@font-face {
    font-family: 'Fraunces';
    src: url('fonts/Fraunces-VariableFont_SOFT,WONK,opsz,wght.woff2') format('woff2');
    font-weight: 100 900;
    font-style: normal;
    font-display: swap;
  }

    @font-face {
      font-family: 'IBM Plex Sans';
      src: url('fonts/IBMPlexSans-VariableFont_wdth,wght.woff2') format('woff2');
      font-weight: 100 700;
      font-style: normal;
      font-display: swap;
    }

    @font-face {
      font-family: 'IBM Plex Mono';
      src: url('fonts/IBMPlexMono-Regular.woff2') format('woff2');
      font-weight: 400;
      font-style: normal;
      font-display: swap;
    }
  
    @font-face {
      font-family: 'IBM Plex Mono';
      src: url('fonts/IBMPlexMono-Medium.woff2') format('woff2');
      font-weight: 500;
      font-style: normal;
      font-display: swap;
    }
  
    @font-face {
      font-family: 'IBM Plex Mono';
      src: url('fonts/IBMPlexMono-SemiBold.woff2') format('woff2');
      font-weight: 600;
      font-style: normal;
      font-display: swap;
    }
  
    :root {
      /* Light theme */
      --ink: #f7f8fa;
      --panel: #ffffff;
      --panel-2: #f1f3f6;
  
      --rule-rgb: 24, 32, 42;
      --rule: rgba(var(--rule-rgb), 0.07);
      --rule-strong: rgba(var(--rule-rgb), 0.14);
  
      --chalk: #18212b;
      --chalk-dim: #667381;
  
      --amber-rgb: 79, 70, 229;
      --amber: rgb(var(--amber-rgb));
      --amber-dim: rgba(var(--amber-rgb), 0.16);
      --amber-hover: #4338ca;
  
      --sage-rgb: 22, 163, 138;
      --sage: rgb(var(--sage-rgb));
  
      --rust-rgb: 220, 74, 74;
      --rust: rgb(var(--rust-rgb));
  
      --slate-rgb: 76, 105, 170;
      --slate: rgb(var(--slate-rgb));
  
      --plum-rgb: 139, 92, 246;
      --plum: rgb(var(--plum-rgb));
  
      --moss-rgb: 132, 145, 55;
      --moss: rgb(var(--moss-rgb));
  
      --radius: 12px;
  
      --placeholder: #9aa4af;
      --text-faint: #87929e;
      --negative-text: #c53d3d;
  
      --on-amber: #ffffff;
      --on-amber-rgb: 255, 255, 255;
    }
  
    [data-theme="dark"] {
      /* Dark theme */
      --ink: #0c1117;
      --panel: #121923;
      --panel-2: #182230;
  
      --rule-rgb: 226, 232, 240;
      --rule: rgba(var(--rule-rgb), 0.07);
      --rule-strong: rgba(var(--rule-rgb), 0.14);
  
      --chalk: #edf2f7;
      --chalk-dim: #98a4b3;
  
      --amber-rgb: 129, 118, 255;
      --amber: rgb(var(--amber-rgb));
      --amber-dim: rgba(var(--amber-rgb), 0.22);
      --amber-hover: #9b92ff;
  
      --sage-rgb: 45, 190, 163;
      --sage: rgb(var(--sage-rgb));
  
      --rust-rgb: 239, 105, 86;
      --rust: rgb(var(--rust-rgb));
  
      --slate-rgb: 122, 151, 216;
      --slate: rgb(var(--slate-rgb));
  
      --plum-rgb: 173, 139, 255;
      --plum: rgb(var(--plum-rgb));
  
      --moss-rgb: 170, 188, 91;
      --moss: rgb(var(--moss-rgb));
  
      --placeholder: #657181;
      --text-faint: #667384;
      --negative-text: #ff9d8d;
  
      --on-amber: #ffffff;
      --on-amber-rgb: 255, 255, 255;
    }

  *{ box-sizing:border-box; }

  html,body{
    margin:0; padding:0;
    background:var(--ink);
    color:var(--chalk);
    font-family:'IBM Plex Sans', sans-serif;
    min-height:100vh;
  }

body {
  background:
    radial-gradient(900px 500px at 50% -120px,
      rgba(var(--amber-rgb), 0.06),
      transparent 70%),
    var(--ink);
    padding: 24px;
}

