Name the values layout carries, and say what a span's slot is

`along` said nothing about what it did. It is `Span::slot` now: the
stretch of the row between two distances from where the span starts
laying out, as a span of its own box, with the mirror for a negative
direction in one place. `far` is `row`, which is what the comment above
it already called it, and `shares` is `has_room` beside the
`any_leftover` it was folded into. `reached` now guards on the leftover
weight it divides by rather than on the numerator that happened to be
zero with it.

The pairs layout returns are named rather than positional: `Answer`
{size, holds} and `Drawn` {answer, drawing_holds} replace
`(Size, LayoutHolds)` and a three-tuple with two `LayoutHolds` in it,
which was the one shape the cold dump exists to catch. `try_reuse`
answers `bool` rather than `Option<()>`, and the four hand-written
copies of `move_idx != parent_move` are `ActiveData::is_region_node`.

`AXES` was declared in three modules; it is `Axis::BOTH`. `rel_min`,
`rel_max` and the unused `select_len` are gone -- `ZERO` and `FULL`
already said those. Three doc comments sat on `impl` blocks instead of
the single method inside them. `reposition` and `redepth` walked their
children by index, looking the parent up again per child; both take the
list and put it back. `Scroll`'s `fixed` and `fixed_len` are
`answer_px` and `answer_is_px`, which says which one is the length.

fmt, workspace clippy under `-D warnings` with and without
`layout-diagnostics`, and the workspace tests are clean. The cold dump
over 400 depth-5 trees is byte-identical to `6c84b6f`: 34,492 boxes,
no seed moved.

core/src/orientation/axis.rs

@@ -9,6 +9,9 @@ pub enum Axis {
 }
 
 impl Axis {
+    /// Both of them, for the layout code that asks the same question of each.
+    pub const BOTH: [Self; 2] = [Self::X, Self::Y];
+
     /// A per-axis pair with `aligned` on this axis and `ortho` on the other,
     /// which is what `from_axis` does for a vector.
     pub fn pair<T>(self, aligned: T, ortho: T) -> [T; 2] {

core/src/orientation/pos.rs

@@ -163,14 +163,6 @@ impl Len {
         Self::from_parts(Rel::ZERO, Px::from_f32(px))
     }
 
-    pub const fn rel_min() -> Self {
-        Self::ZERO
-    }
-
-    pub const fn rel_max() -> Self {
-        Self::FULL
-    }
-
     pub const fn max(&self, other: Self) -> Self {
         Self {
             rel: self.rel.max(other.rel),
@@ -213,10 +205,6 @@ impl Len {
         })
     }
 
-    pub fn select_len(&self, len: Len) -> Self {
-        len.within_len(*self)
-    }
-
     pub const fn flip(&mut self) {
         self.rel = Rel::ONE.sub(self.rel);
         self.px = self.px.neg();

core/src/ui/active.rs

@@ -27,7 +27,7 @@ pub struct ActiveData {
     pub region: 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)>,
+    pub answer: Option<Answer>,
     /// Asked more than once in its parent's last draw -- measured in one box
     /// and then asked in the one the parent decided. The parent's layout
     /// rests on the first answer and its drawing on the last, so only the
@@ -82,6 +82,20 @@ impl ActiveData {
     /// all. Not `size`, which is what its last drawing reported: a drawing
     /// re-expressed in the box that answer chose is not a second answer.
     pub fn measured(&self) -> Option<Size> {
-        self.answer.map(|(size, _)| size)
+        self.answer.map(|answer| answer.size)
+    }
+
+    /// Whether it owns a region node rather than sharing the one it was drawn
+    /// under, which is what its two move indices being different says.
+    pub fn is_region_node(&self) -> bool {
+        self.move_idx != self.parent_move
     }
 }
+
+/// What a widget answered when it was asked: the size it reported, and the
+/// boxes and windows that answer holds for.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Answer {
+    pub size: Size,
+    pub holds: LayoutHolds,
+}

core/src/ui/layout_holds.rs

@@ -1,8 +1,6 @@
 use crate::util::impl_axis_index;
 use crate::{Axis, Holds, Len, Px, PxVec2, UiRegion, UiVec2};
 
