Holds::at(len);
-        }
-        len
-    }
-
     /// The lengths of this widget's own box on `axis` that what it is drawing
     /// holds for -- the same primitives, in the same fractions and offsets
     /// of the box, and the same reported size. A widget that read its length
@@ -464,7 +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
@@ -472,30 +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.frame_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.frame_own[axis as usize] = holds;
+        self.window_own[axis as usize] = holds;
     }
 
     pub fn text_data(&mut self) -> &mut TextData {
@@ -583,70 +549,77 @@ 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.
+/// 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.
     ///
-/// `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(
+    /// 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 {
         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) if *frame.axis(axis) == UiSpan::FULL => {
+            // 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 a part of this widget's own box, in that box's
+                // own lengths, so what it holds for maps back through that
+                // part into a range on this widget's box. A length it pinned
+                // is this widget's length less the part's pixels where the
+                // part is the whole of the box less pixels, which is the one
+                // shape that inverts exactly; any other part pins this
+                // widget's own length.
+                (Part::Of(span), false) => {
+                    let part_len = span.len();
+                    result.extent[n] = holds.extent[n].through(part_len);
+                    result.extent_len[n] = holds.extent_len[n].map(|pinned| match part_len.rel {
+                        Rel::ONE => pinned - Len::from_parts(Rel::ZERO, part_len.px),
+                        _ => self.extent.axis(axis).len(),
+                    });
                 }
-            // 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.
+                // 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
@@ -711,59 +684,58 @@ pub(crate) fn placed_extent(
     placed
 }
 
-/// The part of a widget's own box a `place` names, in the coordinates that
-/// box is in.
-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 length a rule gives a child's frame, per axis: a fraction in it is a
-/// fraction of the frame the child was given, which is the one length the
-/// rule can mean.
-pub(crate) fn narrowed_by(declared: [Option<LayoutLen>; 2], frame: UiRegion) -> [Option<Len>; 2] {
-    AXES.map(|axis| {
-        declared[axis as usize]
-            .map(|len| Len::from_parts(len.rel, len.px).within_len(frame.axis(axis).len()))
-    })
-}
-
-/// The frame a child is asked in and the box 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 +1,42 @@
-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
+    /// length, where saying the same thing in window 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),
+    /// 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::Sized(len) => {
+                let start = extent.start + (extent.len() - len).scale(align.rel());
+                UiSpan::new(start, start + len)
+            }
         }
     }
 }

core/src/ui/render_state.rs

@@ -1,10 +1,10 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
-use crate::ui::painter::{declared_lens, frame_and_extent, narrowed_by, part_of, placed_extent};
+use crate::ui::painter::{declared_lens, frame_and_extent, placed_extent};
 use crate::{
-    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Moves,
-    Painter, Part, PixelRegion, Place, PxVec2, Size, StrongWidget, UiRegion, UiRsc, Weight,
-    WidgetId, Widgets,
+    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx,
+    Moves, Painter, Part, PixelRegion, Place, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc,
+    UiSpan, UiVec2, Weight, WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
 
@@ -20,22 +20,23 @@ pub(super) struct DrawInfo {
     pub parent_move: MoveIdx,
     pub region_node: bool,
     pub mask: MaskIdx,
-    /// The frame in the parent widget's frame coordinates, before
-    /// composition. Its length is the same on every ask of the widget.
-    pub frame: UiRegion,
-    /// That frame composed into `parent_move`'s coordinates, which is what
-    /// the widget's own drawing is written within.
-    pub frame_abs: UiRegion,
-    /// The box the drawing is given, in the frame's own coordinates: the
-    /// part of the parent's own box that `place` names, before the widget's
-    /// answer is placed inside it.
+    /// What a fraction declared or reported under this widget is a fraction
+    /// of, as a length of the window.
+    pub frame: UiVec2,
+    /// The box the widget is asked in, in its parent region node's
+    /// coordinates.
     pub part: UiRegion,
-    /// What of the parent's extent the drawing was given, and what it was
-    /// given at the parent's first ask of it. See [`Place`].
-    pub place: [Place; 2],
-    pub offer_place: [Place; 2],
-    /// The frame in pixels: one multiply from the parent's own, which is
-    /// where every pixel length in layout comes from.
+    /// Where the widget is put, and where it was asked, as parts of the
+    /// parent's box. See [`Place`]. The two are one ask's place until the
+    /// parent puts the answer somewhere else.
+    pub placed: [Place; 2],
+    pub asked: [Place; 2],
+    /// A frame the parent decided for it on each axis, as a length of the
+    /// window, which the widget's own declaration is a fraction of.
+    pub narrow: [Option<Len>; 2],
+    /// Whether the parent already asked about this widget in this draw.
+    pub re_asked: bool,
+    /// The frame in pixels, resolved once against the window.
     pub px: PxVec2,
 }
 
@@ -43,36 +44,20 @@ impl DrawInfo {
     /// The axes where the part is the drawing's box outright, which are the
     /// axes the answer is not placed inside it again.
     fn fill(&self) -> [bool; 2] {
-        self.place.map(Place::fills)
-    }
-
-    /// Whether this ask is the one the widget's answer is kept from: the
-    /// same widget in the place its parent measured it by, however this draw
-    /// came about.
-    ///
-    /// **Open.** A place is a length from where the asking widget's own box
-    /// starts, so two drawings of that widget -- one in the box its parent
-    /// measured it in, one in the box its own answer chose -- ask their
-    /// children in the same places and different boxes, and this cannot tell
-    /// them apart. Shrinker seeds 2 (`repaint`) and 108 (`reorder`) at depth
-    /// 5 are where that shows.
-    fn offer(&self) -> bool {
-        self.place == self.offer_place
+        self.placed.map(Place::fills)
     }
 }
 
