:root{
    --bg:#FAFAF8; --card:#FFFFFF; --ink:#171714;
    --dim:#8A8A85; --faint:#B9B9B4; --line:#E6E6E1; --line-soft:#EFEFEA;
    --accent:#C23A1C;
  }
  *{margin:0;padding:0;box-sizing:border-box}
  html,body{height:100%}
  body{
    background:var(--bg); color:var(--ink);
    font-family:"Inter","Helvetica Neue",-apple-system,BlinkMacSystemFont,"Segoe UI",sans-serif;
    -webkit-font-smoothing:antialiased; overflow:hidden;
  }
  code{font-family:ui-monospace,"SF Mono","Cascadia Code",Consolas,Menlo,monospace;font-size:.88em}
  strong{font-weight:600}

  /* ---------- top bar: menu + context ---------- */
  .topbar{
    position:fixed;top:0;left:0;right:0;z-index:10;
    display:flex;justify-content:space-between;align-items:center;
    padding:20px 28px;
  }
  .menu-btn{
    font-family:inherit;font-size:22px;line-height:1;color:var(--dim);
    background:transparent;border:none;cursor:pointer;padding:2px 8px;
  }
  .menu-btn:hover{color:var(--ink)}
  .menu-btn:focus-visible{outline:1px solid var(--accent);outline-offset:4px}
  .tag-label{
    font-size:11px;letter-spacing:.16em;text-transform:uppercase;color:var(--faint);
  }

  /* ---------- options drawer ---------- */
  .drawer{position:fixed;inset:0;z-index:20;pointer-events:none}
  .drawer.open{pointer-events:auto}
  .drawer-scrim{position:absolute;inset:0;background:rgba(23,23,20,.22);opacity:0;transition:opacity 200ms ease}
  .drawer.open .drawer-scrim{opacity:1}
  .drawer-panel{
    position:absolute;top:0;left:0;bottom:0;width:min(280px,80vw);
    background:var(--card);border-right:1px solid var(--line);
    transform:translateX(-100%);transition:transform 220ms ease;
    display:flex;flex-direction:column;
  }
  .drawer.open .drawer-panel{transform:translateX(0)}
  .drawer-head{
    display:flex;justify-content:space-between;align-items:center;
    padding:20px 24px;border-bottom:1px solid var(--line-soft);
  }
  .drawer-title{font-size:11px;letter-spacing:.16em;text-transform:uppercase;color:var(--dim)}
  .drawer-close{
    font-family:inherit;font-size:22px;line-height:1;color:var(--dim);
    background:transparent;border:none;cursor:pointer;padding:2px 6px;
  }
  .drawer-close:hover{color:var(--ink)}
  .drawer-body{padding:24px;display:flex;flex-direction:column;gap:20px;overflow-y:auto}
  .drawer-foot{margin-top:auto;padding:20px 24px;border-top:1px solid var(--line-soft);
    font-size:9px;letter-spacing:.12em;text-transform:uppercase;color:var(--faint)}
  .menu-label{font-size:10px;letter-spacing:.16em;text-transform:uppercase;color:var(--faint)}
  .menu-select{
    appearance:none;-webkit-appearance:none;
    font-family:inherit;font-size:13px;color:var(--ink);
    background:transparent;border:none;border-bottom:1px solid var(--line);
    border-radius:0;
    padding:0 0 8px 0;margin-top:-14px;cursor:pointer;width:100%;
  }
  .menu-select:focus-visible{outline:1px solid var(--accent);outline-offset:4px}
  .menu-sep{height:1px;background:var(--line-soft)}
  .menu-item{
    font-family:inherit;font-size:13px;letter-spacing:.02em;text-align:left;
    color:var(--ink);background:transparent;border:none;cursor:pointer;
    text-decoration:none;padding:2px 0;
  }
  .menu-item:hover{color:var(--accent)}

  /* ---------- stage ---------- */
  .stage{position:fixed;inset:0;overflow:hidden}

  .slot{position:absolute;top:50%;left:50%;z-index:2;width:min(600px,calc(100vw - 48px));transform:translate(-50%,-50%)}