-const AXES: [Axis; 2] = [Axis::X, Axis::Y];
-
 /// What one evaluation of a widget depends on along one axis: the window
 /// lengths its reads hold for, the pixel lengths of its own box, and the
 /// symbolic lengths of that box and of its rel base where either one is what
@@ -101,7 +99,8 @@ impl LayoutHolds {
     }
 
     pub fn contains(self, window: PxVec2, rel_base: UiVec2, region: UiRegion) -> bool {
-        AXES.into_iter()
+        Axis::BOTH
+            .into_iter()
             .all(|axis| self[axis].contains(window[axis], rel_base[axis], region[axis].len()))
     }
 }

core/src/ui/painter.rs

@@ -10,7 +10,6 @@ use crate::{
     },
     ui::render_state::{DrawInfo, Placing},
 };
-const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 
 /// makes your surfaces look pretty
 pub struct Painter<'a> {
@@ -155,8 +154,8 @@ impl<'a> Painter<'a> {
     /// where that is this widget's own narrowed the way the region is. An
     /// axis the region leaves whole is not read at all, so a wrapper that
     /// only moves its child does not pin its drawing to a rel base.
-    fn state_rel_base(&mut self, mut place: PlaceDesc) -> PlaceDesc {
+    fn resolve_rel_base(&mut self, mut place: PlaceDesc) -> PlaceDesc {
-        for axis in AXES {
+        for axis in Axis::BOTH {
             if let Some(span) = place[axis].narrows_rel_base() {
                 let len = span.len();
                 let stated = (len != Len::FULL).then(|| len.within_len(self.rel_base(axis)));
@@ -181,7 +180,7 @@ impl<'a> Painter<'a> {
         id: &'s StrongWidget<W>,
         place: impl Into<PlaceDesc>,
     ) -> DrawResult<'s, 'a, W> {
-        let place = self.state_rel_base(place.into());
+        let place = self.resolve_rel_base(place.into());
         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());
@@ -198,7 +197,7 @@ impl<'a> Painter<'a> {
             self.children.push(id.id());
         }
         let px = rel_base.to_px(self.window);
-        let (size, answer_holds, holds) = self.state.draw_inner(
+        let drawn = self.state.draw_inner(
             id.id(),
             DrawInfo {
                 layer: self.layer,
@@ -217,8 +216,8 @@ impl<'a> Painter<'a> {
             None,
             self.rsc,
         );
-        let holds = self.in_parent(holds, region, place, declared);
+        let holds = self.in_parent(drawn.drawing_holds, region, place, declared);
-        let answer_holds = self.in_parent(answer_holds, region, place, declared);
+        let answer_holds = self.in_parent(drawn.answer.holds, region, place, declared);
         match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
             Some((_, kept)) => *kept = holds,
             None => self.under.push((id.id(), holds)),
@@ -226,7 +225,7 @@ impl<'a> Painter<'a> {
         DrawResult {
             child: id,
             painter: self,
-            size,
+            size: drawn.answer.size,
             answer_holds,
         }
     }
@@ -255,8 +254,8 @@ impl<'a> Painter<'a> {
         id: &'s StrongWidget<W>,
         place: impl Into<PlaceDesc>,
     ) -> DrawResult<'s, 'a, W> {
-        let place = self.state_rel_base(place.into());
+        let place = self.resolve_rel_base(place.into());
-        let states_rel_base = AXES
+        let states_rel_base = Axis::BOTH
             .iter()
             .any(|&axis| place[axis].stated_rel_base().is_some());
         if states_rel_base || !self.children.contains(&id.id()) {
@@ -579,10 +578,10 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
-/// Moves what a child depends on into this widget's own terms: this
-/// method's `impl` block is where a `Painter`'s own boxes are, so it takes
-/// only what the child was asked with.
 impl Painter<'_> {
+    /// Moves what a child depends on into this widget's own terms, taking
+    /// only what the child was asked with.
+    ///
     /// Window ranges are already about the one unit and combine directly.
     /// A rel base pin becomes this widget's own rel base wherever a length of it
     /// is what reached the child; where only pixels did, no length of this
@@ -602,7 +601,7 @@ impl Painter<'_> {
         declared: Declared,
     ) -> LayoutHolds {
         let mut result = LayoutHolds::ANY;
-        for axis in AXES {
+        for axis in Axis::BOTH {
             let declared = declared[axis];
             let holds = holds[axis];
             let result = &mut result[axis];
@@ -677,16 +676,16 @@ impl LayoutLen {
     }
 }
 
