d10ef94161
Root cause: polish-5 (1e2041e) refactored the applet FLIP-btn position rule to `@include flip-btn-base` but LEFT IT NESTED inside the `#id_applet_my_sign { ... }` outer block of `_billboard.scss`. So the rule resolved to `#id_applet_my_sign .my-sign-applet-card .my-sign-applet-flip-btn` (specificity 0,1,2,0 — 1 ID + 2 classes) — and OUT-CASCADED the hover-reveal rule `.my-sign-applet-card:hover .my-sign-applet-flip-btn` (specificity 0,0,3,0 — 3 classes incl. pseudo, no ID) on the ID axis. Result: opacity:0 from the leftover rule stuck even on real `:hover`, the polish-6 unified positioning rule in `_card-deck.scss` was a redundant 0,0,1,0 also-ran, and the user couldn't see the btn.
Fix: drop the leftover position + mid-flip rules from `_billboard.scss` entirely. The polish-5/6 unified rules in `_card-deck.scss` (`.my-sign-flip-btn, .my-sign-applet-flip-btn, .sea-stage-flip-btn { @include flip-btn-base; z-index: 25; bottom: 0.6rem; left: 0.6rem }` + `.sig-stage-card[data-flipping] .my-sign-flip-btn, .my-sign-applet-card[data-flipping] .my-sign-applet-flip-btn, .sea-stage-card[data-flipping] .sea-stage-flip-btn, ...` + hover-reveal trio) now cover the applet too at clean 0,0,1,0 / 0,0,3,0 / 0,0,2,1 specificities — hover-reveal wins by class count, no ID-context contamination.
Lesson: when refactoring CSS to a shared mixin/placeholder, also LIFT THE SELECTOR OUT of any ID-scoped outer block. Leaving the rule inside `#id_*` inflates its specificity in ways that can shadow other shared rules (esp. hover/state rules that lack an ID). The leftover comment block in `_billboard.scss` now documents the lift-out for future-me / future-DRY-passes.
Tests: 1314/1314 IT+UT total green (74s; pure SCSS rule deletion, no test surface). Visual verify pending user confirmation — the cascade is now clean (hover-reveal wins; transition: opacity 0.3s applies; btn fades in on real card hover + fades out on un-hover).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>