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/scroll.rs

@@ -46,13 +46,23 @@ 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 offset = UiVec2::from_axis(
-            self.axis,
-            UiScalar::from_parts(Rel::ZERO, anchor - self.amt),
-            UiScalar::ZERO,
-        );
-        let mut region = UiRegion::FULL.offset(offset);
-        region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
+        let mut region = UiRegion::FULL;
+        // Content that fills the viewport and has not been scrolled is the
+        // viewport, and is handed back as it came. Writing the same box as
+        // its own length in pixels is the same box in another form, and the
+        // two do not round alike: a part centred in `rel 1` lands a step from
+        // 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;
+        if moved || self.content_len != self.container_len {
+            let offset = UiVec2::from_axis(
+                self.axis,
+                UiScalar::from_parts(Rel::ZERO, anchor - self.amt),
+                UiScalar::ZERO,
+            );
+            region = region.offset(offset);
+            region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
+        }
         painter.widget_aligned(&self.inner, region, RegionAlign::NEAR);
         // 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