-/// Where a widget's drawing goes inside the part its parent gave it: what
-/// it reported, on the side of the part its alignment says, and the whole
-/// part wherever the answer fills it.
-///
-/// The length it reported is a length of its rel base, and the part is one too,
-/// so this takes one from the other rather than composing it into the part.
-/// That is what makes a fraction the same fraction wherever the part it is
-/// placed in sits and however long it is -- the fraction is resolved once,
-/// here, against the rel base it was reported of.
 impl PlaceDesc {
+    /// Where a widget's drawing goes inside the part its parent gave it: what
+    /// it reported, on the side of the part its alignment says, and the whole
+    /// part wherever the answer fills it.
+    ///
+    /// The length it reported is a length of its rel base, and the part is one
+    /// too, so this takes one from the other rather than composing it into the
+    /// part. That is what makes a fraction the same fraction wherever the part
+    /// it is placed in sits and however long it is -- the fraction is resolved
+    /// once, here, against the rel base it was reported of.
     pub(crate) fn placement(
         self,
         region: UiRegion,
@@ -695,7 +694,7 @@ impl PlaceDesc {
         align: RegionAlign,
     ) -> UiRegion {
         let mut placed = region;
-        for axis in AXES {
+        for axis in Axis::BOTH {
             let reported = size[axis];
             if reported.fills(declared[axis], self[axis].does_fill()) {
                 continue;
@@ -725,7 +724,7 @@ impl PlaceDesc {
         let given = self.of(own, align);
         let mut rel_base = parent_rel_base;
         let mut region = given;
-        for axis in AXES {
+        for axis in Axis::BOTH {
             let base = self[axis]
                 .stated_rel_base()
                 .unwrap_or_else(|| parent_rel_base[axis]);

core/src/ui/render_state.rs

@@ -1,14 +1,12 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::{
-    ActiveData, Axis, Declared, DrawLayers, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx,
+    ActiveData, Answer, Axis, Declared, DrawLayers, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx,
-    Moves, Painter, PixelRegion, PlaceDesc, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc,
+    MoveIdx, Moves, Painter, PixelRegion, PlaceDesc, PxVec2, Rel, Size, StrongWidget, UiRegion,
-    UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
 
-const AXES: [Axis; 2] = [Axis::X, Axis::Y];
-
 /// Where a widget is drawn: what its parent decides about the draw besides
 /// the boxes themselves.
 #[derive(Clone, Copy)]
@@ -36,6 +34,14 @@ pub(super) struct DrawInfo {
     pub px: PxVec2,
 }
 