 /// What a widget's children are placed in: its own box, the coordinates its
 /// drawing is in, and what else one ask of a child is decided from.
-struct Placing {
-    id: WidgetId,
-    extent: UiRegion,
-    /// The widget's frame in the coordinates its children compose within:
-    /// `FULL` where it is a region node, since its box is that node.
-    local: UiRegion,
-    px: PxVec2,
-    depth: usize,
-    move_idx: MoveIdx,
-    mask: MaskIdx,
+pub(super) struct Placing {
+    pub id: WidgetId,
+    pub extent: UiRegion,
+    pub frame: UiVec2,
+    pub window: PxVec2,
+    pub depth: usize,
+    pub move_idx: MoveIdx,
+    pub mask: MaskIdx,
 }
 
 pub struct UiRenderState {
@@ -134,24 +119,24 @@ impl UiRenderState {
         self.resized = true;
         let Some(root) = self.old_root else { return };
         let stands = self.active.get(&root).is_some_and(|active| {
-            let px = active.frame.size().to_px(size);
             // Nothing above the root chose anything, so the box it was first
-            // asked about is the whole of its frame.
-            let offer = part_of(UiRegion::FULL, active.offer_place);
-            active.answers_at(px, offer) && active.holds.contains(px, active.extent)
+            // asked about is the whole of its frame. Both its answer and its
+            // drawing have to stand in the new window, since nothing above
+            // it will ask either again.
+            let answer = active
+                .answer
+                .is_some_and(|(_, holds)| holds.contains(size, active.frame, active.part));
+            answer && active.holds.contains(size, active.frame, active.part)
         });
         if !stands {
             widgets.needs_redraw.insert(root);
         }
     }
 
-    /// The root is asked about in the output: the window is where a fraction
-    /// becomes pixels rather than a box of its own, so the root's box is the
-    /// first length threaded down. Its own rules narrow that box, and where
-    /// they do the narrowed box is also the offer -- nothing above it chose
-    /// anything else.
-    fn root_info(&self, region: UiRegion) -> DrawInfo {
-        let px = region.size().to_px(self.output_size);
+    /// The root is asked about in the output. Its own rules narrow both its
+    /// frame and box; nothing above it chose a different one.
+    fn root_info(&self, frame: UiVec2, extent: UiRegion) -> DrawInfo {
+        let px = frame.to_px(self.output_size);
         DrawInfo {
             layer: 0,
             parent: None,
@@ -159,11 +144,12 @@ impl UiRenderState {
             parent_move: MoveIdx::NONE,
             region_node: false,
             mask: MaskIdx::NONE,
-            frame: region,
-            frame_abs: region,
-            part: UiRegion::FULL,
-            place: [Place::Within(Part::All); 2],
-            offer_place: [Place::Within(Part::All); 2],
+            frame,
+            part: extent,
+            placed: [Place::Within(Part::All); 2],
+            asked: [Place::Within(Part::All); 2],
+            narrow: [None; 2],
+            re_asked: false,
             px,
         }
     }
@@ -210,23 +196,24 @@ impl UiRenderState {
         let _layout = diag::timer(TimerKind::FullLayout);
         self.clear(rsc);
         if let Some(id) = root {
-            let region = Self::root_region(id.id(), rsc.widgets());
-            let info = self.root_info(region);
+            let (frame, extent) = Self::root_layout(id.id(), rsc.widgets());
+            let info = self.root_info(frame, extent);
             self.draw_inner(id.id(), info, None, rsc);
         }
     }
 
-    /// The root's frame: the window, narrowed by the root's own rules. Its
-    /// extent is that frame, since nothing above it chose anything else.
-    fn root_region(id: WidgetId, widgets: &Widgets) -> UiRegion {
-        let narrow = narrowed_by(declared_lens(widgets, id), UiRegion::FULL);
+    /// The root's frame and box: the window, taken in by the root's own
+    /// rules. Nothing above it narrowed anything or chose where it goes, so
+    /// its declaration is the whole of what decides either.
+    fn root_layout(id: WidgetId, widgets: &Widgets) -> (UiVec2, UiRegion) {
         frame_and_extent(
             UiRegion::FULL,
-            UiRegion::FULL,
-            narrow,
+            UiVec2::FULL_SIZE,
+            [Place::Within(Part::All); 2],
+            [None; 2],
+            declared_lens(widgets, id),
             widgets.alignment(id),
         )
-        .0
     }
 
     pub(super) fn draw_inner(
@@ -236,55 +223,57 @@ impl UiRenderState {
         mut old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
     ) -> (Size, LayoutHolds, LayoutHolds) {
-        let (frame, part) = (info.frame_abs, info.part);
+        let part = info.part;
         #[cfg(feature = "layout-diagnostics")]
         {
             diag::bump(Counter::DrawRequests);
-            diag::draw_request(id, info.parent, frame, info.px, info.region_node);
+            diag::draw_request(id, info.parent, part, info.px, info.region_node);
         }
         let align = rsc.widgets().alignment(id);
+        let declared = declared_lens(rsc.widgets(), id);
         // Nothing this widget measured can be dirty while it draws: layout is
         // one bottom-up walk, so anything deeper has settled or deferred to
         // its own parent, and a deferred one leaves that parent marked.
         let stale = rsc.widgets().needs_redraw.contains(&id);
-        let retained = match stale {
-            true => None,
-            false => self
-                .retained_answer(id, part, info)
-                .and_then(|answer| self.try_reuse(id, frame, part, info, rsc).map(|_| answer)),
-        };
-        let answer = retained.unwrap_or_else(|| {
+        // The widget draws once, in the box it is asked in, and its answer
+        // is placed inside that box by re-expressing the drawing. The box the
+        // answer chose is never a question: nothing is drawn again in it, so
+        // an answer is kept only with the drawing that gave it, and both
+        // have to hold for the box asked about.
+        let reused = (!stale)
+            .then(|| self.retained_answer(id, part, info))
+            .flatten()
+            .and_then(|answer| {
+                let extent = placed_extent(part, answer.0, declared, info.fill(), align);
+                self.try_reuse(id, part, extent, info, rsc).map(|()| answer)
+            });
+        let answer = reused.unwrap_or_else(|| {
             if old.is_none() {
                 old = self.remove(id, false, rsc);
             }
-            self.draw_at(id, part, info, old.take(), rsc)
+            let answer = self.draw_at(id, part, info, old.take(), rsc);
+            // Where the drawing goes: the part its parent gave it, with the
+            // answer placed inside that part on any axis the parent left
+            // open.
+            let extent = placed_extent(part, answer.0, declared, info.fill(), align);
+            if extent != part {
+                self.relocate(id, extent, info, rsc);
+            }
+            answer
         });
 