  /* No overflow clip here: the peek is opacity-gated (0 at rest, fades in only while
     dragging), so nothing bleeds. The viewport (.stage, overflow:hidden) clips the
     incoming card at the screen edge. Removing the clip fixes iOS Safari, which cut
     the card's box-shadow at rest even with the old padding-clip trick. */
  .stage-clip{position:relative;width:100%;perspective:1200px}
  .drag-row{position:relative;width:100%;will-change:transform;transition:transform 220ms ease,opacity 220ms ease}
  .drag-row.in-right{animation:cardInR 220ms ease}
  .drag-row.in-left{animation:cardInL 220ms ease}
  /* Entry animation slides + fades the NEW card into place once, then RELEASES its
     effect (default animation-fill-mode = none). This is critical: a transform-based
     animation with fill-mode both would persistently pin `transform` and suppress the
     next drag's inline translateX. Without fill-mode, the animation ends and let
     go of transform. */
  @keyframes cardInR{from{opacity:0;transform:translateX(60px)}to{opacity:1;transform:translateX(0)}}
  @keyframes cardInL{from{opacity:0;transform:translateX(-60px)}to{opacity:1;transform:translateX(0)}}

  /* ---------- card ---------- */
  .card{
    background:var(--card);border:1px solid var(--line);
    border-radius:10px;
    box-shadow:0 2px 4px rgba(23,23,20,.05),0 12px 32px rgba(23,23,20,.10);
    height:min(600px,calc(100vh - 176px));
    display:flex;flex-direction:column;
    padding:40px 48px;
    transition:opacity 220ms ease;
    user-select:none;-webkit-user-select:none;
    touch-action:none;-webkit-tap-highlight-color:transparent;
    cursor:grab;
  }
  .card:active{cursor:grabbing}
  /* whole-card flick when flipping over (revealing answer/explanation) */
  .card.flipped{animation:cardFlip 320ms ease}
  @keyframes cardFlip{
    0%{transform:rotateY(-10deg) translateX(14px);opacity:.4}
    100%{transform:rotateY(0) translateX(0);opacity:1}
  }

  /* neighbor card shown while dragging — rigid pair, slides in from the card edge */
  .card-peek{
    position:absolute;top:0;width:100%;
    height:min(600px,calc(100vh - 176px));
    background:var(--card);border:1px solid var(--line);border-radius:10px;
    box-shadow:0 2px 4px rgba(23,23,20,.05),0 12px 32px rgba(23,23,20,.10);
    padding:40px 48px;display:flex;flex-direction:column;
    pointer-events:none;
    opacity:0;
  }
  .card-peek .card-body{flex:1 1 auto;overflow:hidden;margin-top:32px}
  .card-peek .front{font-size:20px;line-height:1.45;font-weight:500;letter-spacing:-.01em}

  .card-body{flex:1 1 auto;overflow-y:auto;scrollbar-width:thin;margin-top:32px}
  .slot.ghost .card-body{overflow:hidden}

  .card-foot{
    flex:0 0 auto;margin-top:24px;padding-top:24px;border-top:1px solid var(--line-soft);
    display:flex;justify-content:space-between;align-items:center;gap:16px;
    min-height:36px;
  }

  .front{font-size:20px;line-height:1.45;font-weight:500;letter-spacing:-.01em}
  .meta{margin-top:16px;font-size:11px;letter-spacing:.16em;text-transform:uppercase;color:var(--accent)}
  .preview{margin-top:16px;font-size:14px;line-height:1.6;color:var(--dim);display:flex}
  .preview .pn{flex:0 0 32px;text-align:right;margin-right:16px;color:var(--faint);font-variant-numeric:tabular-nums}