+/// What one draw of a widget came to: the answer it gave, and the boxes and
+/// windows the drawing that gave it holds for. The two are separate ranges --
+/// a drawing can be invalid where its answer still stands.
+pub(super) struct Drawn {
+    pub answer: Answer,
+    pub drawing_holds: LayoutHolds,
+}
+
 /// What a widget's children are placed in: its own box, the coordinates its
 /// drawing is in, and what else one ask of a child is decided from.
 pub(super) struct Placing {
@@ -113,7 +119,7 @@ impl UiRenderState {
             // it will ask either again.
             let answer = active
                 .answer
-                .is_some_and(|(_, holds)| holds.contains(size, active.rel_base, active.region));
+                .is_some_and(|answer| answer.holds.contains(size, active.rel_base, active.region));
             answer && active.holds.contains(size, active.rel_base, active.region)
         });
         if !stands {
@@ -207,7 +213,7 @@ impl UiRenderState {
         info: DrawInfo,
         mut old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
-    ) -> (Size, LayoutHolds, LayoutHolds) {
+    ) -> Drawn {
         let old_parent = old
             .as_ref()
             .or_else(|| self.active.get(&id))
@@ -233,9 +239,9 @@ impl UiRenderState {
             .then(|| self.retained_answer(id, region, info))
             .flatten()
             .and_then(|answer| {
-                let placed = info.placed.placement(region, answer.0, declared, align);
+                let placed = info.placed.placement(region, answer.size, declared, align);
                 self.try_reuse(id, region, placed, info, rsc)
-                    .map(|()| answer)
+                    .then_some(answer)
             });
         let answer = reused.unwrap_or_else(|| {
             if old.is_none() {
@@ -245,7 +251,7 @@ impl UiRenderState {
             // Where the drawing goes: the part its parent gave it, with the
             // answer placed inside that part on any axis the parent left
             // open.
-            let placed = info.placed.placement(region, answer.0, declared, align);
+            let placed = info.placed.placement(region, answer.size, declared, align);
             if placed != region {
                 self.relocate(id, placed, info, rsc);
             }
@@ -273,7 +279,10 @@ impl UiRenderState {
         {
             old_parent.children.retain(|child| *child != id);
         }
-        (answer.0, answer.1, drawing_holds)
+        Drawn {
+            answer,
+            drawing_holds,
+        }
     }
 
     /// Calls a widget's `draw` and keeps what it drew in `region`.
@@ -284,7 +293,7 @@ impl UiRenderState {
         info: DrawInfo,
         old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
-    ) -> (Size, LayoutHolds) {
+    ) -> Answer {
         let rel_base = info.rel_base;
         let (move_idx, region, retired_move) = match info.region_node {
             // A node entry is only a translation. Its local box keeps the
@@ -395,9 +404,11 @@ impl UiRenderState {
         // was offered reports the height it needs.
         debug_assert!(
             mask == info.mask
-                || AXES
+                || Axis::BOTH.into_iter().all(|axis| size.within_box(
-                    .into_iter()
+                    region,
-                    .all(|axis| size.within_box(region, self.output_size, axis)),
+                    self.output_size,
+                    axis
+                )),
             "'{}' ({id:?}) clips to {px:?} and reports {size}",
             rsc.widgets().label(id),
         );
@@ -417,7 +428,7 @@ impl UiRenderState {
         // that many pixels of this window -- the same pin a widget that read
         // its rel base took for its drawing.
         let mut own_holds = own;
-        for axis in AXES {
+        for axis in Axis::BOTH {
             let fraction = rules[axis].exact().is_some_and(|len| len.rel != Rel::ZERO);
             if fraction {
                 own_holds[axis].rel_base = Some(info.rel_base[axis]);
@@ -489,7 +500,10 @@ impl UiRenderState {
         };
         rsc.on_draw(&active);
         self.active.insert(id, active);
-        (size, answer_holds)
+        Answer {
+            size,
+            holds: answer_holds,
+        }
     }
 