-        // Where the drawing goes, in the frame's own coordinates: the part
-        // its parent gave it, with the answer placed inside that part on any
-        // axis the parent left open. The frame itself does not change, so
-        // nothing under it resolves a fraction a second time.
-        //
-        let extent = placed_extent(
-            part,
-            answer.0,
-            declared_lens(rsc.widgets(), id),
-            info.fill(),
-            align,
-        );
-        self.place(id, extent, info, rsc);
-
         let drawing_holds = self.active[&id].holds;
         let active = self.active.get_mut(&id).unwrap();
         // Whoever asked owns how the boxes were reached: the frame it stated,
-        // and what of its own box it gave the drawing. A local redraw asks
-        // the same question again from these.
-        active.frame_abs = frame;
+        // and what of its own box it asked in. A local redraw asks the same
+        // question again from these.
         active.frame = info.frame;
+        active.narrow = info.narrow;
+        active.re_asked = info.re_asked;
         active.answer = Some(answer);
-        active.offer_place = info.offer_place;
-        active.offer_part = part;
-        active.place = info.place;
+        active.asked = info.asked;
+        active.part = part;
+        active.placed = info.placed;
         active.own_align = align;
         // A subtree can be reused whole under a different parent -- same box,
         // same layer, same region node -- and nothing in the drawing says it
@@ -301,29 +290,6 @@ impl UiRenderState {
         (answer.0, answer.1, drawing_holds)
     }
 
-    /// Recompose retained geometry when the evaluation still holds at this extent.
-    fn place(&mut self, id: WidgetId, extent: UiRegion, info: DrawInfo, rsc: &mut dyn UiRsc) {
-        if self
-            .try_reuse(id, info.frame_abs, extent, info, rsc)
-            .is_some()
-        {
-            return;
-        }
-        assert_eq!(
-            extent,
-            info.part,
-            "'{}' ({id:?}) does not hold for the box its answer chose: part {:?}, wanted {extent:?}, retained {:?} with {:?}",
-            rsc.widgets().label(id),
-            info.part,
-            self.active.get(&id).map(|active| active.extent),
-            self.active.get(&id).map(|active| active.holds),
-        );
-        #[cfg(feature = "layout-diagnostics")]
-        diag::bump(Counter::PlaceRedraws);
-        let old = self.remove(id, false, rsc);
-        self.draw_at(id, extent, info, old, rsc);
-    }
-
     /// Calls a widget's `draw` and keeps what it drew in `extent` of `frame`.
     fn draw_at(
         &mut self,
@@ -333,36 +299,30 @@ impl UiRenderState {
         old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
     ) -> (Size, LayoutHolds) {
-        let frame = info.frame_abs;
-        let (move_idx, local, retired_move) = match info.region_node {
-            // Its box becomes its movable region, so it draws in that
-            // region's coordinates and its box is one entry to rewrite.
+        let frame = info.frame;
+        let (move_idx, extent, retired_move) = match info.region_node {
+            // A node entry is only a translation. Its local box keeps the
+            // same window-unit length as the box in its parent's node.
             true => (
-                self.move_slot(id, info.parent_move, frame),
-                UiRegion::FULL,
+                self.move_slot(id, info.parent_move, translation(extent)),
+                local_region(extent),
                 None,
             ),
             // Keep the old entry alive until every descendant has migrated.
             // Reusing its index sooner could make an old parent look current.
-            false => (info.parent_move, frame, self.slots.remove(&id)),
-        };
-        let (old_children, old_answer, old_offer_part) = match old {
-            Some(old) => (old.children, old.answer, Some(old.offer_part)),
-            None => (Vec::new(), None, None),
+            false => (info.parent_move, extent, self.slots.remove(&id)),
         };
+        let old_children = old.map_or_else(Vec::new, |old| old.children);
         rsc.widgets_mut().needs_redraw.remove(&id);
-        // Only evaluation at the original offer establishes the children's
-        // offers. A placing evaluation must not overwrite that question.
         let px = info.px;
-        let at_offer = info.offer();
 
+        let window = self.output_size;
         let mut painter = Painter {
             state: self,
-            frame: local,
+            frame,
             extent,
             extent_len: [None; 2],
-            answer_extent_len: [None; 2],
-            px,
+            window,
             mask: info.mask,
             layer: info.layer,
             own_layer: info.layer,
@@ -371,13 +331,11 @@ impl UiRenderState {
             primitives: Vec::new(),
             mask_region: None,
             children: Vec::new(),
-            offered: Vec::new(),
-            at_offer,
             size_deps: Vec::new(),
-            frame_own: [Holds::ANY; 2],
+            window_own: [Holds::ANY; 2],
+            frame_own_len: [None; 2],
             under: Vec::new(),
             extent_own: [Holds::ANY; 2],
-            answer_extent_own: [Holds::ANY; 2],
             answer_under: LayoutHolds::ANY,
             depth: info.depth,
             move_idx,
@@ -400,21 +358,18 @@ impl UiRenderState {
             rsc: _,
             frame: _,
             extent: _,
-            px: _,
+            window: _,
             mask,
             textures,
             primitives,
             mask_region,
             extent_own,
-            answer_extent_own,
             extent_len,
-            answer_extent_len,
             answer_under,
             children,
-            offered: _,
-            at_offer: _,
             size_deps,
-            frame_own,
+            window_own,
+            frame_own_len,
             under,
             move_idx,
             layer,
@@ -430,11 +385,23 @@ impl UiRenderState {
         );
         // A rule wins on the axis it names, and the draw answers the rest.
         // Applied here so it is one place rather than every widget that could
-        // carry one, and so the widget under a rule never learns of it.
+        // carry one, and so the widget under a rule never learns of it. The
+        // frame is the answer where the rule gave a length outright: it was
+        // resolved into the frame when the child was asked, and resolving it
+        // again here would take the fraction of a fraction.
         let rules = rsc.widgets().size_rules(id);
+        let ruled = |axis: Axis, reported: LayoutLen| match rules.axis(axis).exact() {
+            None => reported,
+            Some(len) if len.leftover == Weight::ZERO => LayoutLen {
+                rel: info.frame.axis(axis).rel,
+                px: info.frame.axis(axis).px,
+                leftover: Weight::ZERO,
+            },
+            Some(len) => len.within_len(info.frame.axis(axis)),
+        };
         let size = Size {
-            x: rules.x.apply(size.x),
-            y: rules.y.apply(size.y),
+            x: ruled(Axis::X, size.x),
+            y: ruled(Axis::Y, size.y),
         };
         // A widget that clipped its contents to its box drew nothing outside
         // it, so reporting more than the box asks to be placed at a length it
@@ -442,7 +409,10 @@ impl UiRenderState {
         // Overflowing is otherwise ordinary: a text too tall for the box it
         // was offered reports the height it needs.
         debug_assert!(
-            mask == info.mask || AXES.into_iter().all(|axis| within_box(size, px, axis)),
+            mask == info.mask
+                || AXES
+                    .into_iter()
+                    .all(|axis| within_box(size, extent, self.output_size, axis)),
             "'{}' ({id:?}) clips to {px:?} and reports {size}",
             rsc.widgets().label(id),
         );
@@ -454,25 +424,32 @@ impl UiRenderState {
         if let Some(idx) = retired_move {
             self.moves.remove(idx);
         }
-        let drawing_own = LayoutHolds {
-            frame: frame_own,
+        // A rule that is a fraction of the frame is answered with the
+        // frame's own length, so the answer is that frame's and not just
+        // that many pixels of this window -- the same pin a widget that read
+        // its frame took for its drawing.
+        let frame_len = AXES.map(|axis| {
+            let fraction = rules
+                .axis(axis)
+                .exact()
+                .is_some_and(|len| len.rel != Rel::ZERO);
+            match fraction {
+                true => Some(info.frame.axis(axis)),
+                false => frame_own_len[axis as usize],
+            }
+        });
+        let own_holds = LayoutHolds {
+            window: window_own,
+            frame_len,
             extent: extent_own,
             extent_len,
         };
-        let answer_own = LayoutHolds {
-            extent: [
-                drawing_own.extent[0].and(answer_extent_own[0]),
-                drawing_own.extent[1].and(answer_extent_own[1]),
-            ],
-            extent_len: answer_extent_len,
-            ..drawing_own
-        };
-        let answer_holds = answer_own.and(answer_under);
+        let answer_holds = own_holds.and(answer_under);
         let holds = under
             .into_iter()
-            .fold(drawing_own, |holds, (_, child)| holds.and(child));
+            .fold(answer_holds, |holds, (_, child)| holds.and(child));
         debug_assert!(
-            holds.contains(px, extent),
+            holds.contains(self.output_size, info.frame, extent),
             "'{}' ({id:?}) drew in {px:?}, outside the ranges it reported: {holds:?}",
             rsc.widgets().label(id),
         );
@@ -490,11 +467,12 @@ impl UiRenderState {
                         parent_move: move_idx,
                         region_node: false,
                         mask,
-                        frame: UiRegion::FULL,
-                        frame_abs: UiRegion::FULL,
+                        frame: UiVec2::FULL_SIZE,
                         part: UiRegion::FULL,
-                        place: [Place::Within(Part::All); 2],
-                        offer_place: [Place::Within(Part::All); 2],
+                        placed: [Place::Within(Part::All); 2],
+                        asked: [Place::Within(Part::All); 2],
+                        narrow: [None; 2],
+                        re_asked: false,
                         px,
                     },
                     rsc,
@@ -505,14 +483,15 @@ impl UiRenderState {
 