  .theme-toggle{
    position: fixed;
    top: 18px;
    right: 18px;
    z-index: 50;
    width: 38px;
    height: 38px;
    display: flex;
    align-items: center;
    justify-content: center;
    background: var(--panel);
    border: 1px solid var(--rule-strong);
    border-radius: 50%;
    cursor: pointer;
    font-size: 17px;
    line-height: 1;
    transition: border-color .12s ease, transform .12s ease;
  }
  .theme-toggle:hover{ border-color: var(--amber); transform: scale(1.05); }
  .theme-toggle:active{ transform: scale(0.95); }

  .wrap{
    max-width: 860px;
    margin: 0 auto;
    padding: 24px;
    background: var(--panel);
      border: 1px solid var(--rule-strong);
      border-radius: 12px;
  }

  header{
    margin-bottom: 40px;
  }

  .eyebrow{
    font-family:'IBM Plex Mono', monospace;
    font-size: 12.5px;
    letter-spacing: 0.14em;
    text-transform: uppercase;
    color: var(--amber);
    margin: 0 0 10px;
  }

  h1{
    font-family:'Fraunces', serif;
    font-weight: 500;
    font-size: clamp(28px, 4.4vw, 40px);
    line-height: 1.15;
    margin: 0 0 12px;
    color: var(--chalk);
  }

  header p{
    font-size: 15px;
    color: var(--chalk-dim);
    max-width: 54ch;
    line-height: 1.6;
    margin: 0;
  }

  /* ---------- input panel ---------- */

  .panel{
    background: var(--panel);
    border: 1px solid var(--rule-strong);
    border-radius: var(--radius);
    padding: 28px;
    margin-bottom: 28px;
  }

  .field-row{
    display:flex;
    gap: 10px;
    align-items: stretch;
    flex-wrap: wrap;
  }

  .expr-input{
    flex: 1 1 260px;
    background: var(--ink);
    border: 1px solid var(--rule-strong);
    border-radius: 10px;
    color: var(--chalk);
    font-family:'IBM Plex Mono', monospace;
    font-size: 17px;
    padding: 14px 16px;
    outline: none;
    transition: border-color .15s ease, box-shadow .15s ease;
  }
  .expr-input:focus{
    border-color: var(--amber);
    box-shadow: 0 0 0 3px var(--amber-dim);
  }
  .expr-input::placeholder{ color: var(--placeholder); }

  .go-btn{
    background: var(--amber);
    color: var(--on-amber);
    border: none;
    border-radius: 10px;
    font-family:'IBM Plex Mono', monospace;
    font-weight: 600;
    font-size: 17px;
    padding: 14px 26px;
    cursor: pointer;
    transition: transform .1s ease, background .15s ease;
  }
  .go-btn:hover{ background: var(--amber-hover); }
  .go-btn:active{ transform: scale(0.97); }
  .go-btn:disabled{ opacity:.5; cursor:default; }

  .bounds-row{
    display:flex;
    gap: 10px;
    margin-top: 12px;
    flex-wrap: wrap;
    align-items:center;
  }
  .bounds-row label{
    font-family:'IBM Plex Mono', monospace;
    font-size: 12.5px;
    color: var(--chalk-dim);
    display:flex;
    align-items:center;
    gap:6px;
  }
  .bounds-row input[type=text]{
    width: 84px;
    background: var(--ink);
    border: 1px solid var(--rule-strong);
    border-radius: 8px;
    color: var(--chalk);
    font-family:'IBM Plex Mono', monospace;
    font-size: 14px;
    padding: 7px 9px;
    outline:none;
  }
  .bounds-row input[type=text]:focus{ border-color: var(--amber); }

  .quick-bounds{ display:flex; gap:4px; }
  .qb-btn{
    background: transparent;
    border: 1px solid var(--rule-strong);
    color: var(--chalk-dim);
    font-family:'IBM Plex Mono', monospace;
    font-size: 13px;
    padding: 6px 9px;
    border-radius: 8px;
    cursor:pointer;
    transition: all .12s ease;
  }
  .qb-btn:hover{ border-color: var(--amber); color: var(--amber); }