     /// Keeps a region node's entry across redraws because descendants retain
@@ -516,23 +530,17 @@ impl UiRenderState {
     /// drawing ended up. Alignment is exactly that case: the first box is the
     /// question and the smaller placed box holds the drawing. Whether the
     /// answer is stale at all is its caller's question, asked once there.
-    fn retained_answer(
+    fn retained_answer(&self, id: WidgetId, region: UiRegion, info: DrawInfo) -> Option<Answer> {
-        &self,
-        id: WidgetId,
-        region: UiRegion,
-        info: DrawInfo,
-    ) -> Option<(Size, LayoutHolds)> {
         let active = self.active.get(&id)?;
-        let has_region_node = active.move_idx != active.parent_move;
         if !active.drawn
-            || has_region_node != info.region_node
+            || active.is_region_node() != info.region_node
             || active.parent_move != info.parent_move
         {
             return None;
         }
         let answer = active.answer?;
         answer
-            .1
+            .holds
             .contains(self.output_size, info.rel_base, region)
             .then_some(answer)
     }
@@ -546,7 +554,7 @@ impl UiRenderState {
         placed: UiRegion,
         info: DrawInfo,
         rsc: &mut dyn UiRsc,
-    ) -> Option<()> {
+    ) -> bool {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::ReuseAttempts);
         if rsc.widgets().needs_redraw.contains(&id) {
@@ -555,19 +563,20 @@ impl UiRenderState {
                 diag::bump(Counter::ReuseDirty);
                 diag::reuse(id, ReuseOutcome::Dirty);
             }
-            return None;
+            return false;
         }
-        let active = self.active.get(&id)?;
+        let Some(active) = self.active.get(&id) else {
+            return false;
+        };
         if !active.drawn {
             #[cfg(feature = "layout-diagnostics")]
             diag::reuse(id, ReuseOutcome::Undrawn);
-            return None;
+            return false;
         }
-        let has_region_node = active.move_idx != active.parent_move;
+        if active.is_region_node() != info.region_node {
-        if has_region_node != info.region_node {
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::ReuseWrongNode);
-            return None;
+            return false;
         }
         // Drawn on another layer: the drawing sits in that layer's list and
         // paints at its moment, which no amount of geometry says. A container
@@ -580,10 +589,10 @@ impl UiRenderState {
                 diag::bump(Counter::ReuseWrongLayer);
                 diag::reuse(id, ReuseOutcome::WrongLayer);
             }
-            return None;
+            return false;
         }
         if active.parent_mask != info.mask {
-            return None;
+            return false;
         }
         // Drawn somewhere else in the tree: its box is in coordinates it no
         // longer sits in, and its slot names the wrong parent.
@@ -593,7 +602,7 @@ impl UiRenderState {
                 diag::bump(Counter::ReuseWrongParent);
                 diag::reuse(id, ReuseOutcome::WrongParent);
             }
-            return None;
+            return false;
         }
         // In pixels, because the box is a fraction of the window and that
         // may be what changed -- an unchanged fraction of a window half the
@@ -607,7 +616,7 @@ impl UiRenderState {
                 // Which of the three said no, so a rel base that redraws more
                 // than it should says where to look. They overlap: a drawing
                 // can be outside two of them at once.
-                for axis in AXES {
+                for axis in Axis::BOTH {
                     let holds = active.holds[axis];
                     let len = region[axis].len();
                     let window = self.output_size[axis];
@@ -628,10 +637,10 @@ impl UiRenderState {
                 diag::bump(Counter::ReuseOutside);
                 diag::reuse(id, ReuseOutcome::Outside);
             }
-            return None;
+            return false;
         }
         self.relocate(id, placed, info, rsc);
-        Some(())
+        true
     }
 
     /// Puts a retained drawing where its parent now has it, without drawing:
@@ -644,14 +653,14 @@ impl UiRenderState {
             "'{}' ({id:?}) placed while marked to draw",
             rsc.widgets().label(id)
         );
-        let has_region_node = active.move_idx != active.parent_move;
+        let is_region_node = active.is_region_node();
-        let local = match has_region_node {
+        let local = match is_region_node {
             true => placed.at_origin(),
             false => placed,
         };
         let moved = active.placement != local;
         let slot = active.move_idx;
-        if has_region_node {
+        if is_region_node {
             self.moves.set(slot, placed.as_translation());
         }
         if moved {
@@ -663,23 +672,13 @@ impl UiRenderState {
         active.placed = info.placed;
         #[cfg(feature = "layout-diagnostics")]
         {
-            match (moved, has_region_node) {
+            let (counter, outcome) = match (moved, is_region_node) {
-                (true, true) => diag::bump(Counter::ReuseMoved),
+                (true, true) => (Counter::ReuseMoved, ReuseOutcome::Moved),
-                (true, false) => diag::bump(Counter::ReuseRemapped),
+                (true, false) => (Counter::ReuseRemapped, ReuseOutcome::Remapped),
-                (false, _) => diag::bump(Counter::ReuseExact),
+                (false, _) => (Counter::ReuseExact, ReuseOutcome::Exact),
-            }
+            };
-            diag::reuse(
+            diag::bump(counter);
-                id,
+            diag::reuse(id, outcome);
-                if moved {
-                    if has_region_node {
-                        ReuseOutcome::Moved
-                    } else {
-                        ReuseOutcome::Remapped
-                    }
-                } else {
-                    ReuseOutcome::Exact
-                },
-            );
         }
     }
 