         let active = ActiveData {
             id,
-            frame_abs: frame,
             extent,
             frame: info.frame,
-            place: info.place,
-            offer_place: info.offer_place,
-            offer_part: old_offer_part.unwrap_or(extent),
-            // Whoever asked writes the answer, if this was the asking.
-            answer: old_answer,
+            narrow: info.narrow,
+            placed: info.placed,
+            asked: info.asked,
+            part: extent,
+            // Whoever asked writes the answer.
+            answer: None,
+            re_asked: info.re_asked,
             size,
             holds,
             drawn: true,
@@ -575,38 +554,22 @@ impl UiRenderState {
             return None;
         }
         let answer = active.answer?;
-        answer.1.contains(info.px, part).then_some(answer)
+        answer
+            .1
+            .contains(self.output_size, info.frame, part)
+            .then_some(answer)
     }
 
-    /// The pixel lengths of a widget's frame, which is what a local redraw
-    /// needs to ask the question its parent asked.
-    ///
-    /// It is threaded down from the window a length of a box at a time, and
-    /// this takes the same steps back up: a widget's frame is a length of its
-    /// parent's frame, and that chain has no coordinate frame in it, so a
-    /// region node cannot break it -- and it lands on the number a cold
-    /// layout computes rather than near it.
-    fn asked_px(&self, id: WidgetId) -> PxVec2 {
-        let active = &self.active[&id];
-        // Nothing above the root: the window is where a fraction becomes
-        // pixels, which is also the whole of the frame the root is given.
-        let parent_px = match active.parent.and_then(|p| self.active.get(&p)) {
-            Some(parent) => self.asked_px(parent.id),
-            None => self.output_size,
-        };
-        active.frame.size().to_px(parent_px)
-    }
-
-    /// Reuses the actual drawing in a new box if its retained contract holds
-    /// there. Answers retained from a different ask are handled separately.
+    /// Keeps the retained drawing if its contract holds for `part`, the box
+    /// asked about, and puts it at `extent`, where the answer places it.
     fn try_reuse(
         &mut self,
         id: WidgetId,
-        frame: UiRegion,
+        part: UiRegion,
         extent: UiRegion,
         info: DrawInfo,
         rsc: &mut dyn UiRsc,
-    ) -> Option<(Size, LayoutHolds)> {
+    ) -> Option<()> {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::ReuseAttempts);
         if rsc.widgets().needs_redraw.contains(&id) {
@@ -652,10 +615,10 @@ impl UiRenderState {
             }
             return None;
         }
-        // In pixels, because the frame is a fraction of its parent's and
-        // that may be what changed -- an unchanged fraction of a box half the
+        // In pixels, because the box is a fraction of the window and that
+        // may be what changed -- an unchanged fraction of a window half the
         // size is half the widget.
-        if !active.holds.contains(info.px, extent) {
+        if !active.holds.contains(self.output_size, info.frame, part) {
             #[cfg(feature = "layout-diagnostics")]
             {
                 // Which of the three said no, so a frame that redraws more
@@ -664,13 +627,16 @@ impl UiRenderState {
                 let holds = active.holds;
                 for axis in AXES {
                     let n = axis as usize;
-                    if holds.extent_len[n].is_some_and(|pinned| pinned != extent.axis(axis).len()) {
+                    if holds.extent_len[n].is_some_and(|pinned| pinned != part.axis(axis).len()) {
                         diag::bump(Counter::OutsidePinnedLen);
                     }
-                    if !holds.frame[n].contains(info.px.axis(axis)) {
+                    if !holds.window[n].contains(self.output_size.axis(axis))
+                        || holds.frame_len[n].is_some_and(|pinned| pinned != info.frame.axis(axis))
+                    {
                         diag::bump(Counter::OutsideFrame);
                     }
-                    if !holds.extent[n].contains(extent.axis(axis).len().to_px(info.px.axis(axis)))
+                    if !holds.extent[n]
+                        .contains(part.axis(axis).len().to_px(self.output_size.axis(axis)))
                     {
                         diag::bump(Counter::OutsideExtent);
                     }
@@ -680,24 +646,37 @@ impl UiRenderState {
             }
             return None;
         }
-        let extent_moved = active.extent != extent;
-        let moved = active.frame_abs != frame;
-        let (answer, slot) = ((active.size, active.holds), active.move_idx);
-        if moved {
+        self.relocate(id, extent, info, rsc);
+        Some(())
+    }
+
+    /// Puts a retained drawing where its parent now has it, without drawing:
+    /// a widget with a node of its own writes that node's translation, and
+    /// one without re-expresses its own drawing and everything inside it.
+    fn relocate(&mut self, id: WidgetId, extent: UiRegion, info: DrawInfo, rsc: &mut dyn UiRsc) {
+        let active = &self.active[&id];
+        debug_assert!(
+            !rsc.widgets().needs_redraw.contains(&id),
+            "'{}' ({id:?}) placed while marked to draw",
+            rsc.widgets().label(id)
+        );
+        let has_region_node = active.move_idx != active.parent_move;
+        let local = match has_region_node {
+            true => local_region(extent),
+            false => extent,
+        };
+        let moved = active.extent != local;
+        let slot = active.move_idx;
         if has_region_node {
-                self.moves.set(slot, frame);
-            } else {
-                self.recompose_subtree(id, frame, info.parent_move, rsc);
+            self.moves.set(slot, translation(extent));
         }
-        }
-        if extent_moved {
-            self.reposition(id, frame, extent, info, rsc);
+        if moved {
+            self.reposition(id, local, info, rsc);
         }
         self.redepth(id, info.depth);
         let active = self.active.get_mut(&id).unwrap();
-        active.frame_abs = frame;
         active.frame = info.frame;
-        active.place = info.place;
+        active.placed = info.placed;
         #[cfg(feature = "layout-diagnostics")]
         {
             match (moved, has_region_node) {
@@ -718,20 +697,31 @@ impl UiRenderState {
                 },
             );
         }
-        Some(answer)
     }
 
