Re-ask a dirty widget at its offer locally instead of deferring to its parent

A local redraw refused any widget whose given box was not as long as its
offer and marked its parent instead. Under the frame/extent protocol that
is nearly every widget beneath a self-sized container: a span hands its
children its own placement across itself, which is `FULL` while the span
is measured and its answer once it is placed, so the children's offer and
given frames differ on every such axis. A `many` frame at seed 13, depth
8 escalated 43 marks along chains up to seven levels and redrew 508 of
583 active widgets where e44dea3 redraws 159.

Retain the offer's frame beside the given one and ask the offer question
locally: the offer frame composed where the given one is, at the offer's
lengths and placement, then place at the given box where the two differ.
Seed 13 `many` goes from 4.73 ms to 1.28 ms against e44dea3's 0.90, and
294 distinct widgets a frame; size, scroll and repaint are unchanged.

Not sound yet: the suite, the debug oracle and the shrinker at 400 trees
of depth 5 pass, but the oracle at 1000 seeds of depth 6 diverges on seed
532 under reorder and seed 398 under every-size. Both reduce to a
self-sized container whose answer changes under a local redraw; the
reduced plans are in docs/HANDOFF.md of ai-app-2.

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

core/src/layout_diagnostics.rs

@@ -32,6 +32,7 @@ pub(crate) enum Counter {
     SizeReads,
     HintHits,
     HintMisses,
+    RetainedSizeHits,
     ReuseAttempts,
     ReuseExact,
     ReuseMoved,
@@ -41,6 +42,7 @@ pub(crate) enum Counter {
     ReuseOutside,
     ReuseWrongLayer,
     ReuseWrongNode,
+    PlaceRedraws,
     QueuePops,
     DepthReads,
     LocalRedraws,
@@ -52,7 +54,7 @@ pub(crate) enum Counter {
     TextShapes,
     TextBreaks,
     GlyphPlacements,
-    OutsidePinnedLen,
+    OutsidePlacement,
     OutsideFrame,
     OutsideExtent,
 }
@@ -68,6 +70,7 @@ impl Counter {
         "draw-result size reads",
         "hint hits",
         "hint misses",
+        "retained size hits",
         "reuse attempts",
         "reuse exact",
         "reuse moved",
@@ -77,6 +80,7 @@ impl Counter {
         "reuse: outside what it holds for",
         "reuse: another layer",
         "reuse: region-node choice changed",
+        "placed by redrawing",
         "redraw queue pops",
         "depth reads",
         "local redraws",
@@ -88,7 +92,7 @@ impl Counter {
         "text shapes",
         "text line breaks",
         "glyph placements",
-        "reuse outside: the length it was pinned to",
+        "reuse outside: the placement it was pinned to",
         "reuse outside: a frame length",
         "reuse outside: an extent length",
     ];

core/src/ui/active.rs

@@ -1,5 +1,5 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive,
+    DrawRegion, LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, RegionAlign, RetainedPrimitive,
     Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
@@ -9,39 +9,31 @@ use crate::{
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
-    /// Where its drawing goes, in its region node's coordinates.
-    pub extent: 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 box its parent gave it, in `parent_move`'s coordinates: what it
+    /// was asked about, and what a fraction under it is a fraction of. A
+    /// local redraw asks here.
+    pub region: UiRegion,
+    /// Where its drawing sits inside that box, in the box's own coordinates.
+    pub placement: UiRegion,
+    /// The original frame in its parent widget's coordinates. Recomposition
+    /// and pixel-length evaluation both follow this chain.
+    pub given_region: UiRegion,
+    /// The frame it was first asked in, in the same coordinates: the offer's
+    /// frame, which its parent's placing draw may since have narrowed.
+    pub offer_region: UiRegion,
+    /// The lengths of the box its parent first asked about it in, as
+    /// lengths of the box the parent was itself offered. Any later box it
+    /// was given was decided knowing its answer, so this is the question
+    /// asked again -- and a chain of fractions has no frame in it, which is
+    /// why a region node between two widgets cannot break it.
+    pub offer_len: UiVec2,
+    pub offer_placement: [Option<crate::UiSpan>; 2],
     /// The measured answer and its dependencies. A hint-only dependency or
     /// a widget first encountered during placement has no measurement yet.
     pub answer: Option<(Size, LayoutHolds)>,
-    /// Asked more than once in its parent's last draw -- measured in one box
-    /// and then asked in the one the parent decided. The parent's layout
-    /// rests on the first answer and its drawing on the last, so only the
-    /// parent can ask either again.
-    pub re_asked: bool,
-    /// What the widget reported, in window-unit lengths.
+    /// What the widget said it used of its box, the last time it drew.
     pub size: Size,
-    /// The window and extent reads that this drawing holds for, and the
-    /// frame and box it pinned.
+    /// The frame, extent and explicit placement reads that this drawing holds for.
     pub holds: LayoutHolds,
     pub drawn: bool,
     pub parent: Option<WidgetId>,
@@ -50,25 +42,30 @@ pub struct ActiveData {
     /// widget a frame visits and cannot drift while one is being drawn.
     pub depth: usize,
     pub textures: Vec<TextureHandle>,
-    /// Its primitives, each keeping the box it was written in -- in this
-    /// widget's extent coordinates, which is what a move recomposes from.
     pub primitives: Vec<RetainedPrimitive>,
-    pub mask_region: Option<UiRegion>,
+    pub mask_region: Option<DrawRegion>,
+    /// The children whose box is a part of this widget's extent rather than
+    /// of its frame, and which part each was given. Moving the extent
+    /// re-places them through that part, so the drawing need not depend on
+    /// where it sits.
+    pub(crate) extent_children: Vec<(WidgetId, ExtentPlacement)>,
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
     /// The movable region its primitives are positioned through: its own when
     /// opted in, otherwise the nearest ancestor's.
     pub move_idx: MoveIdx,
-    /// The declared lengths whoever drew this widget resolved into its frame.
+    /// The declared lengths whoever drew this widget resolved into its box.
     /// A change to one moves a box this widget cannot fix by drawing again,
     /// and comparing them is what says so.
     pub declared: [Option<LayoutLen>; 2],
+    /// The axes along which its parent chose its box from its own answer,
+    /// so a local redraw asks the question its parent asked.
+    pub decided: [bool; 2],
     /// 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 `extent` uses when this widget
-    /// does not own a region node.
+    /// The movable region whose coordinates `region` uses.
     pub parent_move: MoveIdx,
     /// The mask its drawing is clipped to: one it set itself, or the one it
     /// inherited from whoever drew it.
@@ -82,10 +79,37 @@ pub struct ActiveData {
 }
 
 impl ActiveData {
-    /// 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 box of these pixel
+    /// lengths -- the box it was asked in, where `holds` is about the box its
+    /// answer then chose.
+    pub fn answers_at(&self, px: crate::PxVec2) -> bool {
+        self.answer.is_some_and(|(_, holds)| {
+            holds.contains(
+                px,
+                UiRegion {
+                    x: self.offer_placement[0].unwrap_or(crate::UiSpan::FULL),
+                    y: self.offer_placement[1].unwrap_or(crate::UiSpan::FULL),
+                },
+            )
+        })
+    }
+}
+
+/// What of a container's extent a child was given: the whole of it, for a
+/// wrapper whose box is its child's, or a part of it.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub(crate) enum ExtentPlacement {
+    Inherit,
+    Within(UiRegion),
+}
+
+impl ExtentPlacement {
+    /// The child's frame in the container's frame coordinates, and the slot
+    /// the container chose within it.
+    pub fn resolve(self, extent: UiRegion) -> (UiRegion, [Option<crate::UiSpan>; 2]) {
+        match self {
+            Self::Inherit => (UiRegion::FULL, [Some(extent.x), Some(extent.y)]),
+            Self::Within(part) => (part.within(&extent), [None; 2]),
+        }
     }
 }

core/src/ui/draw_region.rs

@@ -0,0 +1,36 @@
+use crate::{PrimitiveHandle, UiRegion};
+
+/// Retains which box geometry follows when only the extent changes.
+#[derive(Clone, Copy, Debug)]
+pub enum DrawRegion {
+    Frame(UiRegion),
+    Extent(UiRegion),
+}
+
+impl DrawRegion {
+    pub(crate) fn resolve(self, frame: UiRegion, extent: UiRegion) -> UiRegion {
+        match self {
+            Self::Frame(local) => local.within(&frame),
+            Self::Extent(local) => local.within(&extent).within(&frame),
+        }
+    }
+
+    pub(crate) fn map(self, f: impl FnOnce(UiRegion) -> UiRegion) -> Self {
+        match self {
+            Self::Frame(local) => Self::Frame(f(local)),
+            Self::Extent(local) => Self::Extent(f(local)),
+        }
+    }
+}
+
+impl From<UiRegion> for DrawRegion {
+    fn from(region: UiRegion) -> Self {
+        Self::Frame(region)
+    }
+}
+
+#[derive(Debug)]
+pub struct RetainedPrimitive {
+    pub handle: PrimitiveHandle,
+    pub region: DrawRegion,
+}

core/src/ui/layout_holds.rs

@@ -1,79 +1,63 @@
-use crate::{Axis, Holds, Len, PxVec2, UiRegion, UiVec2};
+use crate::{Axis, Holds, PxVec2, UiRegion};
 
-const AXES: [Axis; 2] = [Axis::X, Axis::Y];
-
-/// 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 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. 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.
+/// Dependencies of one evaluation, before the frame and extent are composed.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct LayoutHolds {
-    pub window: [Holds; 2],
-    pub frame_len: [Option<Len>; 2],
+    pub frame: [Holds; 2],
     pub extent: [Holds; 2],
-    pub extent_len: [Option<Len>; 2],
+    pub placement: Option<UiRegion>,
 }
 
 impl LayoutHolds {
     pub const ANY: Self = Self {
-        window: [Holds::ANY; 2],
-        frame_len: [None; 2],
+        frame: [Holds::ANY; 2],
         extent: [Holds::ANY; 2],
-        extent_len: [None; 2],
+        placement: None,
     };
 
     pub fn and(self, other: Self) -> Self {
-        let mut result = Self::ANY;
-        for n in 0..2 {
-            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]
+            self.placement.is_none()
+                || other.placement.is_none()
+                || self.placement == other.placement
         );
-            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]);
+        Self {
+            frame: [
+                self.frame[0].and(other.frame[0]),
+                self.frame[1].and(other.frame[1]),
+            ],
+            extent: [
+                self.extent[0].and(other.extent[0]),
+                self.extent[1].and(other.extent[1]),
+            ],
+            placement: self.placement.or(other.placement),
         }
-        result
     }
 
     pub fn covers(self, other: Self) -> bool {
-        (0..2).all(|n| {
-            self.window[n].lo <= other.window[n].lo
-                && self.window[n].hi >= other.window[n].hi
+        self.placement
+            .is_none_or(|placement| other.placement == Some(placement))
+            && [0, 1].into_iter().all(|n| {
+                self.frame[n].lo <= other.frame[n].lo
+                    && self.frame[n].hi >= other.frame[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, window: PxVec2, frame: UiVec2, extent: UiRegion) -> bool {
-        AXES.into_iter().all(|axis| {
-            let n = axis as usize;
-            let len = extent.axis(axis).len();
-            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)
+    pub fn contains(self, px: PxVec2, placement: UiRegion) -> bool {
+        self.placement.is_none_or(|old| old == placement)
+            && [Axis::X, Axis::Y].into_iter().all(|axis| {
+                self.frame[axis as usize].contains(px.axis(axis))
+                    && self.extent[axis as usize]
+                        .contains(placement.axis(axis).len().to_px(px.axis(axis)))
+            })
+    }
+
+    pub fn in_frame(self, placement: UiRegion) -> [Holds; 2] {
+        [Axis::X, Axis::Y].map(|axis| {
+            self.frame[axis as usize]
+                .and(self.extent[axis as usize].through(placement.axis(axis).len()))
         })
     }
 }

core/src/ui/mod.rs

@@ -10,17 +10,17 @@ use crate::{
 pub const CHAIN_LIMIT: u32 = 64;
 
 mod active;
+mod draw_region;
 mod holds;
 mod layout_holds;
 mod painter;
-mod place;
 mod render_state;
 
 pub use active::*;
+pub use draw_region::*;
 pub use holds::*;
 pub use layout_holds::*;
 pub use painter::{Painter, PrimitiveLike};
-pub use place::*;
 pub use render_state::*;
 
