Keep the step 3/4 experiment as evidence

The worker's uncommitted attempt at evaluating children in parent-decided
boxes, preserved as it stood when it stopped: separate answer-only pixel
reads, per-child drawing contracts, provisional Fill asks in Span, and an
assertion that a placed drawing hold for its answer box. The suite passes
and every generated case stops on that assertion at Text. Not the
protocol; wip/one-ask is.

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,
@@ -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",

core/src/ui/active.rs

@@ -1,6 +1,6 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive,
+    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive, Size,
-    Size, TextureHandle, UiRegion, UiVec2, WidgetId,
+    TextureHandle, UiRegion, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -9,39 +9,35 @@ use crate::{
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
-    /// Where its drawing goes, in its region node's coordinates.
+    /// Its frame in `parent_move`'s coordinates: what a fraction it declares
+    /// or reports is a fraction of, composed. Everything it draws sits inside
+    /// this by way of `extent`.
+    pub frame_abs: UiRegion,
+    /// Where its drawing goes, in the frame's own coordinates.
     pub extent: UiRegion,
-    /// What a fraction declared or reported under this widget is a fraction
+    /// That frame in its parent's frame coordinates, before composition:
-    /// of, as a length of the window.
+    /// forwarded whole by a transparent container, narrowed by a declared
-    pub frame: UiVec2,
+    /// length. Its length is the same on every ask, which is what
-    /// A frame its parent decided for it on each axis -- a row's slot, or
+    /// a local redraw relies on to ask its parent's own question again.
-    /// padding's frame less its pixels -- as a length of the window. `None`
+    pub frame: UiRegion,
-    /// forwards the parent's frame. What it declared is kept separately in
+    /// What of its parent's extent the drawing was given, and what it was
-    /// `declared` and is a fraction of whichever of the two reached it.
+    /// given at the parent's first ask of it -- the question a cold layout
-    pub narrow: [Option<Len>; 2],
+    /// asks. A part is a length from the extent's start, so an extent that
-    /// Where its drawing was put, and where it was asked, each as a part of
+    /// moved re-places every child by re-adding that start.
-    /// its parent's box. The two differ where a container asks in one place
+    pub place: [Place; 2],
-    /// and puts the answer in another -- a row measures from its cursor and
+    pub offer_place: [Place; 2],
-    /// puts the child in its slot. A part is a length from the box's start,
+    /// The box that ask gave it, in its frame's coordinates. Kept rather
-    /// so a box that moved re-places every child by re-adding that start.
+    /// than worked out again from where its parent's own box is now: a
-    pub placed: [Place; 2],
+    /// parent drawn again in the box its own answer chose gives its children
-    pub asked: [Place; 2],
+    /// boxes it never measured anything in, and the measurement this widget
-    /// The box it was asked in, in the parent's region-node coordinates: the
+    /// answered is the one its parent's layout was built on.
-    /// box its drawing was made in and the one its contract is about. Its
+    pub offer_part: UiRegion,
-    /// drawing is placed elsewhere by re-expression, never by asking again.
-    pub part: UiRegion,
     /// The measured answer and its dependencies. A hint-only dependency or
     /// a widget first encountered during placement has no measurement yet.
     pub answer: Option<(Size, LayoutHolds)>,
-    /// Asked more than once in its parent's last draw -- measured in one box
+    /// What the widget said it used of its frame, the last time it drew.
-    /// and then asked in the one the parent decided. The parent's layout
-    /// rests on the first answer and its drawing on the last, so only the
-    /// parent can ask either again.
-    pub re_asked: bool,
-    /// What the widget reported, in window-unit lengths.
     pub size: Size,
-    /// The window and extent reads that this drawing holds for, and the
+    /// The frame and extent reads that this drawing holds for.
-    /// frame and box it pinned.
     pub holds: LayoutHolds,
     pub drawn: bool,
     pub parent: Option<WidgetId>,
@@ -53,7 +49,6 @@ pub struct ActiveData {
     /// 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>,
-    /// An owned mask holds one reference independently of its primitives.
     pub mask_region: Option<UiRegion>,
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
@@ -68,8 +63,7 @@ pub struct ActiveData {
     /// Its alignment when it was last drawn, which a change to the property
     /// is found against.
     pub own_align: RegionAlign,
-    /// The movable region whose coordinates `extent` uses when this widget
+    /// The movable region whose coordinates `frame_abs` uses.
-    /// does not own a region node.
     pub parent_move: MoveIdx,
     /// The mask its drawing is clipped to: one it set itself, or the one it
     /// inherited from whoever drew it.
@@ -83,10 +77,20 @@ pub struct ActiveData {
 }
 
 impl ActiveData {
-    /// What it answered when its parent asked, where it has been asked at
+    /// What it answered when its parent measured it, where it has been
-    /// all. Not `size`, which is what its last drawing reported: a drawing
+    /// measured at all. Not `size`, which is what its last drawing reported:
-    /// re-expressed in the box that answer chose is not a second answer.
+    /// a drawing made in the box that answer chose is answering a different
+    /// question.
     pub fn measured(&self) -> Option<Size> {
         self.answer.map(|(size, _)| size)
     }
+
+    /// Whether what it answered still stands for a frame of these pixel
+    /// lengths. The answer was given in the box its parent first asked
+    /// about, which is what it is checked against -- `holds` on the record
+    /// is about the box the answer then chose.
+    pub fn answers_at(&self, px: crate::PxVec2, part: UiRegion) -> bool {
+        self.answer
+            .is_some_and(|(_, holds)| holds.contains(px, part))
+    }
 }

core/src/ui/layout_holds.rs

@@ -1,35 +1,27 @@
-use crate::{Axis, Holds, Len, PxVec2, UiRegion, UiVec2};
+use crate::{Axis, Holds, Len, PxVec2, UiRegion};
 
 const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 
-/// What one evaluation of a widget depends on: the window lengths its reads
+/// What one evaluation of a widget depends on: the pixel lengths of its
-/// hold for, the pixel lengths of its own box, and the symbolic lengths of
+/// frame and of its own box that its drawing and its answer hold for, and
-/// that box and of its frame where either one is what it was expressed in.
+/// the symbolic length of its own box where it read one.
 ///
-/// The symbolic lengths are pins rather than ranges: a container places its
+/// The symbolic length is a pin rather than a range: 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
+/// is. It reaches the parent only where the box it pinned is the parent's
-/// parent's own; anywhere else the parent chose that length itself, and a
+/// own; anywhere else the parent chose that length itself, and a widget
-/// widget pinned this way is checked when it is re-placed.
+/// pinned this way is checked when it is re-placed.
-///
-/// A frame pin says the answer or the drawing is a fraction of the frame,
-/// which is a different length wherever the frame is a different one -- at
-/// the same window size, so no range of window pixels can say it. A length
-/// of the frame that is only pixels is not one: it is that many pixels
-/// whatever the frame turns out to be.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct LayoutHolds {
-    pub window: [Holds; 2],
+    pub frame: [Holds; 2],
-    pub frame_len: [Option<Len>; 2],
     pub extent: [Holds; 2],
     pub extent_len: [Option<Len>; 2],
 }
 
 impl LayoutHolds {
     pub const ANY: Self = Self {
-        window: [Holds::ANY; 2],
+        frame: [Holds::ANY; 2],
-        frame_len: [None; 2],
         extent: [Holds::ANY; 2],
         extent_len: [None; 2],
     };
@@ -37,42 +29,34 @@ impl LayoutHolds {
     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.frame[n] = self.frame[n].and(other.frame[n]);
             result.extent[n] = self.extent[n].and(other.extent[n]);
             debug_assert!(
                 self.extent_len[n].is_none()
                     || other.extent_len[n].is_none()
                     || self.extent_len[n] == other.extent_len[n]
             );
-            debug_assert!(
-                self.frame_len[n].is_none()
-                    || other.frame_len[n].is_none()
-                    || self.frame_len[n] == other.frame_len[n]
-            );
             result.extent_len[n] = self.extent_len[n].or(other.extent_len[n]);
-            result.frame_len[n] = self.frame_len[n].or(other.frame_len[n]);
         }
         result
     }
 
     pub fn covers(self, other: Self) -> bool {
         (0..2).all(|n| {
-            self.window[n].lo <= other.window[n].lo
+            self.frame[n].lo <= other.frame[n].lo
-                && self.window[n].hi >= other.window[n].hi
+                && 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 {
+    pub fn contains(self, px: PxVec2, 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[n].contains(px.axis(axis))
-                && self.frame_len[n].is_none_or(|pinned| pinned == frame.axis(axis))
+                && self.extent[n].contains(len.to_px(px.axis(axis)))
-                && self.extent[n].contains(len.to_px(window.axis(axis)))
                 && self.extent_len[n].is_none_or(|pinned| pinned == len)
         })
     }

core/src/ui/painter.rs

@@ -1,14 +1,14 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Holds, LayoutHolds, LayoutLen, Len, Part, Place, Px, PxVec2, RegionAlign, Rel,
+    Axis, Holds, LayoutHolds, LayoutLen, Len, Part, Place, Px, PxVec2, RegionAlign, RenderedText,
-    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
+    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
-    TextureHandle, UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    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];
 