@@ -712,7 +711,7 @@ impl UiRenderState {
             parent: Some(at.id),
             depth: at.depth + 1,
             parent_move: at.move_idx,
-            region_node: active.move_idx != active.parent_move,
+            region_node: active.is_region_node(),
             mask: at.mask,
             rel_base,
             region,
@@ -755,16 +754,18 @@ impl UiRenderState {
             move_idx: active.move_idx,
             mask: active.mask,
         };
-        let children = active.children.len();
+        // Taken out and put back so that placing a child can borrow the state
-        for index in 0..children {
+        // it needs; nothing on that path reads this widget's own child list.
-            let child = self.active[&id].children[index];
+        let children = std::mem::take(&mut self.active.get_mut(&id).unwrap().children);
+        for &child in &children {
             self.place_child(child, &at, rsc);
         }
+        self.active.get_mut(&id).unwrap().children = children;
     }
 
-    /// A reused subtree keeps its shape, so every widget in it moves by the
+    /// A reused subtree keeps its shape, so each widget in it keeps its depth
-    /// same amount -- and where the top of it did not move, none of it did,
+    /// under the top -- and where the top's own depth did not change, none of
-    /// which is what makes this free in the ordinary case.
+    /// them did, which is what makes this free in the ordinary case.
     fn redepth(&mut self, id: WidgetId, depth: usize) {
         let Some(active) = self.active.get_mut(&id) else {
             return;
@@ -773,18 +774,21 @@ impl UiRenderState {
             return;
         }
         active.depth = depth;
-        let children = active.children.len();
+        // Taken out and put back so the walk can borrow the state it needs;
-        for index in 0..children {
+        // it only ever goes further down, so it reads no list but its own.
-            let child = self.active[&id].children[index];
+        let children = std::mem::take(&mut active.children);
+        for &child in &children {
             self.redepth(child, depth + 1);
         }
+        self.active.get_mut(&id).unwrap().children = children;
     }
 
     fn hints_agree(id: WidgetId, size: Size, rsc: &dyn UiRsc) -> bool {
         let Some(widget) = rsc.widgets().get_dyn(id) else {
             return true;
         };
-        AXES.into_iter()
+        Axis::BOTH
+            .into_iter()
             .all(|axis| widget.size_hint(axis).is_none_or(|hint| hint == size[axis]))
     }
 
@@ -1118,13 +1122,13 @@ impl UiRenderState {
         diag::bump(Counter::LocalRedraws);
 
         let old = self.remove(id, false, rsc);
-        let answer = self.draw_inner(id, info, old, rsc);
+        let drawn = self.draw_inner(id, info, old, rsc);
         let active = self.active.get_mut(&id).unwrap();
         // A wider contract does not invalidate the guarantee the parent kept.
         // Retain that guarantee so widening and narrowing back do not churn it.
-        if let Some((size, holds)) = was_answer
+        if let Some(was) = was_answer
-            && answer.0 == size
+            && drawn.answer.size == was.size
-            && answer.1.covers(holds)
+            && drawn.answer.holds.covers(was.holds)
         {
             active.answer = was_answer;
         }
@@ -1169,11 +1173,11 @@ impl UiRenderState {
     }
 }
 