-    /// Places one child of `at.id` in the box that widget's own box gives
-    /// it: its part of the extent, with its answer placed inside that part
-    /// where the ask left the axis open.
+    /// Places one child of `at.id` where that widget's own box now has it.
     fn place_child(&mut self, child: WidgetId, at: &Placing, rsc: &mut dyn UiRsc) {
+        let place = self.active[&child].placed;
+        self.place_in(child, at, place, rsc);
+    }
+
+    /// Puts a child of `at.id` in `place` of that widget's box: its answer
+    /// placed inside that part where the place leaves the axis open, the
+    /// drawing re-expressed there.
+    pub(super) fn place_in(
+        &mut self,
+        child: WidgetId,
+        at: &Placing,
+        place: [Place; 2],
+        rsc: &mut dyn UiRsc,
+    ) {
         let active = &self.active[&child];
-        let (frame, part) = Self::re_ask(active, at.extent, active.place);
+        let (frame, part) = Self::ask_again(active, at, place);
         let extent = placed_extent(
             part,
             active.measured().unwrap_or(active.size),
             active.declared,
-            active.place.map(Place::fills),
+            place.map(Place::fills),
             active.own_align,
         );
         let info = DrawInfo {
@@ -742,34 +732,27 @@ impl UiRenderState {
             region_node: active.move_idx != active.parent_move,
             mask: at.mask,
             frame,
-            frame_abs: frame.within(&at.local),
             part,
-            place: active.place,
-            offer_place: active.offer_place,
-            px: frame.size().to_px(at.px),
+            placed: place,
+            asked: active.asked,
+            narrow: active.narrow,
+            re_asked: active.re_asked,
+            px: frame.to_px(at.window),
         };
-        self.place(child, extent, info, rsc);
+        self.relocate(child, extent, info, rsc);
     }
 
-    /// The frame and the box a widget being asked again is given, from what
-    /// it already has and where its parent's box is now. A frame's length is
-    /// the same on every ask, so a declared length is put back where it sits
-    /// in the part rather than resolved from its rule a second time.
-    fn re_ask(
-        active: &ActiveData,
-        parent_extent: UiRegion,
-        place: [Place; 2],
-    ) -> (UiRegion, UiRegion) {
-        let narrow = AXES.map(|axis| {
-            let n = axis as usize;
-            active.declared[n]
-                .is_some()
-                .then(|| active.frame.axis(axis).len())
-        });
+    /// The frame and the box a widget already drawn is given at `place` of
+    /// the box its parent is being taken as. What narrowed its frame and what
+    /// it declared are its own record's, so both are resolved against that
+    /// parent's frame again exactly as the first ask resolved them.
+    fn ask_again(active: &ActiveData, at: &Placing, place: [Place; 2]) -> (UiVec2, UiRegion) {
         frame_and_extent(
-            active.frame,
-            part_of(parent_extent, place),
-            narrow,
+            at.extent,
+            at.frame,
+            place,
+            active.narrow,
+            active.declared,
             active.own_align,
         )
     }
@@ -778,36 +761,21 @@ impl UiRenderState {
     /// is placed as a part of that box, so each one's new box is its retained
     /// part re-added to the new start -- and a child whose own box then did
     /// not change is not touched at all.
-    fn reposition(
-        &mut self,
-        id: WidgetId,
-        frame: UiRegion,
-        extent: UiRegion,
-        info: DrawInfo,
-        rsc: &mut dyn UiRsc,
-    ) {
+    fn reposition(&mut self, id: WidgetId, extent: UiRegion, info: DrawInfo, rsc: &mut dyn UiRsc) {
         let active = self.active.get_mut(&id).unwrap();
-        active.frame_abs = frame;
         active.extent = extent;
-        let local = if info.region_node {
-            UiRegion::FULL
-        } else {
-            frame
-        };
         for primitive in &active.primitives {
             let handle = &primitive.handle;
-            *self.layers[handle.layer].region_mut(handle) =
-                primitive.region.within(&extent).within(&local);
+            *self.layers[handle.layer].region_mut(handle) = primitive.region.within(&extent);
         }
         if let Some(mask_region) = active.mask_region {
-            rsc.ui_mut().masks.get_mut(active.mask).region =
-                mask_region.within(&extent).within(&local);
+            rsc.ui_mut().masks.get_mut(active.mask).region = mask_region.within(&extent);
         }
         let at = Placing {
             id,
             extent,
-            local,
-            px: info.px,
+            frame: info.frame,
+            window: self.output_size,
             depth: info.depth,
             move_idx: active.move_idx,
             mask: active.mask,
@@ -837,38 +805,6 @@ impl UiRenderState {
         }
     }
 
