Do not widen a validity range where nothing rounded

`Holds::through` inverts `px + rel * box`, and allowed three half steps
either side: one for that multiply's rounding and two for the difference
between a length composed down the chain and the same length measured
against the window. The whole of a box has no multiply in it -- `rel` is one
and taking the pixels off again is exact -- so the first half step was being
allowed for a rounding that did not happen, and it compounded: a chain of
widgets each taking the whole of its parent grew the interval half a step a
level. Traced while making the multiply truncate, where the same compounding
moved the interval off the box the drawing was made in and fired the
`Holds` assertion in eleven generated cases.

A range wider than what a drawing holds for is one that admits reusing it
where it does not hold, so this is the unsound direction to be loose in.

Checked: fmt, clippy, 80 suite tests and 16 core unit tests, the release
oracle at 100 seeds, all fifteen shrinker cases at 400 seeds of depth 5
(seed 288 on `region-node` still failing and unchanged by this), and `tabs`,
`view`, `minimal`, `text`, `random` plus the tab replay byte-identical at
1920x1200.

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

core/src/ui/holds.rs

@@ -1,4 +1,4 @@
-use crate::{Len, Px, REL_SHIFT, fixed::div_toward, fixed::narrow};
+use crate::{Len, Px, REL_SHIFT, Rel, fixed::div_toward, fixed::narrow};
 use std::ops::RangeInclusive;
 
 /// The lengths of a box, in pixels, that one drawing of a widget holds for:
@@ -55,15 +55,25 @@ impl Holds {
         if rel == 0 {
             return Self::ANY;
         }
-        // Three half steps either side -- one for the rounding on the way
-        // in, two for the difference between a length composed down the
-        // chain and the same length measured against the window -- and half
-        // of what a `Rel` counts in, to divide by the fraction. Exact until
-        // the division takes it back to the grid.
+        // Half steps either side: two for the difference between a length
+        // composed down the chain and the same length measured against the
+        // window, and one more for the rounding on the way in -- which the
+        // whole of a box did not have, however many pixels were added to it,
+        // since multiplying by one is exact and taking the pixels off again
+        // is too. Allowing for it there anyway compounded, half a step a
+        // level down a chain of widgets each taking the whole of its parent,
+        // and a range wider than what a drawing holds for admits reusing it
+        // where it does not hold. Shifted by half of what a `Rel` counts in,
+        // to divide by the fraction: exact until the division takes it back
+        // to the grid.
         let px = len.px.raw() as i64;
         let half_rel = REL_SHIFT - 1;
-        let lo = ((self.lo.raw() as i64 - px) * 2 - 3) << half_rel;
-        let hi = ((self.hi.raw() as i64 - px) * 2 + 3) << half_rel;
+        let slack = match rel == Rel::ONE.raw() as i64 {
+            true => 2,
+            false => 3,
+        };
+        let lo = ((self.lo.raw() as i64 - px) * 2 - slack) << half_rel;
+        let hi = ((self.hi.raw() as i64 - px) * 2 + slack) << half_rel;
         // Dividing by a negative turns the ends around, so which end each
         // bound comes from is decided before dividing rather than by taking
         // the min and max of four divisions.
@@ -116,6 +126,26 @@ mod tests {
         }
     }
 
+    /// A widget handed the whole of its parent's box, with or without pixels
+    /// taken off it, brings no rounding of its own: only the step between a
+    /// length composed down the chain and the same length measured against
+    /// the window is left to allow for. Widening for the multiply as well
+    /// grew the interval a level at a time down a chain of them.
+    #[test]
+    fn the_whole_of_a_box_widens_by_the_one_step_it_has_to() {
+        let at = Px::from_int(956);
+        let step = |len: Px| Holds {
+            lo: len - Px::STEP,
+            hi: len + Px::STEP,
+        };
+        assert_eq!(Holds::at(at).through(Len::FULL), step(at));
+        let less_eight = Len::from_parts(Rel::ONE, Px::from_int(-8));
+        assert_eq!(
+            Holds::at(at).through(less_eight),
+            step(at + Px::from_int(8))
+        );
+    }
+
     #[test]
     fn a_boundary_the_next_step_along_does_not_admit_it() {
         let boundary = Px::from_int(10);