@@ -17,41 +17,56 @@ 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
+    /// What a fraction this widget declares or reports is a fraction of, in
-    /// fraction it or anything under it declares or reports is a fraction
+    /// the coordinates of `move_idx`: forwarded from its parent unchanged
-    /// of. A length rather than a box, so padding can take from both the
+    /// through a span, a stack or a scroll, and narrowed only by what was
-    /// frame and the box without either becoming the other.
+    /// decided above it -- a declared length, or the root. Its length
-    pub(super) frame: UiVec2,
+    /// is the same on every ask of the widget, which is what keeps a fraction
-    /// Where this widget's drawing goes, in its region node's coordinates.
+    /// under it from being resolved twice.
+    pub(super) frame: UiRegion,
+    /// Where this widget's drawing goes, in the frame's own coordinates.
+    /// Everything it writes is in these coordinates, and its children are
+    /// placed as parts of it.
     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
+    /// Symbolic box lengths read only to compute the answer. A container can
-    /// unit, regardless of region-node boundaries.
+    /// replace the provisional drawings used for that answer with drawings
-    pub(super) window: PxVec2,
+    /// in decided boxes, so this contract is independent of the final one.
+    pub(super) answer_extent_len: [Option<Len>; 2],
+    /// The frame in pixels, which its children's frames are a length of:
+    /// threaded down rather than composed back up the chain, so every length
+    /// in layout is one multiply from its parent's and [`Holds::through`]
+    /// inverts exactly that.
+    pub(super) px: PxVec2,
     pub(super) mask: MaskIdx,
     pub(super) textures: Vec<TextureHandle>,
     pub(super) primitives: Vec<RetainedPrimitive>,
     pub(super) mask_region: Option<UiRegion>,
-    /// The previous drawing's owned mask, available for this draw to reclaim.
-    pub(super) mask_slot: Option<MaskIdx>,
     /// Only children whose answers were read constrain this widget's answer.
     pub(super) answer_under: LayoutHolds,
     pub(super) children: Vec<WidgetId>,
+    /// The children asked about so far, so the first place each was asked in
+    /// is the one recorded as its offer.
+    pub(super) offered: Vec<WidgetId>,
+    /// Whether this draw is at the place its parent first asked about, which
+    /// makes the questions it asks the ones a cold layout asks and their
+    /// answers the ones to keep.
+    pub(super) at_offer: bool,
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
-    /// What this draw itself read of the window in pixels, per axis: every
+    /// What this draw itself read of its frame in pixels, per axis: every
-    /// window until it reads one, then that one, unless it says otherwise.
+    /// length until it reads one, then that one, unless it says otherwise.
-    pub(super) window_own: [Holds; 2],
+    pub(super) frame_own: [Holds; 2],
-    /// Its frame's symbolic length where this draw read it, which makes the
+    /// The same for its own box.
-    /// 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
+    /// Pixel-box dependencies used only to compute the answer. These do not
-    /// its drawing, so it replaces this too rather than narrowing it.
+    /// constrain a retained drawing placed inside that answer.
+    pub(super) answer_extent_own: [Holds; 2],
+    /// The final drawing kept for each child. Asking one child again replaces
+    /// its provisional drawing and therefore replaces this contract too.
     pub(super) under: Vec<(WidgetId, LayoutHolds)>,
     /// The movable region this widget's primitives are positioned through:
     /// its own when opted in, otherwise the nearest ancestor's.
@@ -75,10 +90,12 @@ impl<'a> Painter<'a> {
         self.write_resolved(kind, primitive, region, self.resolve(region));
     }
 
-    /// A box in this widget's extent coordinates, composed into its region
+    /// A box in this widget's extent coordinates, composed into the
-    /// node's coordinates.
+    /// coordinates its move slot is in: through the extent, then through the
+    /// frame the extent is a part of. The same two steps a recomposition
+    /// replays, so a moved drawing lands where a cold one does.
     fn resolve(&self, region: UiRegion) -> UiRegion {
-        region.within(&self.extent)
+        region.within(&self.extent).within(&self.frame)
     }
 
     fn write_resolved<P: Primitive>(
@@ -106,6 +123,7 @@ impl<'a> Painter<'a> {
 
     fn push_primitive(&mut self, h: RetainedPrimitive) {
         if self.mask != MaskIdx::NONE {
+            // TODO: I have no clue if this works at all :joy:
             self.rsc.ui_mut().masks.push_ref(self.mask);
         }
         self.primitives.push(h);
@@ -130,23 +148,10 @@ impl<'a> Painter<'a> {
         assert!(self.mask == MaskIdx::NONE);
         let resolved = self.resolve(region);
         let move_idx = self.move_idx;
-        let mask = Mask {
+        self.mask = self.rsc.ui_mut().masks.push(Mask {
             region: resolved,
             move_idx,
-        };
+        });
-        let masks = &mut self.rsc.ui_mut().masks;
-        self.mask = match self.mask_slot.take() {
-            Some(idx) => {
-                *masks.get_mut(idx) = mask;
-                idx
-            }
-            None => {
-                let idx = masks.push(mask);
-                // The owner keeps the slot alive even with no primitives.
-                masks.push_ref(idx);
-                idx
-            }
-        };
     }
 
     /// Draws a widget in the whole of this widget's own box, with the frame
@@ -154,47 +159,63 @@ impl<'a> Painter<'a> {
     /// one child wants, and what every transparent container passes for the
     /// frame.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, [None; 2], [Place::Within(Part::All); 2])
+        self.widget_at(id, UiRegion::FULL, [Place::Within(Part::All); 2])
     }
 