  .plot-head{
    display:flex;
    align-items:center;
    justify-content:space-between;
    gap: 12px;
    flex-wrap: wrap;
    margin-bottom: 14px;
  }
  .zoom-toolbar{ display:flex; gap:6px; }

  .toggle{
    display:flex; align-items:center; gap:7px;
    font-family:'IBM Plex Mono', monospace;
    font-size: 12.5px;
    color: var(--chalk-dim);
    margin-left: auto;
    cursor:pointer;
    user-select:none;
  }
  .toggle input{ accent-color: var(--amber); }

  /* preview */
  .preview{
    margin-top: 22px;
    padding: 22px 20px;
    background: var(--panel-2);
    border: 1px dashed var(--rule-strong);
    border-radius: 10px;
    min-height: 64px;
    display:flex;
    align-items:center;
    justify-content:center;
    overflow-x:auto;
  }
  .preview .katex{ font-size: 1.35em; color: var(--chalk); }
  .preview .placeholder{
    font-family:'IBM Plex Mono', monospace;
    color: var(--text-faint);
    font-size: 14px;
  }

  .examples{
    margin-top: 18px;
    display:flex;
    flex-wrap:wrap;
    gap: 8px;
  }
  .examples span{
    font-family:'IBM Plex Mono', monospace;
    font-size: 12px;
    color: var(--chalk-dim);
    margin-right:2px;
    align-self:center;
  }
  .ex-btn{
    background: transparent;
    border: 1px solid var(--rule-strong);
    color: var(--chalk-dim);
    font-family:'IBM Plex Mono', monospace;
    font-size: 12.5px;
    padding: 5px 10px;
    border-radius: 999px;
    cursor:pointer;
    transition: all .12s ease;
  }
  .ex-btn:hover{ border-color: var(--amber); color: var(--amber); }

  /* ---------- error ---------- */
  .error-box{
    display:none;
    margin-top: 18px;
    padding: 14px 16px;
    border-left: 3px solid var(--rust);
    background: rgba(var(--rust-rgb),0.1);
    border-radius: 0 8px 8px 0;
    color: var(--negative-text);
    font-size: 14px;
    line-height:1.5;
  }

  .help-toggle{
    display:flex;
    align-items:center;
    justify-content:space-between;
    width: 100%;
    margin-top: 18px;
    padding: 10px 14px;
    background: transparent;
    border: 1px solid var(--rule-strong);
    border-radius: 8px;
    color: var(--chalk-dim);
    font-family:'IBM Plex Sans', sans-serif;
    font-size: 13.5px;
    cursor: pointer;
    transition: border-color .12s ease, color .12s ease;
  }
  .help-toggle:hover{ border-color: var(--amber); color: var(--amber); }
  .help-toggle-icon{
    font-family:'IBM Plex Mono', monospace;
    transition: transform .15s ease;
  }
  .help-toggle-icon.open{ transform: rotate(180deg); }

  .help-panel{
    margin-top: 12px;
    padding: 18px 20px;
    background: var(--panel-2);
    border: 1px solid var(--rule-strong);
    border-radius: 10px;
  }
  .help-section{ margin-bottom: 18px; }
  .help-section:last-child{ margin-bottom: 0; }
  .help-section-title{
    font-family:'IBM Plex Sans', sans-serif;
    font-weight: 600;
    font-size: 13px;
    color: var(--chalk);
    margin-bottom: 8px;
  }
  .help-text{
    font-size: 13px;
    color: var(--chalk-dim);
    line-height: 1.65;
    margin: 0;
  }
  .help-text code{
    font-family:'IBM Plex Mono', monospace;
    background: var(--ink);
    padding: 1px 5px;
    border-radius: 4px;
    color: var(--chalk);
  }
  .help-table{
    width: 100%;
    border-collapse: collapse;
    font-size: 13px;
  }
  .help-table td{
    padding: 5px 10px 5px 0;
    color: var(--chalk-dim);
    border-bottom: 1px solid var(--rule);
    vertical-align: top;
  }
  .help-table td:first-child{
    white-space: nowrap;
    width: 1%;
  }
  .help-table code{
    font-family:'IBM Plex Mono', monospace;
    background: var(--ink);
    padding: 1px 5px;
    border-radius: 4px;
    color: var(--chalk);
    white-space: nowrap;
  }