 #[derive(Default)]

core/src/ui/painter.rs

@@ -1,14 +1,14 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Holds, LayoutHolds, LayoutLen, Len, Part, Place, Px, PxVec2, RegionAlign, Rel,
+    Axis, DrawRegion, ExtentPlacement, Holds, LayoutHolds, LayoutLen, Len, Px, PxVec2, RegionAlign,
     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, Placing},
+    ui::render_state::DrawInfo,
 };
 const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 
@@ -17,40 +17,51 @@ pub struct Painter<'a> {
     pub(super) state: &'a mut UiRenderState,
     pub(super) rsc: &'a mut dyn UiRsc,
 
-    /// This widget's frame, per axis: a length of the window, and what a
-    /// fraction it or anything under it declares or reports is a fraction
-    /// of. A length rather than a box, so padding can take from both the
-    /// frame and the box without either becoming the other.
-    pub(super) frame: UiVec2,
-    /// Where this widget's drawing goes, in its region node's coordinates.
-    pub(super) extent: UiRegion,
-    /// The extent's symbolic length where this draw read it, which makes the
-    /// drawing one that holds for that length alone -- the way reading a
-    /// length in pixels makes it hold for that number of pixels.
-    pub(super) extent_len: [Option<Len>; 2],
-    /// The window in pixels. Frames and boxes become pixels against this one
-    /// unit, regardless of region-node boundaries.
-    pub(super) window: PxVec2,
+    /// The box its parent gave it, in the coordinates of `move_idx`: what a
+    /// fraction of this widget's area is a fraction of, and what every region
+    /// it writes composes within. The same box on the ask that measures and
+    /// the ask that places, which is what keeps a fraction under it from
+    /// being resolved twice.
+    pub(super) region: UiRegion,
+    /// Where this widget's drawing sits inside that box, in the box's own
+    /// coordinates: `FULL` while its answer is not yet known, and the box
+    /// its answer or its parent chose once one of them has.
+    pub(super) placement: UiRegion,
+    /// Whether this draw read its placement, which makes the drawing one
+    /// that holds for that placement alone -- the way reading a length in
+    /// pixels makes it hold for that length.
+    pub(super) reads_placement: bool,
+    /// That box in pixels, which its children's are a length of: threaded
+    /// down from the box this widget was given rather than composed back up
+    /// the chain, so every length in layout is one multiply from its
+    /// parent's and [`Holds::through`] inverts exactly that.
+    pub(super) px: PxVec2,
     pub(super) mask: MaskIdx,
     pub(super) textures: Vec<TextureHandle>,
     pub(super) primitives: Vec<RetainedPrimitive>,
-    pub(super) mask_region: Option<UiRegion>,
+    pub(super) mask_region: Option<DrawRegion>,
+    pub(super) extent_children: Vec<(WidgetId, ExtentPlacement)>,
+    pub(super) extent_own: [Holds; 2],
     /// 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 box each was asked in
+    /// is the one recorded as its offer.
+    pub(super) offered: Vec<WidgetId>,
+    /// The lengths of the box this widget was first asked about in, in
+    /// pixels. Its children's offers are a fraction of it.
+    pub(super) offered_px: PxVec2,
+    /// Whether this draw is in a box of those lengths, 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 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],
-    /// 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)>,
+    /// What this draw itself read of its box in pixels, per axis: every
+    /// length until it reads one, then that one, unless it says otherwise.
+    pub(super) own: [Holds; 2],
+    /// Dependencies of every child drawing, including unmeasured overlays.
+    pub(super) under: LayoutHolds,
     /// The movable region this widget's primitives are positioned through:
     /// its own when opted in, otherwise the nearest ancestor's.
     pub(super) move_idx: MoveIdx,
@@ -63,27 +74,26 @@ pub struct Painter<'a> {
 }
 
 impl<'a> Painter<'a> {
-    fn primitive_at<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
+    fn primitive_at<P: Primitive>(&mut self, primitive: P, region: DrawRegion) {
         let kind = self.rsc.ui_mut().primitives.kind::<P>();
         self.write(kind, primitive, region);
     }
 
     /// Takes the kind, for a caller writing many of one primitive.
-    fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: UiRegion) {
-        self.write_resolved(kind, primitive, region, self.resolve(region));
-    }
-
-    /// 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)
+    fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: DrawRegion) {
+        self.write_resolved(
+            kind,
+            primitive,
+            region,
+            region.resolve(self.region, self.placement),
+        );
     }
 
     fn write_resolved<P: Primitive>(
         &mut self,
         kind: PrimitiveKind<P>,
         primitive: P,
-        region: UiRegion,
+        region: DrawRegion,
         resolved: UiRegion,
     ) {
         #[cfg(feature = "layout-diagnostics")]
@@ -112,107 +122,54 @@ impl<'a> Painter<'a> {
 
     /// Writes a primitive over the whole of this widget's own box.
     pub fn primitive(&mut self, primitive: impl PrimitiveLike) {
+        let at = DrawRegion::Extent(UiRegion::FULL);
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, UiRegion::FULL)
+        self.primitive_at(primitive, at)
     }
 
-    /// Writes a primitive in a part of this widget's own box, in that box's
-    /// coordinates.
-    pub fn primitive_within(&mut self, primitive: impl PrimitiveLike, region: UiRegion) {
+    /// Writes in the frame by default. `DrawRegion::Extent` keeps the local
+    /// geometry attached to this widget's box without reading its placement.
+    pub fn primitive_within(
+        &mut self,
+        primitive: impl PrimitiveLike,
+        region: impl Into<DrawRegion>,
+    ) {
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, region);
+        self.primitive_at(primitive, region.into());
     }
 
-    /// Sets a mask, in this widget's own box's coordinates.
-    pub fn set_mask(&mut self, region: UiRegion) {
+    /// Sets a mask in the selected frame or extent coordinates.
+    pub fn set_mask(&mut self, region: impl Into<DrawRegion>) {
+        let region = region.into();
         self.mask_region = Some(region);
         assert!(self.mask == MaskIdx::NONE);
-        let resolved = self.resolve(region);
-        let move_idx = self.move_idx;
         self.mask = self.rsc.ui_mut().masks.push(Mask {
-            region: resolved,
-            move_idx,
+            region: region.resolve(self.region, self.placement),
+            move_idx: self.move_idx,
         });
     }
 
-    /// Draws a widget in the whole of this widget's own box, with the frame
-    /// forwarded unchanged: what a container that is only a wrapper around
-    /// one child wants, and what every transparent container passes for the
-    /// frame.
+    /// Draws a widget in the whole of this widget's own box: it gets the
+    /// same region -- the same area for its fractions to be of -- and is put
+    /// where this widget was put. What a container that is only a wrapper
+    /// around one child wants, since its box is the child's.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, [None; 2], [Place::Within(Part::All); 2])
+        let own = self.placement;
+        self.widget_at_inner(
+            id,
+            UiRegion::FULL,
+            [Some(own.x), Some(own.y)],
+            Some(ExtentPlacement::Inherit),
+            false,
+        )
     }
 
-    /// Asks a child, saying what its fractions are of and where it is asked.
-    ///
-    /// `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 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>,
-        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 (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, extent);
-        }
-        // A child listed twice would be moved twice.
-        let re_asked = self.children.contains(&id.id());
-        if !re_asked {
-            self.children.push(id.id());
-        }
-        let px = frame.to_px(self.window);
-        let (size, answer_holds, holds) = self.state.draw_inner(
-            id.id(),
-            DrawInfo {
-                layer: self.layer,
-                parent: Some(self.id),
-                depth: self.depth + 1,
-                parent_move: self.move_idx,
-                region_node,
-                mask: self.mask,
-                frame,
-                part: extent,
-                placed: place,
-                asked: place,
-                narrow,
-                re_asked,
-                px,
-            },
-            None,
-            self.rsc,
-        );
-        let holds = self.in_parent(holds, extent, place, narrow, declared);
-        let answer_holds = self.in_parent(answer_holds, extent, place, narrow, declared);
-        match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
-            Some((_, kept)) => *kept = holds,
-            None => self.under.push((id.id(), holds)),
-        }
-        DrawResult {
-            child: id,
-            painter: self,
-            size,
-            answer_holds,
-        }
+    /// What a widget's rules declare its lengths to be, which whoever draws
+    /// it resolves into its box. Reading them depends on nothing -- the box
+    /// that comes of them is kept on the child, and `redraw` compares it
+    /// there.
+    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> [Option<LayoutLen>; 2] {
+        declared_lens(self.rsc.widgets(), id.id())
     }
 
     /// Takes back a child that was drawn only to find out how long it is.
@@ -220,49 +177,204 @@ impl<'a> Painter<'a> {
     /// this frame; what it answered is still something this widget asked.
     pub fn undraw<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
         self.children.retain(|child| *child != id.id());
-        self.under.retain(|(child, _)| *child != id.id());
+        self.extent_children.retain(|(child, _)| *child != id.id());
         self.state.undraw_rec(id.id(), self.rsc);
     }
 