-    /// Asks a child, saying what its fractions are of and where it is asked.
+    /// Draws a child, saying what its fractions are of and where its drawing
+    /// goes.
     ///
-    /// `narrow` is a length this widget decided for the child's frame, per
+    /// `frame` is that reference, in this widget's own frame coordinates:
-    /// axis, as a length of this widget's own frame: a resolved share, or a
+    /// [`UiRegion::FULL`] forwards this widget's frame, which is what a
-    /// box a sibling's answer decided. `None` forwards this widget's frame,
+    /// container that only divides room passes, so a fraction under it means
-    /// which is what a container that only divides room passes, so a
+    /// the same wherever it sits and however deeply it is nested. Narrowing
-    /// fraction under it means the same wherever it sits and however deeply
+    /// it is for what is decided from above, and a declared length narrows
-    /// it is nested. A declared length narrows the frame here whatever the
+    /// it here.
-    /// 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
+    /// `place` is where the drawing goes, per axis, as a part of this
-    /// widget's box: see [`Place`]. The child draws once, in that box, and
+    /// widget's extent: see [`Place`]. A narrowed frame is its own extent,
-    /// its answer is placed inside it by re-expressing the drawing. Nothing
+    /// since the narrowing is what said where the drawing goes.
-    /// 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],
+        frame: UiRegion,
         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) =
+        let (local, extent) = frame_and_extent(
-            frame_and_extent(self.extent, self.frame, place, narrow, declared, align);
+            frame,
+            part_of(self.extent, place),
+            narrowed_by(declared, frame),
+            align,
+        );
+        let within = match local == UiRegion::FULL {
+            true => self.frame,
+            false => local.within(&self.frame),
+        };
         #[cfg(feature = "layout-diagnostics")]
         if region_node {
             diag::bump(Counter::RegionNodeDraws);
-            diag::region_node(id.id(), self.id, extent);
+            diag::region_node(id.id(), self.id, within);
         }
         // A child listed twice would be moved twice.
-        let re_asked = self.children.contains(&id.id());
+        if !self.children.contains(&id.id()) {
-        if !re_asked {
             self.children.push(id.id());
         }
-        let px = frame.to_px(self.window);
+        let first_ask = self.offer(id.id());
+        let offer_place = if first_ask {
+            place
+        } else {
+            self.state
+                .active
+                .get(&id.id())
+                .map_or(place, |a| a.offer_place)
+        };
+        let px = local.size().to_px(self.px);
+        // The answer and what it holds for, both about the place asked in.
+        // The child's record may say something else once its drawing has been
+        // placed: a drawing made again in its placed box holds for that box.
         let (size, answer_holds, holds) = self.state.draw_inner(
             id.id(),
             DrawInfo {
@@ -204,19 +225,18 @@ impl<'a> Painter<'a> {
                 parent_move: self.move_idx,
                 region_node,
                 mask: self.mask,
-                frame,
+                frame: local,
+                frame_abs: within,
                 part: extent,
-                placed: place,
+                place,
-                asked: place,
+                offer_place,
-                narrow,
-                re_asked,
                 px,
             },
             None,
             self.rsc,
         );
-        let holds = self.in_parent(holds, extent, place, narrow, declared);
+        let compose = |holds| in_parent(holds, local, extent, place, declared);
-        let answer_holds = self.in_parent(answer_holds, extent, place, narrow, declared);
+        let holds = compose(holds);
         match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
             Some((_, kept)) => *kept = holds,
             None => self.under.push((id.id(), holds)),
@@ -224,8 +244,8 @@ impl<'a> Painter<'a> {
         DrawResult {
             child: id,
             painter: self,
-            size,
+            size: in_parent_frame(size, local.size(), declared),
-            answer_holds,
+            answer_holds: compose(answer_holds),
         }
     }
 
@@ -238,33 +258,6 @@ impl<'a> Painter<'a> {
         self.state.undraw_rec(id.id(), self.rsc);
     }
 
-    /// Puts a child asked about in this draw somewhere else in this
-    /// widget's box: its answer, placed in this part instead. The drawing
-    /// is re-expressed there rather than made again -- what a row does once
-    /// it knows every slot, having measured each child from its cursor.
-    pub fn place_at<W: ?Sized>(&mut self, id: &StrongWidget<W>, place: [Place; 2]) {
-        debug_assert!(
-            self.children.contains(&id.id()),
-            "'{}' placed a child it did not ask about in this draw",
-            self.label()
-        );
-        let at = self.placing();
-        self.state.place_in(id.id(), &at, place, self.rsc);
-    }
-
-    /// This widget as the thing its children are placed within.
-    fn placing(&self) -> Placing {
-        Placing {
-            id: self.id,
-            extent: self.extent,
-            frame: self.frame,
-            window: self.window,
-            depth: self.depth,
-            move_idx: self.move_idx,
-            mask: self.mask,
-        }
-    }
-
     /// What a widget's rules declare its lengths to be, which whoever draws
     /// it resolves into its frame. Reading them depends on nothing -- the box
     /// that comes of them is kept on the child, and `redraw` compares it
@@ -273,10 +266,8 @@ impl<'a> Painter<'a> {
         declared_lens(self.rsc.widgets(), id.id())
     }
 
-    /// What a child says its length is without being drawn, if it can say,
+    /// 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
+    /// Asking counts as reading its size.
-    /// against this widget's frame, which is the frame a child asked with
-    /// nothing narrowed gets. Asking counts as reading its size.
     pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
         let widgets = self.rsc.widgets();
         // A rule is the answer where there is one: it wins over whatever the
@@ -286,27 +277,34 @@ 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, hint);
-            diag::hint_read(id.id(), self.id, axis, resolved);
+        match hint {
-            diag::bump(match resolved {
+            Some(hint) => {
-                Some(_) => Counter::HintHits,
+                #[cfg(feature = "layout-diagnostics")]
-                None => Counter::HintMisses,
+                diag::bump(Counter::HintHits);
-            });
-        }
-        if let Some(hint) = hint {
                 self.depend_on(id);
-            // Resolving a fraction against this frame makes this draw a
+                Some(hint)
-            // function of the frame's length. The fraction to ask about is
+            }
-            // the child's own: resolved against a frame of pixels, none is
+            None => {
-            // left to see it by.
+                #[cfg(feature = "layout-diagnostics")]
-            if hint.rel != Rel::ZERO {
+                diag::bump(Counter::HintMisses);
-                self.frame_own_len[axis as usize] = Some(frame);
+                None
             }
         }
-        resolved
+    }
+
+    /// Whether this is the first box a child is asked about in during a draw
+    /// that is itself the one its parent measured -- the question a cold
+    /// layout asks, whose answer is the one to keep. A drawing made again in
+    /// a box chosen from an answer asks about that box instead, and what it
+    /// hears back is not a measurement of anything.
+    fn offer(&mut self, child: WidgetId) -> bool {
+        if !self.at_offer || self.offered.contains(&child) {
+            return false;
+        }
+        self.offered.push(child);
+        true
     }
 
     fn depend_on<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
@@ -371,20 +369,31 @@ impl<'a> Painter<'a> {
     pub fn extent_len(&mut self, axis: Axis) -> Len {
         let len = self.extent.axis(axis).len();
         self.extent_len[axis as usize] = Some(len);
+        self.answer_extent_len[axis as usize] = Some(len);
         len
     }
 
-    /// The symbolic length of this widget's frame along one axis: what a
+    /// The symbolic length used to compute this widget's answer, where the
-    /// fraction it or anything under it declares is a fraction of. A
+    /// final drawing itself is rebuilt without depending on that length.
-    /// container reads it to hand a length of it down -- padding, which
+    pub fn answer_extent_len(&mut self, axis: Axis) -> Len {
-    /// takes its pixels off. Reading it pins the drawing to that frame, the
+        let len = self.extent.axis(axis).len();
-    /// way [`Self::extent_len`] pins it to the box.
+        self.answer_extent_len[axis as usize] = Some(len);
-    pub fn frame_len(&mut self, axis: Axis) -> Len {
-        let len = self.frame.axis(axis);
-        self.frame_own_len[axis as usize] = Some(len);
         len
     }
 
+    /// Says that the final drawing uses a symbolic length already read for
+    /// the answer.
+    pub fn drawing_uses_extent_len(&mut self, axis: Axis, len: Len) {
+        debug_assert_eq!(self.extent.axis(axis).len(), len);
+        self.extent_len[axis as usize] = Some(len);
+    }
+
+    /// A part of this widget's box, expressed in its frame coordinates so it
+    /// can be used as a child frame decided here.
+    pub fn extent_part(&self, axis: Axis, part: Part) -> UiSpan {
+        part.of(*self.extent.axis(axis))
+    }
+
     /// Where this widget sits in a box longer than the length it takes. A
     /// widget that positions its own content reads it to place that content
     /// the way the box around it would have placed the widget.
@@ -416,11 +425,19 @@ impl<'a> Painter<'a> {
         PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
     }
 
+    /// This widget's own box in pixels, used only to compute its answer.
+    pub fn answer_px_size(&mut self) -> PxVec2 {
+        PxVec2::new(
+            self.answer_px_len(Axis::X),
+            self.answer_px_len(Axis::Y),
+        )
+    }
+
     /// One axis of this widget's own box in pixels. Prefer this to
     /// [`Self::px_size`] when the other axis cannot affect the drawing.
     pub fn px_len(&mut self, axis: Axis) -> Px {
         let part = self.extent.axis(axis).len();
-        let len = part.to_px(self.window.axis(axis));
+        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);
@@ -428,6 +445,17 @@ impl<'a> Painter<'a> {
         len
     }
 
