Fix stack cover: single sticky folder unit, hide inactive bodies.

Tested all four variants in browser — only active layer body visible while
tabs stay staggered; scroll covers previous layers correctly.

Co-authored-by: Cursor <cursoragent@cursor.com>

shared/stack-layout.css

@@ -18,3 +18,18 @@
   height: 2rem;
   margin-bottom: 3rem;
 }
+
+/* Only the active layer shows its body; tabs always visible */
+.scroll-section:not(.is-active) .body,
+.scroll-section:not(.is-active) .layer-inner,
+.scroll-section:not(.is-active) .frame-body,
+.scroll-section:not(.is-active) .unit-body {
+  visibility: hidden;
+  height: 0;
+  min-height: 0;
+  margin-top: 0 !important;
+  padding: 0;
+  border: none;
+  overflow: hidden;
+  box-shadow: none;
+}