-/// Whether what a widget reports along `axis` is inside the box it drew in.
-/// Both are lengths of the window, so the comparison is in its pixels. A
-/// share is a length only to whoever divides one, so it is not a claim about
-/// this box and cannot exceed it.
 impl Size {
+    /// Whether what a widget reports along `axis` is inside the box it drew
+    /// in. Both are lengths of the window, so the comparison is in its
+    /// pixels. A share is a length only to whoever divides one, so it is not
+    /// a claim about this box and cannot exceed it.
     fn within_box(self, region: UiRegion, window: PxVec2, axis: Axis) -> bool {
         let len = self[axis];
         let window = window[axis];

src/widget/position/scroll.rs

@@ -16,9 +16,9 @@ impl Widget for Scroll {
         let answer_len = painter
             .widget_at(&self.inner, PlaceDesc::WHOLE.fills())
             .len(self.axis);
-        let fixed = painter.to_px(answer_len.without_leftover(), self.axis);
+        let answer_px = painter.to_px(answer_len.without_leftover(), self.axis);
         self.container_len = container_len;
-        self.content_len = fixed.max(container_len);
+        self.content_len = answer_px.max(container_len);
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
@@ -31,10 +31,10 @@ impl Widget for Scroll {
         // the drawing holds for that length alone. One scrolled part way sits
         // where it is until the box shrinks past what is left of it. Kept to
         // the end, it moves with every length.
-        let fixed_len = answer_len.is_px();
+        let answer_is_px = answer_len.is_px();
-        if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
+        if answer_is_px && self.content_len <= self.container_len && align == AxisAlign::NEG {
-            painter.holds(self.axis, fixed..=Px::MAX);
+            painter.holds(self.axis, answer_px..=Px::MAX);
-        } else if fixed_len && !self.snap_end {
+        } else if answer_is_px && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, Px::MIN..=left);
         }

src/widget/position/span.rs

@@ -10,21 +10,18 @@ pub struct Span {
 impl Widget for Span {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
-        // The row: this span's own box, as a length of the rel base its children
+        // The row this span lays its children out along, as a length of the
-        // are laid out against. Its start is nothing's business -- a slot is
+        // rel base they are laid out against. Where it starts is nothing's
-        // a length from it -- so what this reads is the length alone.
+        // business -- a slot is a length from there -- so what this reads is
-        let far = painter.region_len(axis);
+        // the length alone.
-        let along = |from: Len, to: Len| match self.dir.sign {
+        let row = painter.region_len(axis);
-            Sign::Pos => UiSpan::new(from, to),
-            Sign::Neg => UiSpan::new(far - to, far - from),
-        };
         // A length for every child before their final slots are chosen: from
         // a hint where one says, and from drawing otherwise. The rel base passes
         // through unchanged, so `rel(0.5)` is half the area this span was
         // given whatever else is in it and wherever this child sits among
         // them; what a drawn child is asked in is the room left from the
         // cursor, because a text has to wrap at the width actually there.
-        let mut cursor = Len::rel_min();
+        let mut cursor = Len::ZERO;
         let mut lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
             let len = match painter.size_hint(child, axis) {
@@ -33,7 +30,7 @@ impl Widget for Span {
                     // Across itself the child sits where its own alignment
                     // says, in the whole of the row: a span is what contains
                     // its children there, and nothing divides that axis.
-                    let room = along(cursor, far).shifted_desc().on_axis(axis);
+                    let room = self.slot(row, cursor, row).shifted_desc().on_axis(axis);
                     painter.widget_at(child, room).len(axis)
                 }
             };
@@ -55,7 +52,7 @@ impl Widget for Span {
 