-    /// Puts a child asked about in this draw somewhere else in this
-    /// widget's box: its answer, placed in this part instead. The drawing
-    /// is re-expressed there rather than made again -- what a row does once
-    /// it knows every slot, having measured each child from its cursor.
-    pub fn place_at<W: ?Sized>(&mut self, id: &StrongWidget<W>, place: [Place; 2]) {
-        debug_assert!(
-            self.children.contains(&id.id()),
-            "'{}' placed a child it did not ask about in this draw",
-            self.label()
-        );
-        let at = self.placing();
-        self.state.place_in(id.id(), &at, place, self.rsc);
+    /// Draws a child in `region`, relative to this widget's frame. The child
+    /// resolves declared lengths and reports against that frame, then places
+    /// its drawing by its own alignment.
+    ///
+    /// `DrawRegion::Extent` gives a part of where this widget's drawing sits
+    /// instead, for a container whose children belong inside that rather than
+    /// inside the box it was offered. The part is what is kept, so moving the
+    /// extent re-places the child rather than drawing this widget again.
+    pub fn widget_within<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
+        region: impl Into<DrawRegion>,
+    ) -> DrawResult<'s, 'a, W> {
+        match region.into() {
+            DrawRegion::Frame(region) => self.widget_at(id, region, [None; 2]),
+            DrawRegion::Extent(part) => {
+                let within = part.within(&self.placement);
+                self.widget_at_inner(
+                    id,
+                    within,
+                    [None; 2],
+                    Some(ExtentPlacement::Within(part)),
+                    false,
+                )
+            }
+        }
     }
 
-    /// 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,
+    /// Draws a widget in `region`, saying where in it the drawing goes.
+    ///
+    /// `region` is the child's own area: what a fraction it declares or
+    /// reports is a fraction of, and the coordinates the regions it writes
+    /// compose within. It is the same box on the ask that measures and the
+    /// ask that places, which is what stops a fraction under it being
+    /// resolved twice.
+    ///
+    /// `placement` is what of that region the child's drawing takes, per
+    /// axis, wherever this widget is choosing. `None` leaves the axis to the
+    /// child's own answer and alignment, which is what
+    /// [`Self::widget_within`] passes. A span passes the whole row as the
+    /// region, so `rel(0.5)` is half the row wherever the child sits in it,
+    /// and places the child by passing the slot along its axis.
+    pub fn widget_at<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
+        region: UiRegion,
+        placement: [Option<UiSpan>; 2],
+    ) -> DrawResult<'s, 'a, W> {
+        self.widget_at_inner(id, region, placement, None, false)
+    }
+
+    fn widget_at_inner<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
+        region: UiRegion,
+        placement: [Option<UiSpan>; 2],
+        extent: Option<ExtentPlacement>,
+        measuring: bool,
+    ) -> DrawResult<'s, 'a, W> {
+        self.extent_children.retain(|(child, _)| *child != id.id());
+        if let Some(extent) = extent {
+            self.extent_children.push((id.id(), extent));
+        }
+        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, placement) = ask_box(region, declared, align, placement);
+        let within = match local == UiRegion::FULL {
+            true => self.region,
+            false => local.within(&self.region),
+        };
+        #[cfg(feature = "layout-diagnostics")]
+        if region_node {
+            diag::bump(Counter::RegionNodeDraws);
+            diag::region_node(id.id(), self.id, within);
+        }
+        // A child listed twice would be moved twice.
+        if !self.children.contains(&id.id()) {
+            self.children.push(id.id());
+        }
+        let first_ask = self.offer(id.id());
+        let given_len = local.size();
+        let offer_len = match first_ask {
+            true => given_len,
+            false => self
+                .state
+                .active
+                .get(&id.id())
+                .map_or(given_len, |a| a.offer_len),
+        };
+        let offer_region = match first_ask {
+            true => local,
+            false => self
+                .state
+                .active
+                .get(&id.id())
+                .map_or(local, |a| a.offer_region),
+        };
+        let offer_placement = if first_ask {
+            placement
+        } else {
+            self.state
+                .active
+                .get(&id.id())
+                .map_or(placement, |a| a.offer_placement)
+        };
+        let px = given_len.to_px(self.px);
+        let offered_px = offer_len.to_px(self.offered_px);
+        // The answer and what it holds for, both about the box asked in. The
+        // child's record may say something else once its drawing has been
+        // placed: a drawing made again in its placed box holds for that box.
+        let (size, answer_holds, holds) = self.state.draw_inner(
+            id.id(),
+            within,
+            DrawInfo {
+                layer: self.layer,
+                parent: Some(self.id),
+                depth: self.depth + 1,
+                parent_move: self.move_idx,
+                region_node,
                 mask: self.mask,
+                given_region: local,
+                offer_region,
+                offer_len,
+                offer_placement,
+                px,
+                offered_px,
+                placement,
+            },
+            None,
+            measuring,
+            self.rsc,
+        );
+        let in_parent = |holds: LayoutHolds| {
+            let mut result = LayoutHolds::ANY;
+            for axis in AXES {
+                let n = axis as usize;
+                let chosen = placement[n].unwrap_or(UiSpan::FULL).len();
+                match extent {
+                    // Its box is this widget's own, so what its drawing holds
+                    // for is what this widget's extent holds for.
+                    Some(ExtentPlacement::Inherit) if declared[n].is_none() => {
+                        result.frame[n] = holds.frame[n].through(local.axis(axis).len());
+                        result.extent[n] = holds.extent[n];
+                        if holds.placement.is_some() {
+                            result.placement = Some(self.placement);
+                        }
+                    }
+                    // Its box is a part of this widget's extent, so what it
+                    // holds for is a range on that extent and none of it a
+                    // range on the frame. Only the part's length reaches it,
+                    // which is what lets the extent move without a redraw.
+                    Some(ExtentPlacement::Within(part)) if declared[n].is_none() => {
+                        result.extent[n] = holds.frame[n]
+                            .and(holds.extent[n].through(chosen))
+                            .through(part.axis(axis).len());
+                    }
+                    // Its box is a length of this widget's frame: an
+                    // ordinary ask, or a declared length, which is that
+                    // length wherever the box it sits in came from.
+                    _ => {
+                        result.frame[n] = holds.frame[n].through(local.axis(axis).len()).and(
+                            holds.extent[n]
+                                .through(chosen)
+                                .through(local.axis(axis).len()),
+                        );
+                    }
+                }
+            }
+            result
+        };
+        self.under = self.under.and(in_parent(holds));
+        let mut answer_holds = in_parent(answer_holds);
+        // What it reports is a fraction of the box it was given, which is a
+        // part of this widget's extent -- so the same fraction is a different
+        // length once that extent is, and pixels are not. The answer only:
+        // the drawing this holds is re-placed rather than made again.
+        if matches!(extent, Some(ExtentPlacement::Within(_)))
+            && AXES.into_iter().any(|axis| {
+                declared[axis as usize].is_none() && size.axis(axis).rel != crate::Rel::ZERO
+            })
+        {
+            answer_holds.placement = Some(self.placement);
+        }
+        DrawResult {
+            child: id,
+            painter: self,
+            size: in_parent_frame(size, local.size(), declared),
+            answer_holds,
         }
     }
 
-    /// 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
-    /// there.
-    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> [Option<LayoutLen>; 2] {
-        declared_lens(self.rsc.widgets(), id.id())
-    }
-
-    /// 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.
+    /// What a child says its length is without being drawn, if it can say.
+    /// 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
@@ -272,27 +384,81 @@ impl<'a> Painter<'a> {
                 .get_dyn(id.id())
                 .and_then(|widget| widget.size_hint(axis))
         });
-        let frame = self.frame.axis(axis);
-        let resolved = hint.map(|hint| hint.within_len(frame));
         #[cfg(feature = "layout-diagnostics")]
-        {
-            diag::hint_read(id.id(), self.id, axis, resolved);
-            diag::bump(match resolved {
-                Some(_) => Counter::HintHits,
-                None => Counter::HintMisses,
-            });
-        }
-        if let Some(hint) = hint {
+        diag::hint_read(id.id(), self.id, axis, hint);
+        match hint {
+            Some(hint) => {
+                #[cfg(feature = "layout-diagnostics")]
+                diag::bump(Counter::HintHits);
                 self.depend_on(id);
-            // Resolving a fraction against this frame makes this draw a
-            // function of the frame's length. The fraction to ask about is
-            // the child's own: resolved against a frame of pixels, none is
-            // left to see it by.
-            if hint.rel != Rel::ZERO {
-                self.frame_own_len[axis as usize] = Some(frame);
+                Some(hint)
+            }
+            None => {
+                #[cfg(feature = "layout-diagnostics")]
+                diag::bump(Counter::HintMisses);
+                None
             }
         }
-        resolved
+    }
+
+    /// Measures a child's length from its hint, a retained answer, or `draw`.
+    /// A fresh draw evaluates the offer without placing its answer. The caller
+    /// must later place or undraw the child.
+    pub fn measure_len<W: ?Sized>(
+        &mut self,
+        child: &StrongWidget<W>,
+        axis: Axis,
+        region: UiRegion,
+        placement: [Option<UiSpan>; 2],
+    ) -> LayoutLen {
+        let offered = placement;
+        let declared = self.declared_lens(child);
+        let align = self.rsc.widgets().alignment(child.id());
+        let (local, placement) = ask_box(region, declared, align, placement);
+        let first_ask = self.at_offer && !self.offered.contains(&child.id());
+
+        if let Some(hint) = self.size_hint(child, axis) {
+            return hint;
+        }
+        let px = local.size().to_px(self.px);
+        let retained =
+            self.state
+                .retained_size(child.id(), px, placement, self.move_idx, self.rsc.widgets());
+        let Some((size, holds)) = retained else {
+            return self
+                .widget_at_inner(child, region, offered, None, true)
+                .len(axis);
+        };
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::RetainedSizeHits);
+        self.depend_on(child);
+        if first_ask {
+            self.offered.push(child.id());
+            let active = self.state.active.get_mut(&child.id()).unwrap();
+            active.offer_len = local.size();
+            active.offer_region = local;
+            active.offer_placement = placement;
+        }
+        let placement = UiRegion {
+            x: placement[0].unwrap_or(UiSpan::FULL),
+            y: placement[1].unwrap_or(UiSpan::FULL),
+        };
+        let holds = holds.in_frame(placement);
+        for (axis, under) in AXES.into_iter().zip(self.answer_under.frame.iter_mut()) {
+            *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
+        }
+        in_parent_frame(size, local.size(), declared).axis(axis)
+    }
+
+    /// Whether this is the first box a child is asked about in during a draw
+    /// that is itself in the box it was asked in -- the question a cold
+    /// layout asks, whose answer is the one to keep.
+    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>) {
@@ -315,10 +481,11 @@ impl<'a> Painter<'a> {
 
     /// Writes glyphs in the selected frame or extent coordinates.
     // TODO: merge the text methods into the primitive ones.
-    pub fn glyphs(&mut self, text: &RenderedText, origin: UiRegion) {
+    pub fn glyphs(&mut self, text: &RenderedText, origin: impl Into<DrawRegion>) {
+        let origin = origin.into();
         // Glyph offsets and sizes are pixels, which compose additively.
-        // Only the shared origin needs composing through the extent.
-        let resolved = self.resolve(origin);
+        // Only the shared origin needs the frame/extent composition.
+        let resolved = origin.resolve(self.region, self.placement);
         let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {
             let place = |mut region: UiRegion| {
@@ -342,33 +509,30 @@ impl<'a> Painter<'a> {
                     color: text.color,
                     flags: glyph.entry.flags(),
                 },
-                place(origin),
+                origin.map(place),
                 place(resolved),
             );
         }
     }
 