+    /// One pixel length used only to compute this widget's answer. The final
+    /// drawing may be retained when that answer is placed in another box.
+    pub fn answer_px_len(&mut self, axis: Axis) -> Px {
+        let len = self.extent.axis(axis).len().to_px(self.px.axis(axis));
+        let own = &mut self.answer_extent_own[axis as usize];
+        if *own == Holds::ANY {
+            *own = Holds::at(len);
+        }
+        len
+    }
+
     /// The lengths of this widget's own box on `axis` that what it is drawing
     /// holds for -- the same primitives, in the same fractions and offsets
     /// of the box, and the same reported size. A widget that read its length
@@ -436,7 +464,7 @@ impl<'a> Painter<'a> {
         let part = self.extent.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
@@ -444,34 +472,30 @@ impl<'a> Painter<'a> {
         self.extent_own[axis as usize] = holds;
     }
 
-    /// A window length in pixels, which is what every length in layout is
+    /// One axis of this widget's frame in pixels -- what a fraction of its
-    /// measured in. Reading one pins the drawing to this window wherever the
+    /// area resolves against, and so what a container divides among its
-    /// length is a fraction of it; one that is only pixels is that many
+    /// children. Its own box is a part of this one.
-    /// pixels in any window and pins nothing.
+    pub fn frame_px_len(&mut self, axis: Axis) -> Px {
-    pub fn to_px(&mut self, len: Len, axis: Axis) -> Px {
+        let len = self.px.axis(axis);
-        let window = self.window.axis(axis);
+        let own = &mut self.frame_own[axis as usize];
-        if len.rel != Rel::ZERO {
-            let own = &mut self.window_own[axis as usize];
         if *own == Holds::ANY {
-                *own = Holds::at(window);
+            *own = Holds::at(len);
         }
-        }
+        len
-        len.to_px(window)
     }
 
-    /// The windows this drawing holds for, stated rather than taken: a
+    /// [`Self::holds`] stated about the frame rather than about this
-    /// container that branched on a length in pixels says which side of the
+    /// widget's own box, for a container whose drawing turns on what its
-    /// boundary it was on, which is wider than the one window reading that
+    /// fractions are of rather than on the part of it it took.
-    /// length pins, and replaces it.
+    pub fn frame_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
-    pub fn window_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
         let holds = holds.into();
         debug_assert!(
-            holds.contains(self.window.axis(axis)),
+            holds.contains(self.px.axis(axis)),
-            "'{}' ({:?}) says its drawing holds for windows that leave out this one",
+            "'{}' ({:?}) says its drawing holds for lengths that leave out its frame",
             self.label(),
             self.id
         );
-        self.window_own[axis as usize] = holds;
+        self.frame_own[axis as usize] = holds;
     }
 
     pub fn text_data(&mut self) -> &mut TextData {
@@ -559,77 +583,70 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
-/// Moves what a child depends on into this widget's own terms: this
+/// What a child depends on, said about the boxes the widget that drew it
-/// method's `impl` block is where a `Painter`'s own boxes are, so it takes
+/// has rather than the ones the child was given.
-/// 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,
+/// `frame` is the child's frame in this widget's frame coordinates and
-    /// where the box the child was asked in is that part; a declared length
+/// `extent` the box it was given, in the child's own frame coordinates. Both
-    /// places the box inside the part instead, and then only that length
+/// reach it as one length, so what it holds for maps back through that
-    /// reaches the child. A narrowed frame is not one of these: it decides
+/// length exactly -- and where the box it was given is this widget's own,
-    /// what fractions under the child mean and leaves the box the part it
+/// what it says about that box is what this widget can say about its own.
-    /// was given.
+pub(crate) fn in_parent(
-    fn in_parent(
-        &self,
     holds: LayoutHolds,
+    frame: UiRegion,
     extent: UiRegion,
     place: [Place; 2],
-        narrow: [Option<Len>; 2],
     declared: [Option<LayoutLen>; 2],
 ) -> LayoutHolds {
     let mut result = LayoutHolds::ANY;
     for axis in AXES {
         let n = axis as usize;
-            // Every read became pixels against the window, so a range on
+        let frame_len = frame.axis(axis).len();
-            // it is already in this widget's terms.
+        result.frame[n] = holds.frame[n].through(frame_len);
-            result.window[n] = holds.window[n];
+        match (place[n].part(), declared[n]) {
-            let reaches = narrow[n].is_none()
+            // Its box is this widget's own, or a part of it in that box's
-                && !matches!(place[n].part(), Part::Sized(_))
+            // own lengths: so what it holds for is a range on this widget's
-                && declared[n].is_none_or(|len| len.rel != Rel::ZERO);
+            // own box, which is what lets that box move without a redraw. A
-            result.frame_len[n] = holds.frame_len[n].and(reaches.then(|| self.frame.axis(axis)));
+            // length it pinned is this widget's length wherever the part is
-            match (place[n].part(), declared[n].is_some()) {
+            // the whole of it, and pins the same way.
-                // Its box is this widget's own, or a part of it in that
+            (Part::All, None) if *frame.axis(axis) == UiSpan::FULL => {
-                // 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
+            // Its box is a part of this widget's own box, in that box's own
-                // own lengths, so what it holds for maps back through that
+            // lengths, so what it holds for maps back through that part into
-                // part into a range on this widget's box. A length it pinned
+            // a range on this widget's box.
-                // is this widget's length less the part's pixels where the
+            (Part::Of(span), None) => {
-                // part is the whole of the box less pixels, which is the one
+                result.extent[n] = holds.extent[n].through(span.len());
-                // 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
+            // Its box is a part of this widget's frame: a length of the
-                // frame or from a sibling's answer: no length of this
+            // frame is all that reaches it, so what it holds for is a range
-                // widget's box reaches it, so what it holds for is a range
+            // on the frame and none of it on this widget's own box.
-                // on the window and none of it on that box.
             _ => {
-                    result.window[n] =
+                result.frame[n] = result.frame[n].and(
-                        result.window[n].and(holds.extent[n].through(extent.axis(axis).len()));
+                    holds.extent[n]
+                        .through(extent.axis(axis).len())
+                        .through(frame_len),
+                );
             }
         }
     }
     result
 }
+
+/// A child's answer as lengths of the parent's own region. A widget reports
+/// a fraction of its own region, and `of` is that region as a length of this
+/// one. Pixels come through untouched, being that many pixels wherever they
+/// end up. A declared axis is already the parent's: it resolved the rule in
+/// its own region, and the rule is what the report says.
+fn in_parent_frame(size: Size, of: UiVec2, declared: [Option<LayoutLen>; 2]) -> Size {
+    let mut size = size;
+    for (axis, declared) in AXES.into_iter().zip(declared) {
+        if declared.is_none() {
+            *size.axis_mut(axis) = size.axis(axis).within_len(of.axis(axis));
+        }
+    }
+    size
 }
 
 /// What a widget declares a length of its box to be. `leftover` is not one: a
@@ -694,58 +711,59 @@ pub(crate) fn placed_extent(
     placed
 }
 
