Take two roundings out of where a box comes from

Traced what was left of the warm-against-cold difference after fixed point.
It is not accumulation and not one place: it is the same box reached two
ways, and each way rounds where the other does not.

`Scroll` was writing a box it had been given back out as its own length in
pixels. That is the same box in another form, and centring a part in `rel 1`
lands a step from centring it in `px 900`, because halving a difference is
not halving each part of it. Content that fills the viewport and has not been
scrolled is now handed back as it came, which makes the shrinker's `repaint`
and `resize-repaint` cases agree exactly rather than within a step.

`Span` placed each child a step from where the last one ended, so the
rounding of every share was carried along the row. A position is now the
fixed parts before it -- a sum, exact -- plus one rounded share of the room.
Measured: two hundred equal shares of a 1000 px row ended at 999.999 and now
end at 1000, and `tests/cases/layout.rs` pins it at 2, 3, 7, 64 and 200.

What is left is a step per level of nesting between the two ways, which is
what the fuzzers now allow: four of the five shrinker cases pass at one step
and the fifth is five spans deep. Closing it needs one way of asking where a
box is, which is a bigger change than this.

Checked: fmt, clippy, 103 tests, all five shrinker cases at 300 seeds, 100
generated seeds, five examples byte-identical at 1920x1200.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/widget/position/span.rs

@@ -83,6 +83,15 @@ impl Widget for Span {
         // rule beside it already says how long it is, and then reading them
         // answers nothing and makes its size depend on theirs for it.
         let shrinks = !painter.ruled(!axis);
+        // What the fixed parts and the gaps before here take, which is a sum
+        // of lengths and exact, and how much of the leftover weight is
+        // spoken for. A position is one from the other rather than a step
+        // from the last child: the share of the room is rounded, and taking
+        // each from the one before it would carry every rounding along the
+        // row.
+        let mut fixed = UiScalar::rel_min();
+        let mut taken = Weight::ZERO;
+        let room = UiScalar::rel_max() - UiScalar::from_parts(total.rel, total.px);
         let mut start = UiScalar::rel_min();
         let mut ortho = Len::ZERO;
         for (child, len) in self.children.iter().zip(&lens) {
@@ -92,20 +101,17 @@ impl Widget for Span {
             if len.leftover > Weight::ZERO && len.px == Px::ZERO && len.rel == Rel::ZERO && !shares
             {
                 painter.undraw(child);
-                start.px += self.gap;
+                fixed.px += self.gap;
                 continue;
             }
             let mut span = UiSpan::FULL;
             span.start = start;
             if len.leftover > Weight::ZERO && shares {
-                let offset = UiScalar::from_parts(total.rel, total.px);
-                let share = Rel::ratio(len.leftover, total.leftover);
-                let rel_end = UiScalar::from_parts(share, Px::ZERO);
-                let end = (UiScalar::rel_max() + start) - offset;
-                start = rel_end.within(&start.to(end));
+                taken += len.leftover;
             }
-            start.px += len.px;
-            start.rel += len.rel;
+            fixed.px += len.px;
+            fixed.rel += len.rel;
+            start = shared(fixed, taken, total.leftover, room);
             span.end = start;
             let mut region = UiRegion::from_axis(axis, span, UiSpan::FULL);
             if self.dir.sign == Sign::Neg {
@@ -124,7 +130,8 @@ impl Widget for Span {
                     ortho.px = ortho.px.max(used.px);
                 }
             }
-            start.px += self.gap;
+            fixed.px += self.gap;
+            start = shared(fixed, taken, total.leftover, room);
         }
 
         // Carried whole rather than collapsed to one share: a span that sizes
@@ -143,6 +150,16 @@ impl Widget for Span {
     }
 }
 
+/// Where a row has reached: everything fixed before this point, which is a
+/// sum and exact, plus the share of the room the weights so far are worth,
+/// which is one rounding wherever it is asked for.
+fn shared(fixed: UiScalar, taken: Weight, weight: Weight, room: UiScalar) -> UiScalar {
+    if taken == Weight::ZERO {
+        return fixed;
+    }
+    fixed + room.scale(Rel::ratio(taken, weight))
+}
+
 impl Span {
     pub fn empty(dir: Dir) -> Self {
         Self {