-    /// The symbolic length of this widget's own box along one axis, in the
-    /// lengths of its frame that it places its children in. Reading it pins
-    /// the drawing to that length -- and to nothing about where the box
-    /// starts, which is what lets a container move without being drawn
-    /// again. One axis at a time, because a container that divides one axis
-    /// holds for any length of the other.
-    pub fn extent_len(&mut self, axis: Axis) -> Len {
-        let len = self.extent.axis(axis).len();
-        self.extent_len[axis as usize] = Some(len);
-        len
+    /// The box this widget's parent gave it, in the coordinates its own
+    /// primitives are written in -- so a region composed `within` it may be
+    /// drawn directly. Its own box is [`Self::placement`] of this one.
+    pub fn region(&self) -> UiRegion {
+        self.region
     }
 
-    /// The symbolic length of this widget's frame along one axis: what a
-    /// fraction it or anything under it declares is a fraction of. A
-    /// container reads it to hand a length of it down -- padding, which
-    /// takes its pixels off. Reading it pins the drawing to that frame, the
-    /// way [`Self::extent_len`] pins it to the box.
-    pub fn frame_len(&mut self, axis: Axis) -> Len {
-        let len = self.frame.axis(axis);
-        self.frame_own_len[axis as usize] = Some(len);
-        len
+    /// Where this widget's drawing goes inside the box it was given, in that
+    /// box's coordinates: what its own answer took of it, or what its parent
+    /// chose for it. `FULL` on the ask that measures, since nothing has been
+    /// placed yet.
+    ///
+    /// Reading it is what says the drawing depends on it, so a widget that
+    /// positions its own content reads it and is drawn again once its box is
+    /// known, and one that fills whatever it is given never is.
+    pub fn placement(&mut self) -> UiRegion {
+        self.reads_placement = true;
+        self.placement
     }
 
     /// Where this widget sits in a box longer than the length it takes. A
@@ -396,6 +560,14 @@ impl<'a> Painter<'a> {
             .is_some()
     }
 
+    /// The part of this widget's box that something of `size` takes, at the
+    /// near edge. A container that reports one child's size gives every child
+    /// this, so what it draws is inside what it says it occupies.
+    pub fn box_of(&self, size: Size) -> UiRegion {
+        let lens = placed_lens(size, [None; 2], [false; 2]);
+        placed_box(UiRegion::FULL, lens, RegionAlign::NEAR)
+    }
+
     /// This widget's own box in pixels. Reading it makes the drawing one
     /// that holds for this box only, until `holds` says how far it goes.
     pub fn px_size(&mut self) -> PxVec2 {
@@ -405,8 +577,8 @@ impl<'a> Painter<'a> {
     /// 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.window.axis(axis));
+        let part = self.placement.axis(axis).len();
+        let len = part.to_px(self.px.axis(axis));
         let own = &mut self.extent_own[axis as usize];
         if *own == Holds::ANY {
             *own = Holds::at(len);
@@ -419,10 +591,10 @@ impl<'a> Painter<'a> {
     /// of the box, and the same reported size. A widget that read its length
     /// in pixels holds for that one alone until it says otherwise.
     pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
-        let part = self.extent.axis(axis).len();
+        let part = self.placement.axis(axis).len();
         let holds = holds.into();
         debug_assert!(
-            holds.contains(part.to_px(self.window.axis(axis))),
+            holds.contains(part.to_px(self.px.axis(axis))),
             "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
             self.label(),
             self.id
@@ -430,34 +602,30 @@ impl<'a> Painter<'a> {
         self.extent_own[axis as usize] = holds;
     }
 
-    /// 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];
+    /// One axis of the box this widget's parent gave it, in pixels -- what a
+    /// fraction of its area resolves against, and so what a container divides
+    /// among its children. Its own box is a part of this one.
+    pub fn region_px_len(&mut self, axis: Axis) -> Px {
+        let len = self.px.axis(axis);
+        let own = &mut self.own[axis as usize];
         if *own == Holds::ANY {
-                *own = Holds::at(window);
+            *own = Holds::at(len);
         }
-        }
-        len.to_px(window)
+        len
     }
 
-    /// 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>) {
+    /// [`Self::holds`] stated about the region rather than about this
+    /// widget's own box, for a container whose drawing turns on the box it
+    /// was given rather than on the part of it it took.
+    pub fn region_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
         let holds = holds.into();
         debug_assert!(
-            holds.contains(self.window.axis(axis)),
-            "'{}' ({:?}) says its drawing holds for windows that leave out this one",
+            holds.contains(self.px.axis(axis)),
+            "'{}' ({:?}) says its drawing holds for lengths that leave out its region",
             self.label(),
             self.id
         );
-        self.window_own[axis as usize] = holds;
+        self.own[axis as usize] = holds;
     }
 
     pub fn text_data(&mut self) -> &mut TextData {
@@ -545,77 +713,19 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
-/// Moves what a child depends on into this widget's own terms: this
-/// method's `impl` block is where a `Painter`'s own boxes are, so it takes
-/// only what the child was asked with.
-impl Painter<'_> {
-    /// Window ranges are already about the one unit and combine directly.
-    /// A frame pin becomes this widget's own frame wherever a length of it
-    /// is what reached the child; where only pixels did, no length of this
-    /// frame can change the child's and the pin stops here.
-    ///
-    /// Extent validity maps back through the part of this widget's box,
-    /// where the box the child was asked in is that part; a declared length
-    /// places the box inside the part instead, and then only that length
-    /// reaches the child. A narrowed frame is not one of these: it decides
-    /// what fractions under the child mean and leaves the box the part it
-    /// was given.
-    fn in_parent(
-        &self,
-        holds: LayoutHolds,
-        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;
-            // 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. 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 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.window[n] =
-                        result.window[n].and(holds.extent[n].through(extent.axis(axis).len()));
+/// 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));
         }
     }
-        }
-        result
-    }
+    size
 }
 
 /// What a widget declares a length of its box to be. `leftover` is not one: a
@@ -649,89 +759,68 @@ pub(crate) fn fills(reported: LayoutLen, declared: Option<LayoutLen>, decided: b
     reported.leftover != Weight::ZERO || declared.is_some() || decided
 }
 
-/// Where a widget's drawing goes inside the part its parent gave it: what
-/// it reported, on the side of the part its alignment says, and the whole
-/// part wherever the answer fills it.
+/// What of the box it was given a widget's drawing occupies, as lengths of
+/// that box: the size it reported wherever that is a part to be placed, and
+/// the whole of the box wherever the answer fills it.
 ///
-/// The length it reported is a length of its frame, and the part is one too,
-/// so this takes one from the other rather than composing it into the part.
-/// That is what makes a fraction the same fraction wherever the part it is
-/// placed in sits and however long it is -- the fraction is resolved once,
-/// here, against the frame it was reported of.
-pub(crate) fn placed_extent(
-    part: UiRegion,
+/// A reported fraction is a fraction of the box the widget drew in, where a
+/// declared one is a fraction of the box its parent handed down -- a span
+/// reporting `rel(1.0)` means all of what it was given, whatever that was a
+/// fraction of. So this is a length of the box rather than a length composed
+/// into it, and a box in pixels is this step from the given box's pixels.
+pub(crate) fn placed_lens(
     size: Size,
     declared: [Option<LayoutLen>; 2],
-    fill: [bool; 2],
-    align: RegionAlign,
-) -> UiRegion {
-    let mut placed = part;
-    for axis in AXES {
-        let n = axis as usize;
+    decided: [bool; 2],
+) -> UiVec2 {
+    let mut lens = UiVec2::FULL_SIZE;
+    for (axis, (declared, decided)) in AXES.into_iter().zip(declared.into_iter().zip(decided)) {
         let reported = size.axis(axis);
-        if fills(reported, declared[n], fill[n]) {
+        if !fills(reported, declared, decided) {
+            *lens.axis_mut(axis) = Len::from_parts(reported.rel, reported.px);
+        }
+    }
+    lens
+}
+
+/// Where that drawing sits: those lengths taken of the box the widget was
+/// asked in, on the side of it that the widget's alignment says.
+pub(crate) fn placed_box(region: UiRegion, lens: UiVec2, align: RegionAlign) -> UiRegion {
+    let mut placed = region;
+    for axis in AXES {
+        // The whole of the box is already where it sits, and the arithmetic
+        // below is the identity for it.
+        if lens.axis(axis) == Len::FULL {
             continue;
         }
-        let len = Len::from_parts(reported.rel, reported.px);
         let span = placed.axis_mut(axis);
+        let len = lens.axis(axis).within_len(span.len());
         span.start += (span.len() - len).scale(align.axis(axis).rel());
         span.end = span.start + len;
     }
     placed
 }
 
-/// The frame length and the box a child is asked in, in the coordinates the
-/// widget asking draws in.
-///
-/// `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(
-    own: UiRegion,
-    parent_frame: UiVec2,
-    place: [Place; 2],
-    narrow: [Option<Len>; 2],
+/// A declared axis gets a frame of that length, aligned within the parent's
+/// slot (or the offer). Undeclared axes keep the offered frame and chosen
+/// placement, so their reported fractions retain that reference.
+pub(crate) fn ask_box(
+    mut region: UiRegion,
     declared: [Option<LayoutLen>; 2],
     align: RegionAlign,
-) -> (UiVec2, UiRegion) {
-    let part = part_of(own, place, align);
-    let mut frame = parent_frame;
-    let mut extent = part;
-    for axis in AXES {
-        let n = axis as usize;
-        let sized = match place[n].part() {
-            Part::Sized(len) => Some(len),
-            _ => None,
+    placement: [Option<UiSpan>; 2],
+) -> (UiRegion, [Option<UiSpan>; 2]) {
+    let mut placed = [None; 2];
+    for (axis, (len, chosen)) in AXES.into_iter().zip(declared.into_iter().zip(placement)) {
+        let Some(len) = len else {
+            placed[axis as usize] = chosen;
+            continue;
         };
-        let 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());
-            *extent.axis_mut(axis) = UiSpan::new(start, start + len);
+        let span = region.axis_mut(axis);
+        let len = Len::from_parts(len.rel, len.px);
+        let slot = chosen.unwrap_or(*span);
+        span.start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
+        span.end = span.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
+    (region, placed)
 }

core/src/ui/place.rs

@@ -1,73 +0,0 @@
-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,
-    /// 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 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, 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)
-            }
-        }
-    }
-}
-
-/// Where a child goes along one axis, as a part of this widget's box.
-#[derive(Clone, Copy, Debug, PartialEq)]
-pub enum Place {
-    /// The child's answer, aligned inside the part by the child's alignment.
-    Within(Part),
-    /// Exactly the part; the answer is not placed inside it again.
-    Fill(Part),
-}
-
-impl Place {
-    pub(crate) fn part(self) -> Part {
-        match self {
-            Self::Within(part) | Self::Fill(part) => part,
-        }
-    }
-
-    /// Whether the part is the drawing's box outright, rather than the box
-    /// the answer is placed inside.
-    pub(crate) fn fills(self) -> bool {
-        matches!(self, Self::Fill(_))
-    }
-}
-
-/// A primitive as it was written: its box in the widget's own box's
-/// coordinates, which is what a move of that box re-composes from.
-#[derive(Debug)]
-pub struct RetainedPrimitive {
-    pub handle: PrimitiveHandle,
-    pub region: UiRegion,
-}

src/random.rs

@@ -117,35 +117,19 @@ pub struct Branch {
 
 impl Widget for Branch {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
-        let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
-        let measured = painter
-            .widget_at(&self.probe, [None; 2], [Place::Within(Part::All), top])
-            .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 mut top = UiRegion::FULL;
+        top.y.end = top.y.start.offset(Px::from_int(40));
+        let measured = painter.widget_within(&self.probe, top).len(Axis::X);
+        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
 