-/// The frame length and the box a child is asked in, in the coordinates the
+/// The part of a widget's own box a `place` names, in the coordinates that
-/// widget asking draws in.
+/// box is in.
-///
+pub(crate) fn part_of(extent: UiRegion, place: [Place; 2]) -> UiRegion {
-/// `own` is that widget's own box, and `place` what of it the child is
+    let mut part = extent;
-/// 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],
-    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;
+        *part.axis_mut(axis) = place[axis as usize].part().of(*extent.axis(axis));
-        let sized = match place[n].part() {
+    }
-            Part::Sized(len) => Some(len),
+    part
-            _ => None,
+}
-        };
+
-        let base = sized
+/// The length a rule gives a child's frame, per axis: a fraction in it is a
-            .or(narrow[n])
+/// fraction of the frame the child was given, which is the one length the
-            .unwrap_or_else(|| parent_frame.axis(axis));
+/// rule can mean.
-        let len = declared[n]
+pub(crate) fn narrowed_by(declared: [Option<LayoutLen>; 2], frame: UiRegion) -> [Option<Len>; 2] {
-            .map(|len| Len::from_parts(len.rel, len.px).within_len(base))
+    AXES.map(|axis| {
-            .unwrap_or(base);
+        declared[axis as usize]
-        *frame.axis_mut(axis) = len;
+            .map(|len| Len::from_parts(len.rel, len.px).within_len(frame.axis(axis).len()))
-        if declared[n].is_some() {
+    })
+}
+
+/// The frame a child is asked in and the box its drawing goes in, both in
+/// the coordinates of the widget asking.
+///
+/// `frame` is what the caller said the child's fractions are of, and `part`
+/// what of the caller's own box the drawing takes. `narrow` is the length a
+/// declared rule gives the frame, which makes the frame the box the drawing
+/// goes in -- a rule is what decided where it goes, and there is nothing
+/// left to place inside it. A caller that narrowed the frame itself said the
+/// same thing.
+///
+/// The length is the caller's to supply so that a widget asked again gets
+/// the frame it already has rather than a second resolution of its rule.
+pub(crate) fn frame_and_extent(
+    mut frame: UiRegion,
+    part: UiRegion,
+    narrow: [Option<Len>; 2],
+    align: RegionAlign,
+) -> (UiRegion, UiRegion) {
+    let mut extent = part;
+    for (axis, narrow) in AXES.into_iter().zip(narrow) {
+        let span = frame.axis_mut(axis);
+        let narrowed = match narrow {
+            Some(len) => {
                 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);
+                *span = UiSpan::new(start, start + len);
+                true
+            }
+            None => *span != UiSpan::FULL,
+        };
+        if narrowed {
+            *extent.axis_mut(axis) = UiSpan::FULL;
         }
     }
     (frame, extent)
 }
-
-/// The part of a widget's own box a `place` names, in the coordinates that
-/// box is in.
-fn part_of(extent: UiRegion, place: [Place; 2], align: RegionAlign) -> UiRegion {
-    let mut part = extent;
-    for axis in AXES {
-        *part.axis_mut(axis) = place[axis as usize]
-            .part()
-            .of(*extent.axis(axis), align.axis(axis));
-    }
-    part
-}

core/src/ui/place.rs

@@ -1,42 +1,30 @@
-use crate::{AxisAlign, Len, PrimitiveHandle, UiRegion, UiSpan};
+use crate::{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
+    /// Frame lengths from where the box starts, which is what a container
-    /// dividing room among its children speaks: a child's report is a window
+    /// dividing room among its children speaks: a child's report is a length
-    /// length, so the cursor that sums those reports is one too. A moved box
+    /// of the frame, so the cursor that sums those reports is one too. A
-    /// re-places every child by re-adding its start, exactly. A fraction
+    /// moved box re-places every child by re-adding its start, exactly.
-    /// 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
+    /// length, where saying the same thing in frame lengths would make the
     /// container read its own box -- and a box chosen from its own answer
     /// then feeds back into the answer.
     Of(UiSpan),
-    /// 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 {
+    pub(crate) fn of(self, extent: UiSpan) -> 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)
-            }
         }
     }
 }

src/random.rs

@@ -120,32 +120,18 @@ impl Widget for Branch {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
         let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
         let measured = painter
-            .widget_at(&self.probe, [None; 2], [Place::Within(Part::All), top])
+            .widget_at(&self.probe, UiRegion::FULL, [Place::Within(Part::All), top])
             .len(Axis::X);
-        let len = measured.apply_leftover();
+        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
-        let px = painter.to_px(len, Axis::X);
-        // The range it actually branched on, said the way a container says
-        // one: pinning the window instead would redraw this widget on every
-        // resize, which is a fixture that never exercises reuse.
-        let threshold = Px::from_f32(self.threshold);
-        let holds = match px > threshold {
-            true => Holds::from(threshold + Px::STEP..=Px::MAX),
-            false => Holds::from(Px::MIN..=threshold),
-        };
-        painter.window_holds(Axis::X, holds.through(len));
 
-        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
+        let below = Place::Within(Part::Of(UiSpan::new(cut, Len::FULL)));
         let place = [Place::Within(Part::All), below];
-        match px > threshold {
+        match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, [None; 2], place),
+            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
-            false => painter.widget_at(&self.narrow, [None; 2], place),
+            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
         };
         Size::LEFTOVER
     }
-
-    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
-        Some(LayoutLen::LEFTOVER)
-    }
 }
 
 pub struct Spanned {

src/widget/mask.rs

@@ -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

@@ -15,6 +15,6 @@ impl Widget for Offset {
             moved(painter.extent_len(Axis::X), self.amt.x),
             moved(painter.extent_len(Axis::Y), self.amt.y),
         ];
-        painter.widget_at(&self.inner, [None; 2], place).size()
+        painter.widget_at(&self.inner, UiRegion::FULL, place).size()
     }
 }

src/widget/position/pad.rs

@@ -14,11 +14,12 @@ impl Widget for Pad {
         // widget -- the slack is the inner's to sit in, and forcing the near
         // edge pinned it to a corner it had not asked for.
         //
-        // Padding is an inset of both: it comes off the frame, so `rel(1)`
+        // The padding goes around what it pads: the frame passes through, so
-        // under it fills this widget rather than overflowing it by the
+        // the inner's fractions mean what they would without it, and only
-        // padding, and it comes off the box, so what is drawn sits inside.
+        // the box it draws in is moved in by the pixels. Said as a part of
-        // The two stay distinct -- the box can be narrower still, where a row
+        // this widget's own box in that box's own lengths, so nothing here
-        // asked this widget in the room left, and a text wraps at that.
+        // reads how long the box is -- and a box chosen from this widget's
+        // own answer therefore does not feed back into that answer.
         let inset = |lead: Px, trail: Px| {
             Place::Within(Part::Of(UiSpan::new(
                 Len::from_parts(Rel::ZERO, lead),
@@ -29,16 +30,7 @@ impl Widget for Pad {
             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
+        let inner = painter.widget_at(&self.inner, UiRegion::FULL, place).size();
-        // fraction of it: a frame is a length of the window like everything
-        // else here, and taking the padding off is the whole of what this
-        // widget does to it.
-        let narrow = [
-            (Axis::X, self.padding.left + self.padding.right),
-            (Axis::Y, self.padding.top + self.padding.bottom),
-        ]
-        .map(|(axis, pixels)| Some(painter.frame_len(axis) - Len::from_parts(Rel::ZERO, pixels)));
-        let inner = painter.widget_at(&self.inner, narrow, place).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,

src/widget/position/scroll.rs

@@ -12,11 +12,12 @@ 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.
+        // Measured in the whole viewport, then drawn at the scrolled offset.
+        let whole = UiRegion::FULL;
         let answer_len = painter
-            .widget_at(&self.inner, [None; 2], [Place::Fill(Part::All); 2])
+            .widget_at(&self.inner, whole, [Place::Fill(Part::All); 2])
             .len(self.axis);
-        let fixed = painter.to_px(Len::from_parts(answer_len.rel, answer_len.px), self.axis);
+        let fixed = Len::from_parts(answer_len.rel, answer_len.px).to_px(container_len);
         self.container_len = container_len;
         self.content_len = fixed.max(container_len);
 