  .back{margin-top:32px;padding-top:24px;border-top:1px solid var(--line-soft);animation:slideFlip 320ms ease both}
  .back:first-child{margin-top:0;padding-top:0;border-top:none}
  /* flip motion: incoming side slides in from the right + fades, like the card-entry
     motion but for flipping the card over. Applied to a freshly-created .back each
     render, so the transform cannot leak into drag (unlike the .drag-row case). */
  @keyframes slideFlip{from{opacity:0;transform:translate(40px,6px)}to{opacity:1;transform:none}}
  .back-label{font-size:10px;letter-spacing:.16em;text-transform:uppercase;color:var(--faint);margin-bottom:16px}
  .back p{font-size:15px;line-height:1.65}
  .back p + p{margin-top:16px}

  ol.items{list-style:none}
  ol.items li{display:flex;font-size:15px;line-height:1.6;padding:8px 0;border-top:1px solid var(--line-soft)}
  ol.items li:first-child{border-top:none}
  ol.items .n{flex:0 0 32px;text-align:right;margin-right:16px;color:var(--faint);font-variant-numeric:tabular-nums}

  table.cheat{width:100%;border-collapse:collapse}
  table.cheat th{text-align:left;font-size:10px;letter-spacing:.14em;text-transform:uppercase;
    color:var(--dim);font-weight:600;padding:0 16px 8px 0}
  table.cheat td{font-size:13px;line-height:1.5;padding:8px 16px 8px 0;border-top:1px solid var(--line-soft);vertical-align:top}
  table.cheat th:last-child,table.cheat td:last-child{padding-right:0}

  .actions{display:flex;gap:32px}
  .action{
    font-family:inherit;font-size:12px;letter-spacing:.16em;text-transform:uppercase;
    cursor:pointer;color:var(--ink);background:none;border:none;
    padding:8px 0;border-bottom:1px solid transparent;
  }
  .action:hover{border-bottom-color:currentColor}
  .action.known{color:var(--accent)}
  .action .kh{color:var(--faint);margin-left:8px}
  .action:focus-visible{outline:1px solid var(--accent);outline-offset:4px}

  /* primary single action on the front of a card */
  .reveal{
    -webkit-appearance:none;appearance:none;
    font-family:inherit;font-size:12px;letter-spacing:.16em;text-transform:uppercase;
    color:var(--ink);background:transparent;border:1px solid var(--line);border-radius:20px;cursor:pointer;
    padding:10px 22px;width:auto;display:inline-block;
    transition:border-color 160ms ease,color 160ms ease;
  }
  .reveal:hover{border-color:var(--ink);color:var(--ink)}
  .reveal:focus-visible{outline:1px solid var(--accent);outline-offset:2px}

  .hint{font-size:11px;letter-spacing:.16em;text-transform:uppercase;color:var(--faint);text-align:right}

  .bar-right{position:fixed;right:32px;bottom:24px;z-index:10;display:flex;gap:16px;align-items:baseline}
  .position{font-size:12px;letter-spacing:.12em;color:var(--dim);font-variant-numeric:tabular-nums}
  .version{font-size:10px;letter-spacing:.12em;color:var(--faint)}
  .keys{position:fixed;left:32px;bottom:24px;z-index:10;
    font-size:10px;letter-spacing:.12em;text-transform:uppercase;color:#D4D4CF}

  .done{position:absolute;top:50%;left:50%;transform:translate(-50%,-50%);text-align:center;animation:fadeUp 240ms ease both}
  @keyframes fadeUp{from{opacity:0;transform:translateY(8px)}to{opacity:1;transform:none}}
  .done h1{font-size:24px;font-weight:500;letter-spacing:-.01em}
  .done p{margin-top:16px;font-size:11px;letter-spacing:.16em;text-transform:uppercase;color:var(--faint)}

  @media (prefers-reduced-motion:reduce){
    .drag-row.in-right,.drag-row.in-left,.done{animation:none}
    .back{animation:flipFade 180ms ease both}
    .card.flipped{animation:flipFade 180ms ease both}
    .card,.drag-row{transition:none}
  }
  @keyframes flipFade{from{opacity:0}to{opacity:1}}

  @media (max-width:640px){
    .card{padding:24px 24px}
    .topbar{padding:16px 20px}
    .keys{display:none}
    .bar-right{right:24px;bottom:16px}
    /* smaller screen has less margin room — tighten the shadow so it doesn't look clipped at the viewport edges */
    .card,.card-peek{box-shadow:0 2px 3px rgba(23,23,20,.04),0 8px 18px rgba(23,23,20,.07)}
  }