-        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
-        let place = [Place::Within(Part::All), below];
-        match px > threshold {
-            true => painter.widget_at(&self.wide, [None; 2], place),
-            false => painter.widget_at(&self.narrow, [None; 2], place),
+        let mut below = UiRegion::FULL;
+        below.y.start = below.y.start.offset(Px::from_int(40));
+        match px > Px::from_f32(self.threshold) {
+            true => painter.widget_within(&self.wide, below),
+            false => painter.widget_within(&self.narrow, below),
         };
         Size::LEFTOVER
     }
-
-    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
-        Some(LayoutLen::LEFTOVER)
-    }
 }
 
 pub struct Spanned {

src/widget/mask.rs

@@ -6,7 +6,7 @@ pub struct Masked {
 
 impl Widget for Masked {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        painter.set_mask(UiRegion::FULL);
+        painter.set_mask(DrawRegion::Extent(UiRegion::FULL));
         painter.widget(&self.inner);
         // What it occupies is its box, on both axes, for the reason `Scroll`
         // reports the same: it clips what is inside to that box, so it can
@@ -15,8 +15,4 @@ 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

@@ -7,14 +7,7 @@ pub struct Offset {
 
 impl Widget for Offset {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        // The whole of this widget's box, moved: the frame passes through, so
-        // what the child declares or reports means the same as it would
-        // without the offset.
-        let moved = |len: Len, amt: Len| Place::Within(Part::From(UiSpan::new(amt, len + amt)));
-        let place = [
-            moved(painter.extent_len(Axis::X), self.amt.x),
-            moved(painter.extent_len(Axis::Y), self.amt.y),
-        ];
-        painter.widget_at(&self.inner, [None; 2], place).size()
+        let region = UiRegion::FULL.offset(self.amt);
+        painter.widget_within(&self.inner, region).size()
     }
 }

src/widget/position/pad.rs

@@ -13,32 +13,8 @@ impl Widget for Pad {
         // it; where the box is bigger -- a share of a row, a rule over this
         // widget -- the slack is the inner's to sit in, and forcing the near
         // edge pinned it to a corner it had not asked for.
-        //
-        // Padding is an inset of both: it comes off the frame, so `rel(1)`
-        // under it fills this widget rather than overflowing it by the
-        // padding, and it comes off the box, so what is drawn sits inside.
-        // The two stay distinct -- the box can be narrower still, where a row
-        // asked this widget in the room left, and a text wraps at that.
-        let inset = |lead: Px, trail: Px| {
-            Place::Within(Part::Of(UiSpan::new(
-                Len::from_parts(Rel::ZERO, lead),
-                Len::from_parts(Rel::ONE, -trail),
-            )))
-        };
-        let place = [
-            inset(self.padding.left, self.padding.right),
-            inset(self.padding.top, self.padding.bottom),
-        ];
-        // 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();
+        let inside = DrawRegion::Extent(self.padding.region());
+        let inner = painter.widget_within(&self.inner, inside).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,

src/widget/position/scroll.rs

@@ -12,13 +12,15 @@ pub struct Scroll {
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let container_len = painter.px_len(self.axis);
-        // Asked in the whole viewport, then put at the scrolled offset.
-        let answer_len = painter
-            .widget_at(&self.inner, [None; 2], [Place::Fill(Part::All); 2])
-            .len(self.axis);
-        let fixed = painter.to_px(Len::from_parts(answer_len.rel, answer_len.px), self.axis);
+        // Draw in the whole container only when its scrolling-axis length is
+        // not already known, then draw it at the scrolled offset.
+        let whole = UiRegion::FULL;
+        let own = painter.placement();
+        let answer_len =
+            painter.measure_len(&self.inner, self.axis, whole, [Some(own.x), Some(own.y)]);
+        let content = answer_len.apply_leftover();
         self.container_len = container_len;
-        self.content_len = fixed.max(container_len);
+        self.content_len = content.to_px(container_len);
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
@@ -31,9 +33,9 @@ impl Widget for Scroll {
         // the drawing holds for that length alone. One scrolled part way sits
         // where it is until the box shrinks past what is left of it. Kept to
         // the end, it moves with every length.
-        let fixed_len = answer_len.rel == Rel::ZERO && answer_len.leftover == Weight::ZERO;
+        let fixed_len = content.rel == Rel::ZERO;
         if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
-            painter.holds(self.axis, fixed..=Px::MAX);
+            painter.holds(self.axis, self.content_len..=Px::MAX);
         } else if fixed_len && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, Px::MIN..=left);
@@ -44,6 +46,7 @@ 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 region = UiRegion::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
@@ -51,31 +54,26 @@ 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;
-        let content = match moved || self.content_len != self.container_len {
-            true => {
-                let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
-                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 goes is the content
-        // box, scrolled: its drawing moved there, not made again there.
-        painter.place_at(
-            &self.inner,
-            self.axis.pair(Place::Fill(content), Place::Fill(Part::All)),
+        if moved || self.content_len != self.container_len {
+            let offset = UiVec2::from_axis(
+                self.axis,
+                Len::from_parts(Rel::ZERO, anchor - self.amt),
+                Len::ZERO,
             );
+            region = region.offset(offset);
+            region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
+        }
+        // The viewport is the inner's region, so a fraction it declares or
+        // reports is a fraction of what is on screen rather than of the
+        // content box its own answer decided. Where it is put is the content
+        // box, scrolled.
+        painter.widget_at(&self.inner, whole, [Some(region.x), Some(region.y)]);
         // What it occupies is its box, on both axes: it clips its content to
         // that box, so it can neither take less of one nor honestly ask for
         // more. The content's length is what it scrolls through, not what it
         // is.
         Size::LEFTOVER
     }
-
-    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
-        Some(LayoutLen::LEFTOVER)
-    }
 }
 
 impl Scroll {

src/widget/position/span.rs

@@ -10,42 +10,34 @@ pub struct Span {
 impl Widget for Span {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
-        // The row: this span's own box, as a length of the frame its children
-        // are laid out against. Its start is nothing's business -- a slot is
-        // a length from it -- so what this reads is the length alone.
-        let far = painter.extent_len(axis);
+        // The row: this span's own box, as a span of the region it was given.
+        // Its children are laid out along it, and what they declare or report
+        // is a fraction of the region -- the area this span was told it has,
+        // which it passes on unchanged.
+        let own = painter.placement();
+        let row = *own.axis(axis);
+        // Across itself the span's own box is the child's region: a span is
+        // what contains its children there, and nothing divides that axis.
+        // Along it the whole region is, so a fraction means the same thing
+        // for every child however much of the row is left when it is asked.
+        let region = UiRegion::from_axis(axis, UiSpan::FULL, *own.axis(!axis));
         let along = |from: Len, to: Len| match self.dir.sign {
-            Sign::Pos => UiSpan::new(from, to),
-            Sign::Neg => UiSpan::new(far - to, far - from),
+            Sign::Pos => UiSpan::new(row.start + from, row.start + to),
+            Sign::Neg => UiSpan::new(row.end - to, row.end - from),
         };
-        // Across itself the child sits where its own alignment says, in the
-        // whole of the row: a span is what contains its children there, and
-        // nothing divides that axis.
-        let across = Place::Within(Part::All);
-        // A length for every child before their final slots are chosen: 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.
+        let far = row.len();
+        // A length for every child before their final boxes are chosen: from
+        // a hint where one exists, and from drawing otherwise.
         let mut cursor = Len::rel_min();
         let mut lens = Vec::with_capacity(self.children.len());
-        let mut drawn_across = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let len = match painter.size_hint(child, axis) {
-                Some(len) => {
-                    drawn_across.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();
-                    drawn_across.push(Some(size.axis(!axis)));
-                    size.axis(axis)
-                }
-            };
+            // The whole region is the child's, so `rel(0.5)` is half the area
+            // this span was given whatever else is in it and wherever this
+            // child sits among them. What it is placed in is the room left
+            // from the cursor, because a text has to wrap at the width
+            // actually there.
+            let room = axis.pair(Some(along(cursor, far)), None);
+            let len = painter.measure_len(child, axis, region, room);
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
             lens.push(len);
@@ -63,7 +55,7 @@ impl Widget for Span {
         );
 
         // 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.
+        // fixed, as a length of the region rather than a number of pixels.
         let room = far - Len::from_parts(total.rel, total.px);
         // Whether anything is left over is a question in pixels: `rel(0.5)`
         // beside 300 px is full at 600 and overfull at 400. Asked of `room`
@@ -76,12 +68,12 @@ impl Widget for Span {
         // exist at all turns on this.
         let mut shares = false;
         if total.leftover > Weight::ZERO {
-            shares = painter.to_px(room, axis) > Px::ZERO;
+            shares = room.to_px(painter.region_px_len(axis)) > Px::ZERO;
             let holds = match shares {
                 true => Holds::from(Px::STEP..=Px::MAX),
                 false => Holds::from(Px::MIN..=Px::ZERO),
             };
-            painter.window_holds(axis, holds.through(room));
+            painter.region_holds(axis, holds.through(room));
         }
 