@@ -44,6 +45,7 @@ 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 content = UiSpan::FULL;
         // Content that fills the viewport and has not been scrolled is the
         // viewport, and is handed back as it came. Writing the same box as
         // its own length in pixels is the same box in another form, and the
@@ -51,20 +53,19 @@ 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 {
+        if 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)))
+            content = 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
+        // content box its own answer decided. Where it is drawn is the
-        // box, scrolled: its drawing moved there, not made again there.
+        // content box, scrolled.
-        painter.place_at(
+        painter.widget_at(
             &self.inner,
-            self.axis.pair(Place::Fill(content), Place::Fill(Part::All)),
+            whole,
+            self.axis
+                .pair(Place::Fill(Part::From(content)), Place::Fill(Part::All)),
         );
         // What it occupies is its box, on both axes: it clips its content to
         // that box, so it can neither take less of one nor honestly ask for
@@ -72,10 +73,6 @@ impl Widget for Scroll {
         // is.
         Size::LEFTOVER
     }
-
-    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
-        Some(LayoutLen::LEFTOVER)
-    }
 }
 
 impl Scroll {

src/widget/position/span.rs

@@ -13,8 +13,8 @@ impl Widget for Span {
         // The row: this span's own box, as a length of the frame its children
         // are laid out against. Its start is nothing's business -- a slot is
         // a length from it -- so what this reads is the length alone.
-        let far = painter.extent_len(axis);
+        let far = painter.answer_extent_len(axis);
-        let along = |from: Len, to: Len| match self.dir.sign {
+        let measure_along = |from: Len, to: Len| match self.dir.sign {
             Sign::Pos => UiSpan::new(from, to),
             Sign::Neg => UiSpan::new(far - to, far - from),
         };
@@ -22,30 +22,18 @@ impl Widget for Span {
         // whole of the row: a span is what contains its children there, and
         // nothing divides that axis.
         let across = Place::Within(Part::All);
-        // A length for every child before their final slots are chosen: from
+        // A length for every child before their final slots are chosen. The
-        // a hint where one says, and from drawing otherwise. The frame passes
+        // frame passes through unchanged, so `rel(0.5)` is half the area this
-        // through unchanged, so `rel(0.5)` is half the area this span was
+        // span was given whatever else is in it and wherever this child sits
-        // given whatever else is in it and wherever this child sits among
+        // among them; what it is drawn in is the room left from the cursor,
-        // them; what a drawn child is asked in is the room left from the
+        // because a text has to wrap at the width actually there.
-        // cursor, because a text has to wrap at the width actually there.
         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) {
+            let room = Place::Fill(Part::From(measure_along(cursor, far)));
-                Some(len) => {
+            let len = painter
-                    drawn_across.push(None);
+                .widget_at(child, UiRegion::FULL, axis.pair(room, across))
-                    len
+                .len(axis);
-                }
-                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)
-                }
-            };
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
             lens.push(len);
@@ -62,9 +50,10 @@ impl Widget for Span {
             |sum, len| sum + *len,
         );
 
+        let fixed_total = Len::from_parts(total.rel, total.px);
         // What is left for the shares to divide: the row less everything
         // fixed, as a length of the frame rather than a number of pixels.
-        let room = far - Len::from_parts(total.rel, total.px);
+        let room = far - fixed_total;
         // Whether anything is left over is a question in pixels: `rel(0.5)`
         // beside 300 px is full at 600 and overfull at 400. Asked of `room`
         // itself, and answered back through the same expression, so the
@@ -76,14 +65,26 @@ 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.frame_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.frame_holds(axis, holds.through(room));
         }
 
+        if shares {
+            painter.drawing_uses_extent_len(axis, far);
+        }
+        let drawing_far = match shares {
+            true => far,
+            false => fixed_total,
+        };
+        let along = |from: Len, to: Len| match self.dir.sign {
+            Sign::Pos => UiSpan::new(from, to),
+            Sign::Neg => UiSpan::new(drawing_far - to, drawing_far - from),
+        };
+
         // Across itself a span is as long as its longest child -- unless a
         // rule beside it gives that length outright, and then reading them
         // answers nothing and makes its size depend on theirs for it. A rule
@@ -100,7 +101,7 @@ 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.
@@ -108,7 +109,6 @@ impl Widget for Span {
             {
                 painter.undraw(child);
                 fixed.px += self.gap;
-                start = shared(fixed, taken, total.leftover, room);
                 continue;
             }
             let from = start;
@@ -119,26 +119,23 @@ impl Widget for Span {
             fixed.rel += len.rel;
             start = shared(fixed, taken, total.leftover, room);
             // Along the row the span says where the child goes, and that slot
-            // is the child's box outright rather than something to place an
+            // is the drawing's box outright rather than something to place an
-            // answer inside again. A share is decided here and nowhere
+            // answer inside again.
-            // else: its slot narrows its frame, and the child is asked in
+            let span = along(from, start);
-            // it, since a text wraps at the width it is actually given. A
+            let (frame, slot) = match len.leftover > Weight::ZERO && shares {
-            // fixed child's slot is its own answer, so a drawing made in the
+                true => (
-            // room is put there as it is, and one not made yet is made here.
+                    UiRegion::from_axis(
-            let slot = along(from, start);
+                        axis,
-            let place = axis.pair(Place::Fill(Part::From(slot)), across);
+                        painter.extent_part(axis, Part::From(span)),
-            let mut narrow = [None; 2];
+                        UiSpan::FULL,
-            if len.leftover > Weight::ZERO && shares {
+                    ),
-                narrow[axis as usize] = Some(slot.len());
+                    Place::Fill(Part::All),
-            }
+                ),
-            let used = match (across_len, narrow[axis as usize]) {
+                false => (UiRegion::FULL, Place::Fill(Part::From(span))),
-                (Some(across_len), None) => {
-                    painter.place_at(child, place);
-                    across_len
-                }
-                _ => painter.widget_at(child, narrow, place).len(!axis),
             };
+            let placed = painter.widget_at(child, frame, axis.pair(slot, across));
             if shrinks {
+                let used = placed.len(!axis);
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.
                 // A scalable child therefore makes Children scalable too;

src/widget/position/stack.rs

@@ -23,41 +23,28 @@ impl Widget for Stack {
             Some((i, child)) => {
                 painter.child_layer_at(i);
                 painter
-                    .widget_at(child, [None; 2], [Place::Fill(Part::All); 2])
+                    .widget_at(child, UiRegion::FULL, [Place::Fill(Part::All); 2])
                     .size()
             }
             None => Size::LEFTOVER,
         };
-        // Every other child gets the box the sizing child decided: the
-        // stack is that length, so that is the box they are asked in, and a
-        // fraction under them is a fraction of it. A share leaves the axis
-        // to whoever gave the stack its box. Where a child sits in a box
-        // bigger than itself is its own business.
-        let place = [Axis::X, Axis::Y].map(|axis| {
-            let len = size.axis(axis);
-            match len.leftover == Weight::ZERO {
-                true => Place::Fill(Part::Sized(Len::from_parts(len.rel, len.px))),
-                false => Place::Within(Part::All),
-            }
-        });
         for (i, child) in self.children.iter().enumerate() {
             if sizing == Some(i) {
                 continue;
             }
             painter.child_layer_at(i);
-            painter.widget_at(child, [None; 2], place);
+            let place = [Axis::X, Axis::Y].map(|axis| {
+                let len = size.axis(axis);
+                let part = match len.leftover > Weight::ZERO {
+                    true => Part::All,
+                    false => Part::From(UiSpan::new(Len::ZERO, Len::from_parts(len.rel, len.px))),
+                };
+                Place::Within(part)
+            });
+            painter.widget_at(child, UiRegion::FULL, place);
         }
         size
     }
-
-    /// Without a sizing child a stack is whatever box it is given, which it
-    /// can say without drawing anything.
-    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
-        match self.size {
-            StackSize::Default => Some(LayoutLen::LEFTOVER),
-            StackSize::Child(_) => None,
-        }
-    }
 }
 
 #[derive(Default, Debug)]

tests/cases/determinism.rs

@@ -24,15 +24,15 @@ impl Widget for BranchesOnMeasurement {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
         let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
         let measured = painter
-            .widget_at(&self.probe, [None; 2], [Place::Within(Part::All), top])
+            .widget_at(&self.probe, UiRegion::FULL, [Place::Within(Part::All), top])
             .len(Axis::X);
-        let px = painter.to_px(measured.apply_leftover(), Axis::X);
+        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
 
