14th pre-validation detector + a preventive skill rule.
User-reported 2026-05-10 "Bistro" mobile food app shipped with root
padding [0,16,0,16] AND a "Today's Specials" section padding [0,24].
Effective gutter = 40px on a 375px page → only 295px of usable
content width. Reads as "too much padding" / pinched.
Two pieces:
1. layout.md AESTHETIC HYGIENE block now teaches "page gutter goes
on ONE layer, not both" — pick root horizontal padding OR
per-section horizontal padding, not both. Default convention:
root carries the gutter, sections set vertical-only padding.
Hero / banner / image-bleed sections then sit edge-to-edge by
simply NOT adding horizontal padding (root's gutter shows
through). Preventive teaching at prompt time.
2. detectStackedHorizontalPadding (info-only, detect-only). Walks
every mobile-shaped root (width 320–480 + tall + multi-child),
compares root horizontal padding against each direct child's
horizontal padding; flags the section as the offender when both
are > 0. Page-shape filter mirrors detectEdgeSectionPadding so
the legitimate component-internal padding stacking pattern
(chip → badge → icon, etc.) doesn't trip it. Severity is INFO
because a section may legitimately want a deeper inset for
visual emphasis — let the user/agent decide via audit panel.
Side-quest: scripts/ab-corpus/check-stacked-padding.ts ships with
this commit so the next stacked-padding-style detector calibration
can survey corpus frequency without rebuilding the harness.
20dbbf227a