         // Across itself a span is as long as its longest child -- unless a
@@ -100,7 +92,7 @@ impl Widget for Span {
         let mut taken = Weight::ZERO;
         let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
-        for ((child, &len), &across_len) in self.children.iter().zip(&lens).zip(&drawn_across) {
+        for (child, len) in self.children.iter().zip(&lens) {
             // 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.
@@ -117,27 +109,14 @@ impl Widget for Span {
             fixed.px += len.px;
             fixed.rel += len.rel;
             start = shared(fixed, taken, total.leftover, room);
-            // Along the row the span says where the child goes, and that slot
-            // is the child's box outright rather than something to place an
-            // 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 (across_len, narrow[axis as usize]) {
-                (Some(across_len), None) => {
-                    painter.place_at(child, place);
-                    across_len
-                }
-                _ => painter.widget_at(child, narrow, place).len(!axis),
-            };
+            // Along the row the span says where the child goes; across it the
+            // child sits where its own alignment says. Its region is the
+            // whole of what this span was given either way, which is what its
+            // fractions are of.
+            let placed =
+                painter.widget_at(child, region, axis.pair(Some(along(from, start)), None));
             if shrinks {
+                let used = placed.len(!axis);
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.
                 // A scalable child therefore makes Children scalable too;

src/widget/position/stack.rs

@@ -13,51 +13,32 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
-        // Whichever child sizes the stack is given the stack's whole box --
-        // the stack is the length that child asked for, so placing that
-        // answer inside the box it decided would apply it twice.
+        // Whichever child sizes the stack keeps the stack's whole region as
+        // its own -- the stack is the length that child asked for, so taking
+        // the fraction of the stack's box again would take it twice -- and is
+        // put where the stack itself is put.
         let size = match sizing.and_then(|i| self.children.get(i).map(|c| (i, c))) {
             // On the layer that child ends up on, so the ask below is a reuse
             // rather than a second drawing of it somewhere else: a retained
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter
-                    .widget_at(child, [None; 2], [Place::Fill(Part::All); 2])
-                    .size()
+                painter.widget(child).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);
-            painter.widget_at(child, [None; 2], place);
+            // Every other child has the stack's own box for its region, since
+            // the stack is what contains it, and where it sits in one bigger
+            // than itself is its own business.
+            painter.widget_within(child, DrawRegion::Extent(UiRegion::FULL));
         }
         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)]

src/widget/text/mod.rs

@@ -80,7 +80,7 @@ impl TextView {
         // hair under that line, and the break made in it is not the break a
         // cold layout makes there.
         let size = Size::from_px(PxVec2::ceil_from_f32(tex.size));
-        painter.glyphs(tex, region);
+        painter.glyphs(tex, DrawRegion::Extent(region));
         (region, size)
     }
 

tests/cases/determinism.rs

@@ -21,18 +21,16 @@ struct BranchesOnMeasurement {
 
 impl Widget for BranchesOnMeasurement {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
-        let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
-        let measured = painter
-            .widget_at(&self.probe, [None; 2], [Place::Within(Part::All), top])
-            .len(Axis::X);
-        let px = painter.to_px(measured.apply_leftover(), Axis::X);
+        let mut top = UiRegion::FULL;
+        top.y.end = top.y.start.offset(Px::from_int(40));
+        let measured = painter.widget_within(&self.probe, top).len(Axis::X);
+        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
 
-        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
-        let place = [Place::Within(Part::All), below];
+        let mut below = UiRegion::FULL;
+        below.y.start = below.y.start.offset(Px::from_int(40));
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, [None; 2], place),
-            false => painter.widget_at(&self.narrow, [None; 2], place),
+            true => painter.widget_within(&self.wide, below),
+            false => painter.widget_within(&self.narrow, below),
         };
         Size::LEFTOVER
     }

tests/cases/layout.rs

@@ -83,10 +83,11 @@ fn a_text_in_a_span_wraps_at_the_room_left_rather_than_the_whole_row() {
     assert!(crowded > whole_row, "{crowded} against {whole_row}");
 }
 
-/// Padding is an inset: it narrows the frame a fraction resolves against and
-/// adds itself back to the padded widget's reported length.
+/// The same reading through a pad: its inset is the whole box less the
+/// padding, so half of the inset plus the padding is half the box plus one
+/// padding, not two.
 #[test]
-fn a_pad_puts_its_padding_around_a_fraction_of_the_whole_box() {
+fn a_pad_reports_a_fraction_of_its_inset_as_a_fraction_of_its_box() {
     let mut h = Harness::new((400, 100));
     let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut h.rsc);
     let padded = (inner,).span(Dir::RIGHT).pad(10).add(&mut h.rsc);
@@ -95,85 +96,10 @@ 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), (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]
 fn a_span_ruled_across_itself_does_not_measure_its_children_there() {
     let mut h = Harness::new((400, 200));
@@ -500,7 +426,7 @@ 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 region = h.render.moves.resolve(active.move_idx, active.extent);
+    let region = h.render.moves.resolve(active.parent_move, active.region);
     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());
@@ -811,13 +737,3 @@ fn a_fixed_child_is_centered_in_its_wrappers_share() {
     assert_corners!(h, wrapper, (200, 0), (900, 400));
     assert_corners!(h, leaf, (500, 150), (600, 250));
 }
-
-/// The root's frame is the window and its rule is a fraction of that, which
-/// is one resolution and not two: nothing above it narrowed anything.
-#[test]
-fn a_root_with_a_fraction_rule_is_that_fraction_of_the_window() {
-    let mut h = Harness::new((900, 200));
-    let root = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
-    h.set_root(root);
-    assert_eq!(h.region(&root).unwrap().size().x, Px::from_int(450));
-}

tests/cases/retained.rs

@@ -156,9 +156,9 @@ fn a_span_child_that_declares_its_length_is_drawn_once() {
     h.set_root((hinted, asked).span(Dir::RIGHT));
 
     assert_eq!(told_draws.get(), 1);
-    // Asked once, from the cursor; its slot is its answer and the drawing is
-    // moved there.
-    assert_eq!(asked_draws.get(), 1);
+    // Only the available length changes: positioning the final slot does
+    // not invalidate a numeric size read.
+    assert_eq!(asked_draws.get(), 2);
 }
 
 #[test]
@@ -213,12 +213,9 @@ struct FromHint {
 impl Widget for FromHint {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
-        let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
-        painter.widget_at(
-            &self.inner,
-            [None; 2],
-            [Place::Within(Part::All), Place::Within(Part::From(top))],
-        );
+        let mut region = UiRegion::FULL;
+        region.y.end = region.y.start.offset(len.px);
+        painter.widget_within(&self.inner, region);
         Size::LEFTOVER
     }
 }
@@ -255,9 +252,10 @@ impl Widget for ReadsBox {
 /// Reads its box across one axis only, so its drawing holds for a taller
 /// box on its own and only a wider one is worth a draw.
 ///
-/// Both of these report a quarter of what they read. 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.
+/// Both of these report a quarter of what they read, without saying that the
+/// drawing holds there too, so each length they are asked at costs two draws:
+/// one to answer, and one in the quarter-sized box that answer places them
+/// in. The counts below are in those pairs.
 struct ReadsWidth {
     draws: Rc<Cell<usize>>,
 }
@@ -307,51 +305,6 @@ fn a_span_ruled_across_itself_moves_its_child_without_redrawing_it() {
     assert_eq!(h.render.active[&span.id()].size.y, LayoutLen::rel(1.0));
 }
 
-/// A row places its children as lengths from where its own box starts, so a
-/// child that grew moves the ones after it and nothing else: each of them is
-/// the same box in a new place, which the retained drawing follows without
-/// being made again. Both kinds of length: one the row resolves from a rule,
-/// and one it takes from what the child reported.
-#[test]
-fn a_row_moves_what_follows_a_child_that_grew_rather_than_drawing_it() {
-    for declared in [false, true] {
-        let mut h = Harness::new((400, 200));
-        let first = rect(Color::RED).width(50).add(&mut h.rsc);
-        let ruled = Rc::new(Cell::new(0));
-        let second = Counted {
-            draws: ruled.clone(),
-            size: Size::LEFTOVER,
-            reads_box: false,
-        };
-        let second = match declared {
-            true => second.width(rel(0.25)).add(&mut h.rsc),
-            false => second.width(60).add(&mut h.rsc),
-        };
-        let (third, reported) = counted(&mut h, Size::from((70, 20)), false);
-        h.set_root((first, second, third).span(Dir::RIGHT).width(rel(1.0)));
-        let (was_ruled, was_reported) = (ruled.get(), reported.get());
-        // A quarter of the row is a quarter of the row, wherever it sits in
-        // it and whatever the first child takes.
-        let width = match declared {
-            true => 100,
-            false => 60,
-        };
-        assert_corners!(h, second, (50, 0), (50 + width, 200));
-
-        h.set_len(first, Axis::X, 80);
-        h.frame();
-
-        assert_eq!(ruled.get(), was_ruled, "the ruled child was drawn again");
-        assert_eq!(
-            reported.get(),
-            was_reported,
-            "the reported child was drawn again"
-        );
-        assert_corners!(h, second, (80, 0), (80 + width, 200));
-        assert_corners!(h, third, (80 + width, 90), (150 + width, 110));
-    }
-}
-
 /// The output is the root of the box chain, so a resize is a box that changed
 /// length like any other -- there is not a second rule for the window. A
 /// drawing that holds for one length is drawn again whichever box moved.
@@ -383,7 +336,7 @@ fn a_resize_redraws_what_read_its_box() {
     h.resize((800, 100));
     h.frame();
 
-    assert_eq!(draws.get(), settled + 1);
+    assert_eq!(draws.get(), settled + 2);
 }
 
 #[test]
@@ -403,7 +356,7 @@ fn a_resize_only_redraws_read_axes() {
 
     h.resize((800, 300));
     h.frame();
-    assert_eq!(draws.get(), settled + 1, "width changes its answer");
+    assert_eq!(draws.get(), settled + 2, "width changes its answer");
 }
 
 /// A window is measured onto the grid like everything else, so a resize too
@@ -430,7 +383,7 @@ fn a_resize_within_one_step_is_not_a_resize() {
 
     h.resize((400.0 + step, 200.0));
     h.frame();
-    assert_eq!(draws.get(), settled + 1);
+    assert_eq!(draws.get(), settled + 2);
 }
 
 /// The same for a box that changes because a sibling did: what is compared
@@ -796,6 +749,36 @@ fn primitive_bounds(h: &Harness, id: WidgetId) -> Vec<PixelRegion> {
         .collect()
 }
 