  /* ---------- results ---------- */
  .results{ display:none; }

  .result-summary{
    background: var(--panel);
    border: 1px solid var(--rule-strong);
    border-radius: var(--radius);
    padding: 26px 28px;
    margin-bottom: 22px;
  }
  .result-summary .label{
    font-family:'IBM Plex Mono', monospace;
    font-size: 11.5px;
    letter-spacing: 0.1em;
    text-transform: uppercase;
    color: var(--chalk-dim);
    margin-bottom: 10px;
  }
  .result-summary .formula{
    font-size: 1.3em;
    overflow-x:auto;
    padding-bottom: 4px;
  }
  .result-summary .formula .katex{ color: var(--amber); }

  .formula-row{
    display:flex;
    align-items:center;
    gap: 10px;
  }
  .formula-row .formula{ flex: 1 1 auto; overflow-x:auto; }
  .copy-tex-btn{
    flex: 0 0 auto;
    background: transparent;
    border: 1px solid var(--rule-strong);
    color: var(--chalk-dim);
    font-family:'IBM Plex Mono', monospace;
    font-size: 11px;
    padding: 5px 9px;
    border-radius: 999px;
    cursor:pointer;
    transition: all .12s ease;
    white-space: nowrap;
  }
  .copy-tex-btn:hover{ border-color: var(--amber); color: var(--amber); }
  .copy-tex-btn.copied{ border-color: var(--sage); color: var(--sage); }

  .badge{
    display:inline-block;
    font-family:'IBM Plex Mono', monospace;
    font-size: 11px;
    padding: 3px 9px;
    border-radius: 999px;
    margin-top:10px;
    background: rgba(var(--sage-rgb),0.15);
    color: var(--sage);
    border: 1px solid rgba(var(--sage-rgb),0.35);
  }
  .badge.warn{
    background: rgba(var(--amber-rgb),0.12);
    color: var(--amber);
    border-color: var(--amber-dim);
  }
  .badge.divergent{
    background: rgba(var(--rust-rgb),0.14);
    color: var(--negative-text);
    border-color: rgba(var(--rust-rgb),0.4);
  }

  .definite-value{
    margin-top: 18px;
    padding-top: 18px;
    border-top: 1px solid var(--rule);
  }

  .result-summary-head{
    display:flex;
    align-items:center;
    justify-content:space-between;
    gap: 12px;
  }
  .share-btn{
    background: transparent;
    border: 1px solid var(--rule-strong);
    color: var(--chalk-dim);
    font-family:'IBM Plex Mono', monospace;
    font-size: 12px;
    padding: 6px 11px;
    border-radius: 999px;
    cursor:pointer;
    transition: all .12s ease;
    white-space: nowrap;
  }
  .share-btn:hover{ border-color: var(--amber); color: var(--amber); }
  .share-btn.copied{ border-color: var(--sage); color: var(--sage); }

  .plot-legend{
    display:flex;
    gap: 18px;
    margin: 10px 2px 0;
    font-family:'IBM Plex Mono', monospace;
    font-size: 12.5px;
    color: var(--chalk-dim);
  }
  .plot-legend .dot{
    display:inline-block;
    width:9px; height:9px;
    border-radius:50%;
    margin-right:6px;
  }
  .legend-toggle{
    display:flex;
    align-items:center;
    cursor:pointer;
    user-select:none;
  }
  .legend-toggle input{ margin-right:6px; accent-color: var(--amber); }

  .breakdown-caption{
    margin: 12px 2px 0;
    font-family:'IBM Plex Mono', monospace;
    font-size: 12.5px;
    color: var(--chalk-dim);
    display:flex;
    align-items:center;
    gap: 6px;
    flex-wrap: wrap;
  }
  .breakdown-caption .pos{ color: var(--amber); }
  .breakdown-caption .neg{ color: var(--negative-text); }
  .breakdown-caption .net{ color: var(--chalk); font-weight:600; }