-    /// Replays the original local compositions, including their rounding order.
-    /// A region node terminates the walk because its contents name its slot.
-    fn recompose_subtree(
-        &mut self,
-        id: WidgetId,
-        frame: UiRegion,
-        parent_move: MoveIdx,
-        rsc: &mut dyn UiRsc,
-    ) {
-        let active = self.active.get_mut(&id).unwrap();
-        active.frame_abs = frame;
-        if active.move_idx != parent_move {
-            self.moves.set(active.move_idx, frame);
-            return;
-        }
-        let extent = active.extent;
-        for primitive in &active.primitives {
-            let handle = &primitive.handle;
-            *self.layers[handle.layer].region_mut(handle) =
-                primitive.region.within(&extent).within(&frame);
-        }
-        if let Some(local) = active.mask_region {
-            rsc.ui_mut().masks.get_mut(active.mask).region = local.within(&extent).within(&frame);
-        }
-        let children = active.children.len();
-        for index in 0..children {
-            let child = self.active[&id].children[index];
-            let local = self.active[&child].frame;
-            self.recompose_subtree(child, local.within(&frame), parent_move, rsc);
-        }
-    }
-
     fn hints_agree(id: WidgetId, size: Size, rsc: &dyn UiRsc) -> bool {
         let Some(widget) = rsc.widgets().get_dyn(id) else {
             return true;
@@ -939,13 +875,14 @@ impl UiRenderState {
             id,
             ActiveData {
                 id,
-                frame_abs: UiRegion::FULL,
                 extent: UiRegion::FULL,
-                frame: UiRegion::FULL,
-                place: [Place::Within(Part::All); 2],
-                offer_place: [Place::Within(Part::All); 2],
-                offer_part: UiRegion::FULL,
+                frame: UiVec2::FULL_SIZE,
+                narrow: [None; 2],
+                placed: [Place::Within(Part::All); 2],
+                asked: [Place::Within(Part::All); 2],
+                part: UiRegion::FULL,
                 answer: None,
+                re_asked: false,
                 size,
                 holds: LayoutHolds::ANY,
                 drawn: false,
@@ -1126,9 +1063,8 @@ impl UiRenderState {
     pub fn window_region(&self, id: &impl IdLike) -> Option<PixelRegion> {
         let active = self.active.get(&id.id())?;
         active.drawn.then(|| {
-            let placed = active.extent.within(&active.frame_abs);
             self.moves
-                .resolve(active.parent_move, placed)
+                .resolve(active.move_idx, active.extent)
                 .to_px(self.output_size)
         })
     }
@@ -1145,11 +1081,16 @@ impl UiRenderState {
         // Its parent resolved its declared lengths into its box and decided
         // whether to draw it at all, so a change to either is the parent's
         // to draw -- with the mark left on, so the parent draws it rather
-        // than keeping it.
+        // than keeping it. So is a widget the parent asked twice: its
+        // layout rests on an answer this widget cannot give again alone.
         let declared_changed = declared_lens(rsc.widgets(), id) != active.declared;
         let alignment_changed = rsc.widgets().alignment(id) != active.own_align;
         if let Some(parent) = active.parent
-            && (declared_changed || alignment_changed || !active.drawn || active.answer.is_none())
+            && (declared_changed
+                || alignment_changed
+                || active.re_asked
+                || !active.drawn
+                || active.answer.is_none())
         {
             // Both stay marked: the parent because it has this to draw, and
             // this because the parent must draw it rather than keep what it
@@ -1165,10 +1106,10 @@ impl UiRenderState {
         // box is its own to work out again against the output. Every other
         // widget was given one.
         let Some(parent) = active.parent else {
-            let region = Self::root_region(id, rsc.widgets());
+            let (frame, extent) = Self::root_layout(id, rsc.widgets());
             let info = DrawInfo {
                 mask: active.parent_mask,
-                ..self.root_info(region)
+                ..self.root_info(frame, extent)
             };
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::LocalRedraws);
@@ -1176,13 +1117,14 @@ impl UiRenderState {
             self.draw_inner(id, info, old, rsc);
             return true;
         };
-        let px = self.asked_px(id);
-        let (was_answer, was_holds) = (active.answer, active.holds);
-        let needs_replacement = active.place != active.offer_place;
-        // Re-ask the question its answer came from, not a later placement of
-        // that answer. A parent may move the retained drawing into a box the
-        // answer chose without making that box a new question.
-        let parent_extent = self.active[&parent].extent;
+        let (was_answer, was_holds, was_place) = (active.answer, active.holds, active.placed);
+        // The question its parent asked, asked again: the same place of the
+        // box the parent was asked in, which is the box the parent's own
+        // draw ran in and what its children's parts are of. Where the
+        // parent's answer put its own drawing is not a question anybody
+        // asked, and nothing is asked in it here either.
+        let parent_at = self.placing_of(parent, self.active[&parent].part);
+        let (frame, part) = Self::ask_again(active, &parent_at, active.asked);
         let info = DrawInfo {
             layer: active.layer,
             parent: active.parent,
@@ -1190,12 +1132,13 @@ impl UiRenderState {
             parent_move: active.parent_move,
             region_node: rsc.widgets().is_region_node(id),
             mask: active.parent_mask,
-            frame: active.frame,
-            frame_abs: active.frame_abs,
-            part: Self::re_ask(active, parent_extent, active.offer_place).1,
-            place: active.offer_place,
-            offer_place: active.offer_place,
-            px,
+            frame,
+            part,
+            placed: active.asked,
+            asked: active.asked,
+            narrow: active.narrow,
+            re_asked: false,
+            px: frame.to_px(self.output_size),
         };
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::LocalRedraws);
@@ -1211,10 +1154,12 @@ impl UiRenderState {
         {
             active.answer = was_answer;
         }
-        if active.holds.covers(was_holds) && was_holds.contains(px, active.extent) {
+        if active.holds.covers(was_holds)
+            && was_holds.contains(self.output_size, active.frame, active.extent)
+        {
             active.holds = was_holds;
         }
-        if needs_replacement || active.answer != was_answer || active.holds != was_holds {
+        if active.answer != was_answer || active.holds != was_holds {
             // The parent retains both the answer and the drawing's validity;
             // even an unchanged size can narrow the range safe for a resize.
             #[cfg(feature = "layout-diagnostics")]
@@ -1223,18 +1168,61 @@ impl UiRenderState {
                 diag::bump(Counter::ReaderEdges);
             }
             self.mark(parent, rsc.widgets_mut());
+        } else {
+            // The answer stands, so where the parent put it stands: the
+            // fresh drawing goes back there -- the same place, of the box
+            // the parent's answer chose rather than the one it was asked in.
+            let at = self.placing_of(parent, self.active[&parent].extent);
+            self.place_in(id, &at, was_place, rsc);
         }
         true
     }
+
+    /// A drawn widget as the thing its children are placed within, with
+    /// `extent` as the box their parts are of: the box it was asked in for
+    /// asking one of them again, the box its answer chose for placing one.
+    fn placing_of(&self, id: WidgetId, extent: UiRegion) -> Placing {
+        let active = &self.active[&id];
+        Placing {
+            id,
+            extent,
+            frame: active.frame,
+            window: self.output_size,
+            depth: active.depth,
+            move_idx: active.move_idx,
+            mask: active.mask,
+        }
+    }
 }
 