+#[test]
+fn frame_geometry_and_extent_geometry_keep_their_references() {
+    struct Both(Rc<Cell<usize>>);
+    impl Widget for Both {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.0.set(self.0.get() + 1);
+            painter.primitive_within(RectPrimitive::color(Color::RED), UiRegion::FULL);
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::LEFTOVER
+        }
+    }
+    for node in [false, true] {
+        let mut h = Harness::new((400, 200));
+        let first = rect(Color::GREEN).width(100).add(&mut h.rsc);
+        let draws = Rc::new(Cell::new(0));
+        let both = Both(draws.clone()).add(&mut h.rsc);
+        h.rsc.widgets_mut().set_region_node(both, node);
+        h.set_root((first, both).span(Dir::RIGHT));
+        let count = draws.get();
+        h.set_len(first, Axis::X, 200);
+        h.frame();
+        assert_eq!(draws.get(), count);
+        let bounds = primitive_bounds(&h, both.id());
+        assert_eq!(bounds[0].top_left.x, Px::ZERO);
+        assert_eq!(bounds[0].bot_right.x, Px::from_int(400));
+        assert_eq!(bounds[1].top_left.x, Px::from_int(200));
+        assert_eq!(bounds[1].bot_right.x, Px::from_int(400));
+    }
+}
+
 #[test]
 fn changing_an_inherited_extent_keeps_the_original_measurement_offer() {
     fn build(h: &mut Harness, width: i32, text: &str) -> (WeakWidget<Text>, WeakWidget<Rect>) {
@@ -875,14 +858,7 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
     }
     impl Widget for Frame {
         fn draw(&mut self, painter: &mut Painter) -> Size {
-            painter.widget_at(
-                &self.child,
-                [None; 2],
-                [
-                    Place::Within(Part::From(self.region.x)),
-                    Place::Within(Part::From(self.region.y)),
-                ],
-            );
+            painter.widget_within(&self.child, self.region);
             Size::LEFTOVER
         }
     }
@@ -890,7 +866,7 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
     impl Widget for Painted {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             self.0.set(self.0.get() + 1);
-            painter.set_mask(UiRegion::FULL);
+            painter.set_mask(DrawRegion::Extent(UiRegion::FULL));
             painter.primitive(RectPrimitive::color(Color::BLUE));
             Size::LEFTOVER
         }
@@ -939,6 +915,43 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
     }
 }
 
+#[test]
+fn a_span_does_not_place_its_measurement_before_assigning_the_childs_slot() {
+    struct MeasuredBox(Rc<Cell<usize>>);
+
+    impl Widget for MeasuredBox {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.0.set(self.0.get() + 1);
+            painter.px_size();
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::from((100, 50))
+        }
+    }
+
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = MeasuredBox(draws.clone()).add(&mut h.rsc);
+    h.set_root((leaf,).span(Dir::RIGHT).width(rel(1.0)).height(rel(1.0)));
+
+    assert_eq!(draws.get(), 3);
+    assert_corners!(h, leaf, (0, 75), (100, 125));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf.id()).unwrap()]
+    );
+    h.frame();
+    assert_eq!(draws.get(), 3);
+
+    h.resize((600, 300));
+    h.frame();
+    assert_eq!(draws.get(), 6);
+    assert_corners!(h, leaf, (0, 125), (100, 175));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf.id()).unwrap()]
+    );
+}
+
 #[test]
 fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
     struct Glyphs {
@@ -953,7 +966,8 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
                 UiSpan::new(Len::rel(0.23) + Len::px(-7.125), Len::FULL),
                 UiSpan::new(Len::rel(0.37) + Len::px(3.25), Len::FULL),
             );
-            painter.glyphs(text, origin);
+            painter.glyphs(text, DrawRegion::Frame(origin));
+            painter.glyphs(text, DrawRegion::Extent(origin));
             Size::LEFTOVER
         }
     }
@@ -966,11 +980,8 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [Some(self.region.x.len()), None],
-                [
-                    Place::Fill(Part::From(self.extent.x)),
-                    Place::Fill(Part::From(self.extent.y)),
-                ],
+                self.region,
+                [Some(self.extent.x), Some(self.extent.y)],
             );
             Size::LEFTOVER
         }
@@ -1117,7 +1128,7 @@ fn widening_and_restoring_a_contract_does_not_invalidate_its_reader() {
     assert_eq!(leaf_draws.get(), settled + 1);
 }
 #[test]
-fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
+fn padding_and_stack_frames_follow_the_extent_without_drawing_again() {
     struct Observed<W> {
         widget: W,
         draws: Rc<Cell<usize>>,
@@ -1136,11 +1147,8 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
-                [
-                    Place::Fill(Part::From(self.extent.x)),
-                    Place::Fill(Part::From(self.extent.y)),
-                ],
+                UiRegion::FULL,
+                [Some(self.extent.x), Some(self.extent.y)],
             );
             Size::LEFTOVER
         }
@@ -1171,18 +1179,13 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
             h.set_root(root);
             (root, leaf, fixed, draws)
         };
-        // The same box in three places. A pad places its child as lengths of
-        // its own box measured from where that box starts, so moving it is
-        // nothing to the pad -- where changing its length is a different
-        // question, and does draw it again.
-        let at = |start: f32| {
-            let span = |start: Len| UiSpan::new(start, start + Len::rel(0.4));
-            UiRegion::new(span(Len::rel(start) + Len::px(3.125)), span(Len::px(11.25)))
-        };
         let mut warm = Harness::new((403, 211));
-        let (root, leaf, fixed, draws) = plant(&mut warm, at(0.13));
-        for start in [0.13, -0.17, 0.31] {
-            let extent = at(start);
+        let (root, leaf, fixed, draws) = plant(&mut warm, UiRegion::FULL);
+        for (start, end) in [(0.13, 0.83), (-0.17, 1.23), (0.31, 0.67)] {
+            let extent = UiRegion::new(
+                UiSpan::new(Len::rel(start) + Len::px(3.125), Len::rel(end)),
+                UiSpan::new(Len::px(11.25), Len::rel(end)),
+            );
             let before = draws.get();
             warm.rsc[root].extent = extent;
             warm.frame();
@@ -1224,13 +1227,13 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
+                UiRegion::FULL,
                 [
-                    Place::Fill(Part::From(UiSpan::new(
+                    Some(UiSpan::new(
                         Len::px(self.start),
                         Len::px(self.start + 200.0),
-                    ))),
-                    Place::Fill(Part::From(UiSpan::FULL)),
+                    )),
+                    Some(UiSpan::FULL),
                 ],
             );
             Size::LEFTOVER
@@ -1264,14 +1267,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
     impl Widget for Container {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter
-                .widget_at(
-                    &self.child,
-                    [None; 2],
-                    [
-                        Place::Within(Part::From(self.region.x)),
-                        Place::Within(Part::From(self.region.y)),
-                    ],
-                )
+                .widget_within(&self.child, DrawRegion::Extent(self.region))
                 .size()
         }
     }
@@ -1286,11 +1282,8 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 painter
                     .widget_at(
                         &self.child,
-                        [None; 2],
-                        [
-                            Place::Fill(Part::From(self.extent.x)),
-                            Place::Fill(Part::From(self.extent.y)),
-                        ],
+                        UiRegion::FULL,
+                        [Some(self.extent.x), Some(self.extent.y)],
                     )
                     .size(),
             );

tests/cases/scroll.rs

@@ -60,69 +60,6 @@ fn a_wheel_scrolls_the_content_and_stops_at_its_end() {
     assert_corners!(h, top, (0, 0), (400, 200));
 }
 
-#[test]
-fn fixed_content_and_a_share_fill_one_viewport() {
-    let mut h = Harness::new((900, 100));
-    let content = rect(Color::RED)
-        .width(LayoutLen {
-            px: Px::from_int(600),
-            rel: Rel::ZERO,
-            leftover: Weight::ONE,
-        })
-        .add(&mut h.rsc);
-    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
-    h.set_root(scroll);
-
-    assert_corners!(h, content, (0, 0), (900, 100));
-}
-
-#[test]
-fn fixed_content_wider_than_the_viewport_still_scrolls() {
-    let mut h = Harness::new((900, 100));
-    let content = rect(Color::RED).width(1200).add(&mut h.rsc);
-    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
-    h.set_root(scroll);
-
-    assert_corners!(h, content, (-300, 0), (900, 100));
-}
-
-#[test]
-fn a_lone_share_fills_without_scrolling() {
-    let mut h = Harness::new((900, 100));
-    let content = rect(Color::RED).width(LayoutLen::LEFTOVER).add(&mut h.rsc);
-    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
-    h.set_root(scroll);
-
-    assert_corners!(h, content, (0, 0), (900, 100));
-}
-
-#[test]
-fn wrapping_content_beside_a_fixed_length_is_stable_warm_and_cold() {
-    fn plant(h: &mut Harness) -> (WidgetId, WidgetId) {
-        let fixed = rect(Color::RED).width(600).add(&mut h.rsc);
-        let text = wtext("Wrapping shapes one source into as many lines as the box leaves room for, so a paragraph's height is an answer and not a setting.")
-            .size(16)
-            .wrap(true)
-            .width(LayoutLen::LEFTOVER)
-            .add(&mut h.rsc);
-        let content = (fixed, text).span(Dir::RIGHT).add(&mut h.rsc);
-        let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
-        h.set_root(scroll);
-        (text.id(), content.id())
-    }
-
-    let mut warm = Harness::new((900, 300));
-    let (text, content) = plant(&mut warm);
-    warm.rsc.widgets_mut().get_dyn_mut(text);
-    warm.frame();
-
-    let mut cold = Harness::new((900, 300));
-    let (cold_text, cold_content) = plant(&mut cold);
-
-    assert_eq!(warm.region(&text), cold.region(&cold_text));
-    assert_eq!(warm.region(&content), cold.region(&cold_content));
-}
-
 /// A widget that clips to its box may not report more than the box: its
 /// parent would place the part it cut off, and the framework would put a
 /// drawing longer than its box somewhere. `Masked` is the second of these
@@ -134,7 +71,7 @@ fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
 
     impl Widget for Clipper {
         fn draw(&mut self, painter: &mut Painter) -> Size {
-            painter.set_mask(UiRegion::FULL);
+            painter.set_mask(painter.region());
             painter.widget(&self.0).size()
         }
     }
@@ -145,19 +82,3 @@ fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
     h.set_root(clipper);
     h.frame();
 }
-
-/// Content that fits sits in the viewport, not in a box of the window's
-/// length anchored at the viewport's start. `Part::From` takes window
-/// lengths, so a `rel(1.0)` span in one is the window, and only a scroll
-/// filling the window would land right.
-#[test]
-fn content_that_fits_is_placed_in_the_viewport_and_not_in_the_window() {
-    let mut h = Harness::new((400, 400));
-    let head = rect(Color::RED).height(100).add(&mut h.rsc);
-    let inner = rect(Color::BLUE).height(50).add(&mut h.rsc);
-    let scroll = Scroll::new(inner.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
-    h.set_root((head, scroll).span(Dir::DOWN));
-
-    assert_corners!(h, scroll, (0, 100), (400, 400));
-    assert_corners!(h, inner, (0, 225), (400, 275));
-}

tests/cases/unsettled.rs

@@ -14,180 +14,6 @@ use iris::harness::Harness;
 use iris::prelude::*;
 use iris::random::Branch;
 