  .plot-footer{
    display:flex;
    justify-content:space-between;
    align-items:center;
    flex-wrap:wrap;
    gap: 14px;
    margin-top: 10px;
  }

  .value-table{
    display:flex;
    align-items:center;
    gap: 8px;
    flex-wrap:wrap;
    font-family:'IBM Plex Mono', monospace;
    font-size: 12.5px;
    color: var(--chalk-dim);
  }
  .value-table-label{ white-space:nowrap; }
  .value-table input{
    width: 74px;
    background: var(--ink);
    border: 1px solid var(--rule-strong);
    border-radius: 8px;
    color: var(--chalk);
    font-family:'IBM Plex Mono', monospace;
    font-size: 12.5px;
    padding: 6px 8px;
    outline:none;
  }
  .value-table input:focus{ border-color: var(--amber); }
  .value-out{ color: var(--chalk); white-space:nowrap; }
  .value-out em{ font-style:normal; color: var(--chalk-dim); }

  .check-hint{
    font-size: 13.5px;
    color: var(--chalk-dim);
    line-height:1.6;
    margin: 0 0 16px;
  }
  .check-hint code{
    font-family:'IBM Plex Mono', monospace;
    background: var(--panel-2);
    padding: 1px 5px;
    border-radius: 4px;
  }
  .check-result{
    margin-top: 14px;
    padding: 12px 14px;
    border-radius: 8px;
    font-size: 14px;
    line-height:1.5;
  }
  .check-result.correct{
    background: rgba(var(--sage-rgb),0.12);
    border: 1px solid rgba(var(--sage-rgb),0.35);
    color: var(--sage);
  }
  .check-result.incorrect{
    background: rgba(var(--rust-rgb),0.1);
    border: 1px solid rgba(var(--rust-rgb),0.35);
    color: var(--negative-text);
  }

  .disclaimer{
    margin-top: 28px;
    font-size: 12px;
    color: var(--text-faint);
    line-height:1.6;
    text-align:center;
  }

  .theorem-note{
    background: var(--panel-2);
    border-left: 3px solid var(--amber);
    border-radius: 0 10px 10px 0;
    padding: 14px 18px;
    margin-bottom: 22px;
  }
  .theorem-note-title{
    font-family:'IBM Plex Sans', sans-serif;
    font-weight: 600;
    font-size: 14px;
    color: var(--amber);
    margin-bottom: 6px;
  }
  .theorem-note-detail{
    font-size: 13.5px;
    color: var(--chalk-dim);
    line-height: 1.65;
  }
  .theorem-note-detail .katex{ color: var(--chalk); font-size: 1.02em; }

  .sample-points-table{
    width: 100%;
    border-collapse: collapse;
    margin: 14px 0 0;
    font-family:'IBM Plex Mono', monospace;
    font-size: 12.5px;
  }
  .sample-points-table th{
    text-align: left;
    color: var(--chalk-dim);
    font-weight: 500;
    padding: 4px 10px 6px 0;
    border-bottom: 1px solid var(--rule-strong);
  }
  .sample-points-table td{
    padding: 5px 10px 5px 0;
    color: var(--chalk);
    border-bottom: 1px solid var(--rule);
  }
  .sample-points-table td:first-child{ color: var(--chalk-dim); white-space: nowrap; }

  /* derivation ladder — the signature element */
  .ladder-heading{
    font-family:'Fraunces', serif;
    font-size: 20px;
    font-weight: 500;
    margin: 6px 0 18px 4px;
    color: var(--chalk);
  }

  .ladder{
    position: relative;
    padding-left: 44px;
  }
  .ladder::before{
    content:'';
    position:absolute;
    left: 15px;
    top: 8px;
    bottom: 8px;
    width: 2px;
    background: linear-gradient(var(--amber-dim), var(--rule-strong) 92%);
  }