 /// Whether what a widget reports along `axis` is inside the box it drew in.
-/// A share is a length only to whoever divides one, so it is not a claim
-/// about this box and cannot exceed it.
-fn within_box(size: Size, px: PxVec2, axis: Axis) -> bool {
+/// Both are lengths of the window, so the comparison is in its pixels. A
+/// share is a length only to whoever divides one, so it is not a claim about
+/// this box and cannot exceed it.
+fn within_box(size: Size, extent: UiRegion, window: PxVec2, axis: Axis) -> bool {
     let len = size.axis(axis);
-    let box_len = px.axis(axis);
-    len.leftover != Weight::ZERO || box_len.mul(len.rel) + len.px <= box_len
+    let window = window.axis(axis);
+    len.leftover != Weight::ZERO
+        || Len::from_parts(len.rel, len.px).to_px(window) <= extent.axis(axis).len().to_px(window)
+}
+
+/// A box in a fresh region node keeps its window-unit length and starts at
+/// that node's origin.
+fn local_region(region: UiRegion) -> UiRegion {
+    let size = region.size();
+    UiRegion::new(
+        UiSpan::new(Len::ZERO, size.x),
+        UiSpan::new(Len::ZERO, size.y),
+    )
+}
+
+/// A region node changes only the origin. A full relative span anchored at
+/// the box start composes as that translation in both the CPU and shader.
+fn translation(region: UiRegion) -> UiRegion {
+    UiRegion {
+        x: UiSpan::new(region.x.start, region.x.start + Len::FULL),
+        y: UiSpan::new(region.y.start, region.y.start + Len::FULL),
+    }
 }
 
 impl Default for UiRenderState {

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::Of(UiSpan::new(cut, Len::FULL)));
+        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
         let place = [Place::Within(Part::All), below];
-        match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
-            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

@@ -14,12 +14,11 @@ 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. Said as a part of
-        // this widget's own box in that box's own lengths, 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),
@@ -30,7 +29,16 @@ impl Widget for Pad {
             inset(self.padding.left, self.padding.right),
             inset(self.padding.top, self.padding.bottom),
         ];
-        let inner = painter.widget_at(&self.inner, UiRegion::FULL, place).size();
+        // 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

@@ -12,12 +12,11 @@ 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 fixed = Len::from_parts(answer_len.rel, answer_len.px).to_px(container_len);
+        let fixed = painter.to_px(Len::from_parts(answer_len.rel, answer_len.px), self.axis);
         self.container_len = container_len;
         self.content_len = fixed.max(container_len);
 
@@ -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

@@ -13,8 +13,8 @@ impl Widget for Span {
         // The row: this span's own box, as a length of the frame its children
         // are laid out against. Its start is nothing's business -- a slot is
         // a length from it -- so what this reads is the length alone.
-        let far = painter.answer_extent_len(axis);
-        let measure_along = |from: Len, to: Len| match self.dir.sign {
+        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),
         };
@@ -22,18 +22,34 @@ impl Widget for Span {
         // 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::Fill(Part::From(measure_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(Part::From(along(cursor, far)));
+                    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);
@@ -50,10 +66,9 @@ impl Widget for Span {
             |sum, len| sum + *len,
         );
 
-        let fixed_total = Len::from_parts(total.rel, total.px);
         // What is left for the shares to divide: the row less everything
         // fixed, as a length of the frame rather than a number of pixels.
-        let room = far - fixed_total;
+        let room = far - Len::from_parts(total.rel, total.px);
         // Whether anything is left over is a question in pixels: `rel(0.5)`
         // beside 300 px is full at 600 and overfull at 400. Asked of `room`
         // itself, and answered back through the same expression, so the
@@ -65,26 +80,14 @@ 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));
         }
 
-        if shares {
-            painter.drawing_uses_extent_len(axis, far);
-        }
-        let drawing_far = match shares {
-            true => far,
-            false => fixed_total,
-        };
-        let along = |from: Len, to: Len| match self.dir.sign {
-            Sign::Pos => UiSpan::new(from, to),
-            Sign::Neg => UiSpan::new(drawing_far - to, drawing_far - from),
-        };
-
         // Across itself a span is as long as its longest child -- unless a
         // rule beside it gives that length outright, and then reading them
         // answers nothing and makes its size depend on theirs for it. A rule
@@ -101,7 +104,8 @@ 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.
@@ -119,23 +123,26 @@ 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 span = along(from, start);
-            let (frame, slot) = match len.leftover > Weight::ZERO && shares {
-                true => (
-                    UiRegion::from_axis(
-                        axis,
-                        painter.extent_part(axis, Part::From(span)),
-                        UiSpan::FULL,
-                    ),
-                    Place::Fill(Part::All),
-                ),
-                false => (UiRegion::FULL, Place::Fill(Part::From(span))),
+            // 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),
             };
-            let placed = painter.widget_at(child, frame, axis.pair(slot, across));
             if shrinks {
-                let used = placed.len(!axis);
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.
                 // A scalable child therefore makes Children scalable too;

src/widget/position/stack.rs

@@ -23,28 +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);
-            let place = [Axis::X, Axis::Y].map(|axis| {
-                let len = size.axis(axis);
-                let part = match len.leftover > Weight::ZERO {
-                    true => Part::All,
-                    false => Part::From(UiSpan::new(Len::ZERO, Len::from_parts(len.rel, len.px))),
-                };
-                Place::Within(part)
-            });
-            painter.widget_at(child, UiRegion::FULL, place);
+            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