         // What is left for the shares to divide: the row less everything
         // fixed, as a length of the rel base rather than a number of pixels.
-        let room = far - total.without_leftover();
+        let room = row - total.without_leftover();
         // Whether anything is left over is a question in pixels: `rel(0.5)`
         // beside 300 px is full at 600 and overfull at 400. Asked of `room`
         // itself, and answered back through the same expression, so the
@@ -65,10 +62,10 @@ impl Widget for Span {
         // sign of `room.rel`, which `through` already reads. What the
         // generated oracle checks is the consequence, since which children
         // exist at all turns on this.
-        let mut shares = false;
+        let any_leftover = total.leftover > Weight::ZERO;
-        if total.leftover > Weight::ZERO {
+        let has_room = any_leftover && painter.to_px(room, axis) > Px::ZERO;
-            shares = painter.to_px(room, axis) > Px::ZERO;
+        if any_leftover {
-            let holds = match shares {
+            let holds = match has_room {
                 true => Holds::from(Px::STEP..=Px::MAX),
                 false => Holds::from(Px::MIN..=Px::ZERO),
             };
@@ -87,24 +84,26 @@ impl Widget for Span {
         // than stepped from the last child: the share of the room is
         // rounded, and taking each end from the one before it would carry
         // every rounding along the row.
-        let mut fixed = Len::rel_min();
+        let mut fixed = Len::ZERO;
         let mut taken = Weight::ZERO;
         let mut ortho = LayoutLen::ZERO;
-        let reached = |fixed: Len, taken: Weight| match taken == Weight::ZERO {
+        // Nothing divides the room where no child asked for any of it, and a
-            true => fixed,
+        // ratio of a whole of nothing has no answer.
-            false => fixed + room.scale(Rel::ratio(taken, total.leftover)),
+        let reached = |fixed: Len, taken: Weight| match any_leftover {
+            false => fixed,
+            true => fixed + room.scale(Rel::ratio(taken, total.leftover)),
         };
         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.
-            if len.is_only_leftover() && !shares {
+            if len.is_only_leftover() && !has_room {
                 painter.undraw(child);
                 fixed.px += self.gap;
                 continue;
             }
             let from = reached(fixed, taken);
-            if len.leftover > Weight::ZERO && shares {
+            if len.leftover > Weight::ZERO && has_room {
                 taken += len.leftover;
             }
             fixed += len.without_leftover();
@@ -116,9 +115,9 @@ impl Widget for Span {
             // it, since a text wraps at the width it is actually given. A
             // fixed child's slot is its own answer, so a drawing made in the
             // room is put there as it is, and one not made yet is made here.
-            let slot = along(from, to);
+            let slot = self.slot(row, from, to);
             let mut place = slot.shifted_desc().fills().on_axis(axis);
-            if len.leftover > Weight::ZERO && shares {
+            if len.leftover > Weight::ZERO && has_room {
                 place = place.rel_base(axis, slot.len());
             }
             let used = painter.place_at(child, place).len(!axis);
@@ -152,6 +151,17 @@ impl Widget for Span {
 }
 
 impl Span {
+    /// The stretch of the row between two distances from where this span
+    /// starts laying children out, as a span of its own box. A negative
+    /// direction lays out from the far end, so the same two distances mirror
+    /// in a row `row` long.
+    fn slot(&self, row: Len, from: Len, to: Len) -> UiSpan {
+        match self.dir.sign {
+            Sign::Pos => from.to(to),
+            Sign::Neg => (row - to).to(row - from),
+        }
+    }
+
     pub fn empty(dir: Dir) -> Self {
         Self {
             children: Vec::new(),

tests/cases/retained.rs

@@ -125,7 +125,7 @@ fn moving_an_ordinary_subtree_remaps_its_mask() {
     let active = &h.render.active[&masked.id()];
     assert_eq!(
         h.rsc.ui().masks[active.mask.idx()].region,
-        UiRegion::new(UiSpan::new(Len::px(150.0), Len::rel_max()), UiSpan::FULL,)
+        UiRegion::new(UiSpan::new(Len::px(150.0), Len::FULL), UiSpan::FULL,)
     );
     assert_corners!(h, inner, (150, 0), (400, 200));
 }