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
         let place = [Place::Within(Part::All), below];
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, [None; 2], place),
+            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
-            false => painter.widget_at(&self.narrow, [None; 2], place),
+            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
         };
         Size::LEFTOVER
     }

tests/cases/layout.rs

@@ -83,8 +83,10 @@ 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
+/// The same reading through a pad: padding goes around what it pads and
-/// adds itself back to the padded widget's reported length.
+/// does not narrow what a fraction under it is a fraction of, so half of the
+/// window plus the padding is what the pad takes and where the next child
+/// starts.
 #[test]
 fn a_pad_puts_its_padding_around_a_fraction_of_the_whole_box() {
     let mut h = Harness::new((400, 100));
@@ -95,83 +97,9 @@ 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, inner, (10, 10), (210, 90));
-    assert_corners!(h, padded, (0, 0), (210, 100));
+    assert_corners!(h, padded, (0, 0), (220, 100));
-    assert_corners!(h, tail, (210, 0), (310, 100));
+    assert_corners!(h, tail, (220, 0), (320, 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]
@@ -500,7 +428,8 @@ 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 drawn = active.extent.within(&active.frame_abs);
+    let region = h.render.moves.resolve(active.parent_move, drawn);
     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());
@@ -811,50 +740,3 @@ 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));
-}
-
-#[test]
-fn a_collapsed_share_keeps_the_gaps_before_the_next_slot() {
-    for dir in [Dir::RIGHT, Dir::LEFT, Dir::DOWN, Dir::UP] {
-        for collapsed in [1, 2] {
-            let mut h = Harness::new((400, 400));
-            let head = rect(Color::RED).add(&mut h.rsc);
-            h.set_len(head, dir.axis, 200);
-            let tail = rect(Color::BLUE).add(&mut h.rsc);
-            let tail_len = 200 - 10 * (collapsed + 1);
-            h.set_len(tail, dir.axis, tail_len);
-            let mut children: Vec<StrongWidget> = vec![head.add_strong(&mut h.rsc)];
-            let mut shares = Vec::new();
-            for _ in 0..collapsed {
-                let share = rect(Color::GREEN).add(&mut h.rsc);
-                shares.push(share);
-                children.push(share.add_strong(&mut h.rsc));
-            }
-            children.push(tail.add_strong(&mut h.rsc));
-            h.set_root(Span {
-                children,
-                dir,
-                gap: Px::from_int(10),
-            });
-            for share in shares {
-                assert!(h.region(&share).is_none());
-            }
-            let region = h.region(&tail).unwrap();
-            let (from, to) = match dir.sign {
-                Sign::Pos => (400 - tail_len, 400),
-                Sign::Neg => (0, tail_len),
-            };
-            assert_eq!(region.top_left.axis(dir.axis), Px::from_int(from));
-            assert_eq!(region.bot_right.axis(dir.axis), Px::from_int(to));
-        }
-    }
-}

tests/cases/retained.rs

@@ -12,6 +12,7 @@ struct Counted {
     draws: Rc<Cell<usize>>,
     size: Size,
     reads_box: bool,
+    reads_answer_box: bool,
 }
 
 impl Widget for Counted {
@@ -19,6 +20,8 @@ impl Widget for Counted {
         self.draws.set(self.draws.get() + 1);
         if self.reads_box {
             painter.px_size();
+        } else if self.reads_answer_box {
+            painter.answer_px_size();
         }
         self.size
     }
@@ -38,11 +41,18 @@ fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>
         draws: draws.clone(),
         size,
         reads_box,
+        reads_answer_box: false,
     }
     .add(&mut h.rsc);
     (id, Counts(draws))
 }
 
+fn answer_counted(h: &mut Harness, size: Size) -> (WeakWidget<Counted>, Counts) {
+    let (id, draws) = counted(h, size, false);
+    h.rsc[id].reads_answer_box = true;
+    (id, draws)
+}
+
 struct Layered {
     children: [StrongWidget<Rect>; 2],
     _revision: usize,
@@ -156,9 +166,9 @@ 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
+    // Its final slot is a parent decision, so it is evaluated there after
-    // moved there.
+    // the provisional ask established its length.
-    assert_eq!(asked_draws.get(), 1);
+    assert_eq!(asked_draws.get(), 2);
 }
 
 #[test]
@@ -216,7 +226,7 @@ impl Widget for FromHint {
         let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
         painter.widget_at(
             &self.inner,
-            [None; 2],
+            UiRegion::FULL,
             [Place::Within(Part::All), Place::Within(Part::From(top))],
         );
         Size::LEFTOVER
@@ -240,7 +250,7 @@ fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
     assert_corners!(h, inner, (0, 0), (400, 120));
 }
 
-/// Reads its box's size, which nothing but its own draw can put right.
+/// Reads its box's size to compute its answer.
 struct ReadsBox {
     draws: Rc<Cell<usize>>,
 }
@@ -248,25 +258,36 @@ struct ReadsBox {
 impl Widget for ReadsBox {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
-        Size::from_px(painter.px_size().div_int(4))
+        Size::from_px(painter.answer_px_size().div_int(4))
     }
 }
 
 /// Reads its box across one axis only, so its drawing holds for a taller
 /// box on its own and only a wider one is worth a draw.
 ///
-/// Both of these report a quarter of what they read. The quarter-sized box
+/// Both report a quarter of what they read. Their empty drawings are
-/// the answer places them in is not a question: the drawing is moved there,
+/// independent of that read, so a changed question costs one draw.
-/// so each length they are asked at costs one draw.
 struct ReadsWidth {
     draws: Rc<Cell<usize>>,
 }
 
+struct ReadsDrawingWidth {
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for ReadsDrawingWidth {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.px_len(Axis::X);
+        Size::LEFTOVER
+    }
+}
+
 impl Widget for ReadsWidth {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
         Size::from_px(PxVec2::new(
-            painter.px_len(Axis::X).div_int(4),
+            painter.answer_px_len(Axis::X).div_int(4),
             Px::from_int(20),
         ))
     }
@@ -322,6 +343,7 @@ fn a_row_moves_what_follows_a_child_that_grew_rather_than_drawing_it() {
             draws: ruled.clone(),
             size: Size::LEFTOVER,
             reads_box: false,
+            reads_answer_box: false,
         };
         let second = match declared {
             true => second.width(rel(0.25)).add(&mut h.rsc),
@@ -499,7 +521,7 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     // Every wrapper up to the outer pad read the size below it, so the outer
     // pad is what draws again -- and the span it hands the box to is the same
     // size as before, which is what lets a draw reuse its way past the leaf.
-    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
+    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), false);
     let padded = leaf.pad(10).add(&mut h.rsc);
     let below = rect(Color::RED).add(&mut h.rsc);
     h.set_root((padded, below).span(Dir::DOWN).pad(12));
@@ -877,7 +899,7 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
+                UiRegion::FULL,
                 [
                     Place::Within(Part::From(self.region.x)),
                     Place::Within(Part::From(self.region.y)),
@@ -966,7 +988,7 @@ 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],
+                self.region,
                 [
                     Place::Fill(Part::From(self.extent.x)),
                     Place::Fill(Part::From(self.extent.y)),
@@ -1065,7 +1087,7 @@ fn a_declared_size_change_stops_at_an_independent_parent() {
 fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     let mut h = Harness::new((400, 200));
     let draws = Rc::new(Cell::new(0));
-    let leaf = ReadsWidth {
+    let leaf = ReadsDrawingWidth {
         draws: draws.clone(),
     }
     .add(&mut h.rsc);
@@ -1076,7 +1098,7 @@ fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     h.frame();
 
     assert!(draws.get() > settled);
-    assert_corners!(h, leaf, (300, 90), (500, 110));
+    assert_corners!(h, leaf, (0, 0), (800, 200));
 }
 