@@ -12,7 +12,6 @@ struct Counted {
     draws: Rc<Cell<usize>>,
     size: Size,
     reads_box: bool,
-    reads_answer_box: bool,
 }
 
 impl Widget for Counted {
@@ -20,8 +19,6 @@ impl Widget for Counted {
         self.draws.set(self.draws.get() + 1);
         if self.reads_box {
             painter.px_size();
-        } else if self.reads_answer_box {
-            painter.answer_px_size();
         }
         self.size
     }
@@ -41,18 +38,11 @@ fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>
         draws: draws.clone(),
         size,
         reads_box,
-        reads_answer_box: false,
     }
     .add(&mut h.rsc);
     (id, Counts(draws))
 }
 
-fn answer_counted(h: &mut Harness, size: Size) -> (WeakWidget<Counted>, Counts) {
-    let (id, draws) = counted(h, size, false);
-    h.rsc[id].reads_answer_box = true;
-    (id, draws)
-}
-
 struct Layered {
     children: [StrongWidget<Rect>; 2],
     _revision: usize,
@@ -166,9 +156,9 @@ fn a_span_child_that_declares_its_length_is_drawn_once() {
     h.set_root((hinted, asked).span(Dir::RIGHT));
 
     assert_eq!(told_draws.get(), 1);
-    // Its final slot is a parent decision, so it is evaluated there after
-    // the provisional ask established its length.
-    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]
@@ -226,7 +216,7 @@ impl Widget for FromHint {
         let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
         painter.widget_at(
             &self.inner,
-            UiRegion::FULL,
+            [None; 2],
             [Place::Within(Part::All), Place::Within(Part::From(top))],
         );
         Size::LEFTOVER
@@ -250,7 +240,7 @@ fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
     assert_corners!(h, inner, (0, 0), (400, 120));
 }
 
-/// Reads its box's size to compute its answer.
+/// Reads its box's size, which nothing but its own draw can put right.
 struct ReadsBox {
     draws: Rc<Cell<usize>>,
 }
@@ -258,36 +248,25 @@ struct ReadsBox {
 impl Widget for ReadsBox {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
-        Size::from_px(painter.answer_px_size().div_int(4))
+        Size::from_px(painter.px_size().div_int(4))
     }
 }
 
 /// Reads its box across one axis only, so its drawing holds for a taller
 /// box on its own and only a wider one is worth a draw.
 ///
-/// Both report a quarter of what they read. Their empty drawings are
-/// independent of that read, so a changed question costs one draw.
+/// 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>>,
 }
 
-struct ReadsDrawingWidth {
-    draws: Rc<Cell<usize>>,
-}
-
-impl Widget for ReadsDrawingWidth {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        self.draws.set(self.draws.get() + 1);
-        painter.px_len(Axis::X);
-        Size::LEFTOVER
-    }
-}
-
 impl Widget for ReadsWidth {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
         Size::from_px(PxVec2::new(
-            painter.answer_px_len(Axis::X).div_int(4),
+            painter.px_len(Axis::X).div_int(4),
             Px::from_int(20),
         ))
     }
@@ -343,7 +322,6 @@ fn a_row_moves_what_follows_a_child_that_grew_rather_than_drawing_it() {
             draws: ruled.clone(),
             size: Size::LEFTOVER,
             reads_box: false,
-            reads_answer_box: false,
         };
         let second = match declared {
             true => second.width(rel(0.25)).add(&mut h.rsc),
@@ -521,7 +499,7 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     // Every wrapper up to the outer pad read the size below it, so the outer
     // pad is what draws again -- and the span it hands the box to is the same
     // size as before, which is what lets a draw reuse its way past the leaf.
-    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), false);
+    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
     let padded = leaf.pad(10).add(&mut h.rsc);
     let below = rect(Color::RED).add(&mut h.rsc);
     h.set_root((padded, below).span(Dir::DOWN).pad(12));
@@ -899,7 +877,7 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
+                [None; 2],
                 [
                     Place::Within(Part::From(self.region.x)),
                     Place::Within(Part::From(self.region.y)),
@@ -988,7 +966,7 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                self.region,
+                [Some(self.region.x.len()), None],
                 [
                     Place::Fill(Part::From(self.extent.x)),
                     Place::Fill(Part::From(self.extent.y)),
@@ -1087,7 +1065,7 @@ fn a_declared_size_change_stops_at_an_independent_parent() {
 fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     let mut h = Harness::new((400, 200));
     let draws = Rc::new(Cell::new(0));
-    let leaf = ReadsDrawingWidth {
+    let leaf = ReadsWidth {
         draws: draws.clone(),
     }
     .add(&mut h.rsc);
@@ -1098,7 +1076,7 @@ fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     h.frame();
 
     assert!(draws.get() > settled);
-    assert_corners!(h, leaf, (0, 0), (800, 200));
+    assert_corners!(h, leaf, (300, 90), (500, 110));
 }
 
 #[test]
@@ -1158,7 +1136,7 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
+                [None; 2],
                 [
                     Place::Fill(Part::From(self.extent.x)),
                     Place::Fill(Part::From(self.extent.y)),
@@ -1233,7 +1211,7 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
     struct Measured;
     impl Widget for Measured {
         fn draw(&mut self, painter: &mut Painter) -> Size {
-            let width = painter.answer_px_len(Axis::X);
+            let width = painter.px_len(Axis::X);
             painter.primitive(RectPrimitive::color(Color::BLUE));
             Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
         }
@@ -1246,7 +1224,7 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
+                [None; 2],
                 [
                     Place::Fill(Part::From(UiSpan::new(
                         Len::px(self.start),
@@ -1288,7 +1266,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    UiRegion::FULL,
+                    [None; 2],
                     [
                         Place::Within(Part::From(self.region.x)),
                         Place::Within(Part::From(self.region.y)),
@@ -1308,7 +1286,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 painter
                     .widget_at(
                         &self.child,
-                        UiRegion::FULL,
+                        [None; 2],
                         [
                             Place::Fill(Part::From(self.extent.x)),
                             Place::Fill(Part::From(self.extent.y)),
@@ -1333,10 +1311,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 } else {
                     Size::from((80, 27))
                 };
-                let (leaf, _) = match fractional {
-                    true => counted(h, size, false),
-                    false => answer_counted(h, size),
-                };
+                let (leaf, _) = counted(h, size, !fractional);
                 let child = Container {
                     child: leaf.add_strong(&mut h.rsc),
                     region,

tests/cases/scroll.rs

@@ -145,3 +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

@@ -733,6 +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
@@ -846,3 +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(),