/* ════════════════════════════════════════════════════════
   COMPONENT — STICKY FILTER BAR
   Filter row that locks beneath the fixed header once the
   user scrolls past it. Used by fiction.html, nonfiction.html,
   and any future list page.

   Markup pattern:
     <section class="sticky-filter" data-sticky-filter>
       <div class="sticky-filter-inner">
         <div class="sticky-filter-label">Genre</div>
         <div class="btn-group" role="group" aria-label="Filter by genre">
           <button class="btn btn--filter btn--sm" aria-pressed="true" data-filter="all">All <span class="count">14</span></button>
           …
         </div>
       </div>
     </section>

   Behavior owned by the .is-sticky class, which the JS
   helper toggles when the filter bar's natural position
   passes under the header. We use `position: sticky` so the
   bar lives in the document flow when in view, and `top` is
   tied to --header-h so they stack perfectly without overlap.
   ════════════════════════════════════════════════════════ */

.sticky-filter {
  position: sticky;
  top: var(--header-h);
  z-index: var(--z-sticky);
  background: color-mix(in srgb, var(--paper) 92%, transparent);
  border-bottom: 1px solid transparent;
  /* Backdrop blur kicks in once it's stuck to the header so
     the title text scrolling behind it stays legible. */
  transition:
    background-color var(--dur-base) var(--ease-out),
    border-color var(--dur-base) var(--ease-out),
    box-shadow var(--dur-base) var(--ease-out);
}
.sticky-filter.is-stuck {
  background: color-mix(in srgb, var(--paper) 96%, transparent);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom-color: color-mix(in srgb, var(--ink) 8%, transparent);
  box-shadow: var(--shadow-1);
}

.sticky-filter-inner {
  max-width: var(--max-container);
  margin: 0 auto;
  padding: 12px var(--gutter);
  padding-left: calc(var(--gutter) + var(--gutter-content));
  display: flex;
  align-items: center;
  gap: 20px;
  min-height: var(--filter-h);
  transition: padding-left var(--dur-base) var(--ease-out);
}
/* When stuck under the header, drop the content indent so the
   filter's left edge lines up with the wordmark above it. */
.sticky-filter.is-stuck .sticky-filter-inner {
  padding-left: var(--gutter);
}
.sticky-filter-label {
  font-family: var(--mono);
  font-size: var(--t-69);
  letter-spacing: 0.22em;
  text-transform: uppercase;
  color: var(--marginalia);
  flex: 0 0 auto;
}

/* Filter buttons live in a horizontally-scrollable track on
   narrow screens so we don't wrap awkwardly. The track hides
   its scrollbar (Firefox + WebKit). On wider screens we let
   the buttons wrap to a second row instead. */
.sticky-filter .btn-group {
  flex: 1 1 auto;
  min-width: 0;
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
  align-items: center;
}

@media (max-width: 760px) {
  .sticky-filter {
    top: var(--header-h-mobile);
  }
  .sticky-filter-inner {
    padding: 10px var(--gutter-mobile);
    gap: 14px;
    flex-direction: column;
    align-items: flex-start;
    min-height: 0;
  }
  .sticky-filter-label {
    margin-bottom: 2px;
  }
  /* Switch wrap → horizontal scroll so all options stay reachable */
  .sticky-filter .btn-group {
    flex-wrap: nowrap;
    overflow-x: auto;
    overscroll-behavior-x: contain;
    width: 100%;
    padding-bottom: 2px;
    /* Hide scrollbar — track stays interactive */
    scrollbar-width: none;
  }
  .sticky-filter .btn-group::-webkit-scrollbar { display: none; }
  .sticky-filter .btn--filter { flex: 0 0 auto; }
}

@media (prefers-reduced-motion: reduce) {
  .sticky-filter { transition: none; }
}