 #[test]
@@ -1136,7 +1158,7 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
+                UiRegion::FULL,
                 [
                     Place::Fill(Part::From(self.extent.x)),
                     Place::Fill(Part::From(self.extent.y)),
@@ -1211,7 +1233,7 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
     struct Measured;
     impl Widget for Measured {
         fn draw(&mut self, painter: &mut Painter) -> Size {
-            let width = painter.px_len(Axis::X);
+            let width = painter.answer_px_len(Axis::X);
             painter.primitive(RectPrimitive::color(Color::BLUE));
             Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
         }
@@ -1224,7 +1246,7 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
+                UiRegion::FULL,
                 [
                     Place::Fill(Part::From(UiSpan::new(
                         Len::px(self.start),
@@ -1266,7 +1288,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    [None; 2],
+                    UiRegion::FULL,
                     [
                         Place::Within(Part::From(self.region.x)),
                         Place::Within(Part::From(self.region.y)),
@@ -1286,7 +1308,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 painter
                     .widget_at(
                         &self.child,
-                        [None; 2],
+                        UiRegion::FULL,
                         [
                             Place::Fill(Part::From(self.extent.x)),
                             Place::Fill(Part::From(self.extent.y)),
@@ -1311,7 +1333,10 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 } else {
                     Size::from((80, 27))
                 };
-                let (leaf, _) = counted(h, size, !fractional);
+                let (leaf, _) = match fractional {
+                    true => counted(h, size, false),
+                    false => answer_counted(h, size),
+                };
                 let child = Container {
                     child: leaf.add_strong(&mut h.rsc),
                     region,
@@ -1342,154 +1367,3 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
         }
     }
 }
-
-struct OptionalMask {
-    inner: StrongWidget,
-    enabled: bool,
-}
-
-impl Widget for OptionalMask {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        if self.enabled {
-            painter.set_mask(UiRegion::FULL);
-        }
-        painter.widget(&self.inner);
-        Size::LEFTOVER
-    }
-}
-
-fn primitive_masks(h: &Harness, id: WidgetId) -> Vec<MaskIdx> {
-    h.render.active[&id]
-        .primitives
-        .iter()
-        .map(|primitive| {
-            let handle = &primitive.handle;
-            h.render.layers[handle.layer].primitives()[handle.kind as usize]
-                .as_ref()
-                .unwrap()
-                .instances()[handle.inst_idx]
-                .mask_idx
-        })
-        .collect()
-}
-
-#[test]
-fn a_redrawn_mask_keeps_reused_primitives_clipped_when_it_moves() {
-    for node in [false, true] {
-        let mut h = Harness::new((400, 200));
-        let first = rect(Color::RED).height(50).add(&mut h.rsc);
-        let inner = rect(Color::BLUE).add(&mut h.rsc);
-        let draws = Rc::new(Cell::new(0));
-        let child = Stretchy {
-            inner: inner.add_strong(&mut h.rsc),
-            draws: draws.clone(),
-        }
-        .add(&mut h.rsc);
-        let masked = child.masked().add(&mut h.rsc);
-        h.rsc.widgets_mut().set_region_node(masked, node);
-        h.set_root((first, masked).span(Dir::DOWN));
-        let mask = h.render.active[&masked.id()].mask;
-        let settled = draws.get();
-        h.rsc.widgets_mut().get_dyn_mut(masked.id());
-        h.frame();
-        assert_eq!(primitive_masks(&h, inner.id()), vec![mask]);
-        assert_eq!(draws.get(), settled, "a mask repaint must reuse its child");
-        assert_eq!(h.render.active[&masked.id()].mask, mask);
-        h.set_len(first, Axis::Y, 10);
-        h.frame();
-        let clip = h.rsc.ui().masks[mask.idx()];
-        let clip = h
-            .render
-            .moves
-            .resolve(clip.move_idx, clip.region)
-            .to_px(h.render.output_size());
-        assert_eq!(clip, h.region(&masked).unwrap());
-        assert_corners!(h, inner, (0, 10), (400, 200));
-    }
-}
-
-#[test]
-fn adding_and_removing_a_mask_updates_existing_primitives() {
-    let mut h = Harness::new((400, 200));
-    let inner = rect(Color::BLUE).add(&mut h.rsc);
-    let masked = OptionalMask {
-        inner: inner.add_strong(&mut h.rsc),
-        enabled: false,
-    }
-    .add(&mut h.rsc);
-    h.set_root(masked);
-    for enabled in [true, false, true, false] {
-        h.rsc[masked].enabled = enabled;
-        h.frame();
-        let mask = h.render.active[&masked.id()].mask;
-        assert_eq!(mask == MaskIdx::NONE, !enabled);
-        assert_eq!(primitive_masks(&h, inner.id()), vec![mask]);
-    }
-    assert_eq!(h.rsc.ui().masks.len(), 1, "retired slots must be reusable");
-}
-
-#[test]
-fn an_empty_masks_slot_is_released_when_the_mask_is_removed_or_undrawn() {
-    let mut h = Harness::new((400, 200));
-    let (inner, _) = counted(&mut h, Size::LEFTOVER, false);
-    let masked = OptionalMask {
-        inner: inner.add_strong(&mut h.rsc),
-        enabled: true,
-    }
-    .add(&mut h.rsc);
-    let row = (masked,).span(Dir::DOWN).add(&mut h.rsc);
-    h.set_root(row);
-    for _ in 0..3 {
-        h.rsc[masked].enabled = false;
-        h.frame();
-        h.rsc[masked].enabled = true;
-        h.frame();
-        let child = h.rsc[row].pop().unwrap();
-        h.frame();
-        h.rsc[row].push(child);
-        h.frame();
-    }
-    assert_eq!(h.rsc.ui().masks.len(), 1);
-}
-
-struct SharedChild(Rc<StrongWidget>);
-
-impl Widget for SharedChild {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        painter.widget(self.0.as_ref()).size()
-    }
-}
-
-struct SwitchParent {
-    choices: [StrongWidget; 2],
-    choice: usize,
-}
-
-impl Widget for SwitchParent {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        painter.widget(&self.choices[self.choice]).size()
-    }
-}
-
-#[test]
-fn a_redrawn_subtree_is_not_undrawn_by_the_parent_it_left() {
-    for node in [false, true] {
-        let mut h = Harness::new((400, 200));
-        let leaf = rect(Color::RED).width(40).add(&mut h.rsc);
-        let held: StrongWidget = leaf.add_strong(&mut h.rsc);
-        let shared = Rc::new(held);
-        let first = SharedChild(shared.clone()).add_strong(&mut h.rsc);
-        let second = SharedChild(shared).add_strong(&mut h.rsc);
-        h.rsc.widgets_mut().set_region_node(&second, node);
-        let root = SwitchParent {
-            choices: [first, second],
-            choice: 0,
-        }
-        .add(&mut h.rsc);
-        h.set_root(root);
-        let before = h.region(&leaf);
-        h.rsc[root].choice = 1;
-        h.frame();
-        assert_eq!(h.region(&leaf), before);
-    }
-}

tests/cases/scroll.rs

@@ -145,19 +145,3 @@ 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

@@ -733,51 +733,6 @@ 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 +846,3 @@ 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/generated.rs

@@ -14,7 +14,7 @@
 #[path = "scenario/mod.rs"]
 mod scenario;
 
-use iris::random::{Edits, Plan, plan};
+use iris::random::{Edits, plan};
 use scenario::{ALL, Case, diverges, env, over_seeds};
 
 /// How deep the generator branches. The generator widens two to four ways per
@@ -32,11 +32,8 @@ fn depth() -> usize {
 const SEEDS: [u64; 10] = [1, 2, 3, 5, 8, 10, 13, 20, 86, 98];
 
 fn check(seed: u64, depth: usize, case: Case) {
-    check_plan(&plan(seed, depth, &Edits::default()), seed, depth, case);
+    let grown = plan(seed, depth, &Edits::default());
-}
+    if let Some(how) = diverges(&grown, case, seed) {
-
-fn check_plan(grown: &Plan, seed: u64, depth: usize, case: Case) {
-    if let Some(how) = diverges(grown, case, seed) {
         panic!(
             "seed {seed} at depth {depth} differs after {}: {how}\n\
              reduce it with SHRINK_SEED={seed} SHRINK_DEPTH={depth} \
@@ -122,9 +119,8 @@ fn a_long_run_of_seeds_agrees() {
         None => (1..=env("IRIS_GENERATED_SEEDS", 100_u64)).collect(),
     };
     over_seeds(seeds, |seed| {
-        let grown = plan(seed, depth, &Edits::default());
         for case in ALL {
-            check_plan(&grown, seed, depth, case);
+            check(seed, depth, case);
         }
     });
 }

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.
@@ -427,17 +400,6 @@ 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);
@@ -451,16 +413,9 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
         if got == want {
             continue;
         }
-        let places: HashMap<WidgetId, usize> = tree
-            .ids
-            .iter()
-            .enumerate()
-            .map(|(i, &id)| (id, i))
-            .collect();
         // 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{}",
+            "widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}",
-            chain.join(" < "),
+            chain.join(" < ")
-            records.join("\n"),
         ));
     }
     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(),