-fn assert_same_regions(
-    warm: &Harness,
-    warm_ids: &[WidgetId],
-    cold: &Harness,
-    cold_ids: &[WidgetId],
-) {
-    let mut wrong = Vec::new();
-    for (i, (&w, &c)) in warm_ids.iter().zip(cold_ids).enumerate() {
-        let (got, want) = (warm.region(&w), cold.region(&c));
-        if got != want {
-            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
-        }
-    }
-    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
-}
-
-/// Ten widgets, shrunk from seed 2 at depth 5. The stack is as tall as its
-/// first child, so its other children belong in that one-line box. A cold
-/// layout used to keep the span's answer from the larger measuring box while
-/// a repaint asked it in the stack's final box.
-fn plant_stack_in_its_sizing_childs_box(h: &mut Harness) -> Vec<WidgetId> {
-    let sizing = wtext("one line, overflowing whatever it is given")
-        .size(16)
-        .wrap(false)
-        .add(&mut h.rsc);
-    let filler = rect(Color::CYAN.alpha(252)).add(&mut h.rsc);
-    let plain = wtext("one line, overflowing whatever it is given")
-        .size(16)
-        .wrap(false)
-        .add(&mut h.rsc);
-    let span = (filler, plain).span(Dir::DOWN).add(&mut h.rsc);
-    let pad = Pad {
-        padding: Padding::ZERO,
-        inner: span.add_strong(&mut h.rsc),
-    }
-    .add(&mut h.rsc);
-    let probe = rect(Color::RED).add(&mut h.rsc);
-    let wide = rect(Color::YELLOW.alpha(252)).add(&mut h.rsc);
-    let narrow = rect(Color::RED).add(&mut h.rsc);
-    let branch = Branch {
-        probe: probe.add_strong(&mut h.rsc),
-        wide: wide.add_strong(&mut h.rsc),
-        narrow: narrow.add_strong(&mut h.rsc),
-        threshold: 55.0,
-    }
-    .add(&mut h.rsc);
-    let stack = Stack {
-        children: vec![
-            sizing.add_strong(&mut h.rsc),
-            pad.add_strong(&mut h.rsc),
-            branch.add_strong(&mut h.rsc),
-        ],
-        size: StackSize::Child(0),
-    }
-    .add(&mut h.rsc);
-    h.rsc
-        .widgets_mut()
-        .set_size_rules(stack.id(), Some(LayoutLen::LEFTOVER), None);
-    h.set_root(stack);
-    vec![
-        sizing.id(),
-        filler.id(),
-        plain.id(),
-        span.id(),
-        pad.id(),
-        probe.id(),
-        wide.id(),
-        narrow.id(),
-        branch.id(),
-        stack.id(),
-    ]
-}
-
-#[test]
-fn repainting_a_stack_uses_the_box_its_sizing_child_decided() {
-    let mut warm = Harness::new((900, 1200));
-    let ids = plant_stack_in_its_sizing_childs_box(&mut warm);
-    for &id in &ids {
-        warm.rsc.widgets_mut().get_dyn_mut(id);
-    }
-    warm.frame();
-
-    let mut cold = Harness::new((900, 1200));
-    let cold_ids = plant_stack_in_its_sizing_childs_box(&mut cold);
-
-    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-}
-
-/// Ten widgets, shrunk from seed 108 at depth 5. The nested reverse spans
-/// evaluate the branch in successively narrower boxes. The answer from the
-/// final, decided box must be the one retained after every span is reordered.
-fn plant_branch_in_nested_reverse_spans(
-    h: &mut Harness,
-    reordered: bool,
-) -> (Vec<WidgetId>, [WeakWidget<Span>; 3]) {
-    let pair = |first: StrongWidget, second: StrongWidget| match reordered {
-        true => vec![second, first],
-        false => vec![first, second],
-    };
-    let probe = rect(Color::RED.alpha(63)).add(&mut h.rsc);
-    let wide = rect(Color::RED).add(&mut h.rsc);
-    let narrow = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
-    let branch = Branch {
-        probe: probe.add_strong(&mut h.rsc),
-        wide: wide.add_strong(&mut h.rsc),
-        narrow: narrow.add_strong(&mut h.rsc),
-        threshold: 483.0,
-    }
-    .add(&mut h.rsc);
-    let wrapped = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
-    let down = Span {
-        children: pair(
-            branch.add_strong(&mut h.rsc),
-            wrapped.add_strong(&mut h.rsc),
-        ),
-        dir: Dir::DOWN,
-        gap: Px::ZERO,
-    }
-    .add(&mut h.rsc);
-    let inner_filler = rect(Color::CYAN.alpha(63)).add(&mut h.rsc);
-    let inner = Span {
-        children: pair(
-            down.add_strong(&mut h.rsc),
-            inner_filler.add_strong(&mut h.rsc),
-        ),
-        dir: Dir::LEFT,
-        gap: Px::ZERO,
-    }
-    .height(LayoutLen::rel(1.0))
-    .add(&mut h.rsc);
-    let outer_filler = rect(Color::GREEN.alpha(63)).add(&mut h.rsc);
-    let outer = Span {
-        children: pair(
-            inner.add_strong(&mut h.rsc),
-            outer_filler.add_strong(&mut h.rsc),
-        ),
-        dir: Dir::LEFT,
-        gap: Px::ZERO,
-    }
-    .height(LayoutLen::rel(1.0))
-    .add(&mut h.rsc);
-    h.set_root(outer);
-    (
-        vec![
-            probe.id(),
-            wide.id(),
-            narrow.id(),
-            branch.id(),
-            wrapped.id(),
-            down.id(),
-            inner_filler.id(),
-            inner.id(),
-            outer_filler.id(),
-            outer.id(),
-        ],
-        [down, inner, outer],
-    )
-}
-
-#[test]
-fn reordering_nested_spans_keeps_the_answer_from_the_decided_box() {
-    let mut warm = Harness::new((900, 1200));
-    let (ids, spans) = plant_branch_in_nested_reverse_spans(&mut warm, false);
-    for span in spans {
-        warm.rsc[span].children.rotate_left(1);
-    }
-    warm.frame();
-
-    let mut cold = Harness::new((900, 1200));
-    let (cold_ids, _) = plant_branch_in_nested_reverse_spans(&mut cold, true);
-
-    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-}
-
 /// Six widgets, shrunk from a 402-widget tree the fuzzer found. Nothing about
 /// the tree changes -- every widget is marked for redraw and the frame is
 /// taken again -- so no box may move, and a warm frame has to land where a
@@ -733,51 +559,6 @@ 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
@@ -891,179 +672,3 @@ fn adding_text_to_a_reverse_row_keeps_its_shared_height() {
     let (_, other, _) = build(&mut cold, true);
     assert_eq!(warm.region(&shared), cold.region(&other));
 }
-
-/// Nine widgets, shrunk from seed 946 at depth 6. The column is a share of
-/// the row while its rect has room to draw and a fixed width once it has
-/// not, so the row asks it twice: in the room, where it answers a share,
-/// and in its slot, where it answers its text's width. Emptying the column
-/// changes only the first answer. A local redraw that asked only the second
-/// question kept the row as it was; the column has to defer to the row.
-fn plant_column_that_is_a_share_only_while_its_rect_fits(
-    h: &mut Harness,
-    emptied: bool,
-) -> (Vec<WidgetId>, WeakWidget<Span>, Vec<StrongWidget>) {
-    let first = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
-    let filler = rect(Color::CYAN.alpha(126)).add(&mut h.rsc);
-    let second = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
-    let mut spare: Vec<StrongWidget> =
-        vec![filler.add_strong(&mut h.rsc), second.add_strong(&mut h.rsc)];
-    let mut children: Vec<StrongWidget> = vec![first.add_strong(&mut h.rsc)];
-    if !emptied {
-        children.append(&mut spare);
-    }
-    let column = Span {
-        children,
-        dir: Dir::DOWN,
-        gap: Px::ZERO,
-    }
-    .height(159)
-    .add(&mut h.rsc);
-    let left = rect(Color::MAGENTA.alpha(189)).add(&mut h.rsc);
-    let right = rect(Color::BLUE.alpha(0)).add(&mut h.rsc);
-    let row = Span {
-        children: vec![
-            left.add_strong(&mut h.rsc),
-            column.add_strong(&mut h.rsc),
-            right.add_strong(&mut h.rsc),
-        ],
-        dir: Dir::RIGHT,
-        gap: Px::ZERO,
-    }
-    .add(&mut h.rsc);
-    let end = rect(Color::MAGENTA.alpha(189)).add(&mut h.rsc);
-    let root = Span {
-        children: vec![end.add_strong(&mut h.rsc), row.add_strong(&mut h.rsc)],
-        dir: Dir::LEFT,
-        gap: Px::ZERO,
-    }
-    .add(&mut h.rsc);
-    h.set_root(root);
-    (
-        vec![
-            first.id(),
-            filler.id(),
-            second.id(),
-            column.id(),
-            left.id(),
-            right.id(),
-            row.id(),
-            end.id(),
-            root.id(),
-        ],
-        column,
-        spare,
-    )
-}
-
-#[test]
-fn emptying_a_column_the_row_asked_twice_asks_the_row_again() {
-    let mut warm = Harness::new((900, 1200));
-    let (ids, column, _spare) =
-        plant_column_that_is_a_share_only_while_its_rect_fits(&mut warm, false);
-    warm.frame();
-    // Kept alive: dropping the last share of a widget frees its id.
-    let _removed: Vec<StrongWidget> = warm.rsc[column].children.drain(1..).collect();
-    warm.frame();
-
-    let mut cold = Harness::new((900, 1200));
-    let (cold_ids, _, _spare) =
-        plant_column_that_is_a_share_only_while_its_rect_fits(&mut cold, true);
-    cold.frame();
-
-    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-}
-
-/// 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

@@ -1,42 +0,0 @@
-//! 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

@@ -103,11 +103,6 @@ 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
@@ -124,13 +119,12 @@ pub enum Case {
     Shuffle(Shuffle),
 }
 
-pub const ALL: [Case; 16] = [
+pub const ALL: [Case; 15] = [
     Case::Repaint,
     Case::RepaintSome,
     Case::Resize,
     Case::ResizeRepaint,
     Case::ResizeSize,
-    Case::SizeResize,
     Case::Size,
     Case::EverySize,
     Case::Align,
@@ -152,7 +146,6 @@ 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",
@@ -177,15 +170,6 @@ 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) {
@@ -300,7 +284,7 @@ fn change(case: Case, warm: &mut Harness, tree: &mut Tree, plan: &Plan, rng: &mu
             warm.frame();
             return out;
         }
-        Case::Size | Case::ResizeSize | Case::SizeResize => Edits {
+        Case::Size | Case::ResizeSize => Edits {
             sizes: some_sizes(warm, tree, rng),
             ..Default::default()
         },
@@ -400,17 +384,6 @@ 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.
@@ -427,29 +400,12 @@ 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));
@@ -460,7 +416,6 @@ 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];
@@ -469,22 +424,11 @@ 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  {}\n{}",
-            chain.join(" < "),
-            records.join("\n"),
+            "widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}",
+            chain.join(" < ")
         ));
     }
     match drawn {

tests/shrink.rs

@@ -70,14 +70,10 @@ 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 {
-            if diverges(&grown, case, seed).is_none() {
+            let Some(how) = diverges(&grown, case, seed) else {
                 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(),