A retained part is re-expressed as a fraction of its new box by dividing by
the old box's extent, and that extent is one whenever the box spans the whole
of its parent's -- which is the common shape. An integer division is the most
expensive thing in `apply_scalar` and it ran twice per span.
`perf stat -e instructions:u` over 500 frames: `many` 1,938,264,572 to
1,886,265,821, `scroll` 452,517,906 to 444,792,314.
Tried first and reverted: short-circuiting a fraction of nought or one, at
either end of the box. That is not the common case, and the two comparisons
cost 17% more than the divisions they were meant to save.
Checked: fmt, clippy, 105 tests, three shrinker cases at 300 seeds, 100
generated seeds.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
iris-aiandClaude Opus 5
4f5e27cba9