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.

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();
-        if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
-            painter.holds(self.axis, fixed..=Px::MAX);
-        } else if fixed_len && !self.snap_end {
+        let answer_is_px = answer_len.is_px();
+        if answer_is_px && self.content_len <= self.container_len && align == AxisAlign::NEG {
+            painter.holds(self.axis, answer_px..=Px::MAX);
+        } 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
-        // 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.region_len(axis);
-        let along = |from: Len, to: Len| match self.dir.sign {
-            Sign::Pos => UiSpan::new(from, to),
-            Sign::Neg => UiSpan::new(far - to, far - from),
-        };
+        // The row this span lays its children out along, as a length of the
+        // rel base they are laid out against. Where it starts is nothing's
+        // business -- a slot is a length from there -- so what this reads is
+        // the length alone.
+        let row = painter.region_len(axis);
         // 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;
-        if total.leftover > Weight::ZERO {
-            shares = painter.to_px(room, axis) > Px::ZERO;
-            let holds = match shares {
+        let any_leftover = total.leftover > Weight::ZERO;
+        let has_room = any_leftover && painter.to_px(room, axis) > Px::ZERO;
+        if any_leftover {
+            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 {
-            true => fixed,
-            false => fixed + room.scale(Rel::ratio(taken, total.leftover)),
+        // Nothing divides the room where no child asked for any of it, and a
+        // ratio of a whole of nothing has no answer.
+        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(),