  .step{
    position: relative;
    margin-bottom: 22px;
    background: var(--panel);
    border: 1px solid var(--rule-strong);
    border-radius: 10px;
    padding: 16px 20px;
  }
  .step::before{
    content: attr(data-n);
    position: absolute;
    left: -44px;
    top: 14px;
    width: 30px;
    height: 30px;
    border-radius: 50%;
    background: var(--ink);
    border: 2px solid var(--amber);
    color: var(--amber);
    font-family:'IBM Plex Mono', monospace;
    font-size: 13px;
    font-weight:600;
    display:flex;
    align-items:center;
    justify-content:center;
  }
  .step-title{
    font-family:'IBM Plex Sans', sans-serif;
    font-weight: 600;
    font-size: 14.5px;
    color: var(--chalk);
    margin-bottom: 6px;
  }
  .step-detail{
    font-size: 14.5px;
    color: var(--chalk-dim);
    line-height:1.7;
    overflow-x:auto;
  }
  .step-detail .katex{ color: var(--chalk); font-size: 1.05em; }

  .graph-note{
    margin-top: 26px;
    font-family:'IBM Plex Mono', monospace;
    font-size: 12.5px;
    color: var(--text-faint);
    text-align:center;
  }

  footer{
    margin-top: 50px;
    padding-top: 22px;
    border-top: 1px solid var(--rule);
    font-family:'IBM Plex Mono', monospace;
    font-size: 12px;
    color: var(--text-faint);
    display:flex;
    justify-content:space-between;
    flex-wrap:wrap;
    gap: 8px;
  }
  footer a{ color: var(--chalk-dim); }

  @media (max-width: 520px){
    .ladder{ padding-left: 34px; }
    .step::before{ left: -34px; width:26px; height:26px; font-size:11px; }
  }

  .spinner{
    display:inline-block;
    width: 15px; height:15px;
    border: 2px solid rgba(var(--on-amber-rgb),0.35);
    border-top-color: var(--on-amber);
    border-radius:50%;
    animation: spin .7s linear infinite;
    vertical-align: -3px;
    margin-right: 7px;
  }
  @keyframes spin{ to{ transform: rotate(360deg); } }

  /* ==========================================================================
     ADDED for the derivative calculator's step<->formula linking (feature
     request: hover/click a solution step, highlight the exact piece of
     "Your input" it's about). Nothing above this line was touched —
     appended only, so the original stylesheet stays diffable against the
     integral calculator's.
     ========================================================================== */
  .step{ cursor: pointer; }
  .step.step-active{
    background: rgba(var(--amber-rgb),0.05);
  }
  [id^="deriv-step-"]{
    border-radius: 3px;
    transition: background .12s ease;
  }
  [id^="deriv-step-"].formula-highlight{
    background: rgba(var(--amber-rgb),0.22);
  }
  .critical-points-list{
    font-family: 'IBM Plex Mono', monospace;
    font-size: .85rem;
    color: var(--chalk-dim, var(--chalk));
    margin: 6px 0 0;
  }
  .critical-points-list b{ color: var(--chalk); font-weight: 600; }
  .sample-points-table tr.user-added td{ color: var(--amber); }

  /* ==========================================================================
     ADDED: on-screen math keyboard (mouse/touch formula entry above #expr).
     Nothing above this line was touched — appended only, same convention
     as the step<->formula linking block above.
     ========================================================================== */

  .mk-mount{ margin-bottom: 10px; }

  .mk-keyboard{
    display: flex;
    flex-direction: column;
    gap: 6px;
  }

  .mk-row{
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(40px, 1fr));
    gap: 6px;
  }

  .mk-row--menus{
    grid-template-columns: repeat(auto-fit, minmax(120px, 1fr));
  }

  .mk-key{
    background: transparent;
    border: 1px solid var(--rule-strong);
    color: var(--chalk);
    font-family: 'IBM Plex Mono', monospace;
    font-size: 15px;
    line-height: 1;
    min-height: 40px;
    padding: 8px 6px;
    border-radius: 8px;
    cursor: pointer;
    transition: background .1s ease, border-color .1s ease, transform .06s ease;
  }
  .mk-key:hover{ border-color: var(--amber); color: var(--amber); }
  .mk-key:active{ transform: scale(.94); background: var(--amber-dim); }
  .mk-key:focus-visible{ outline: 2px solid var(--amber); outline-offset: 1px; }

  .mk-key--accent{
    background: var(--amber);
    border-color: var(--amber);
    color: var(--on-amber);
    font-weight: 600;
  }
  .mk-key--accent:hover{ color: var(--on-amber); filter: brightness(1.06); }
  .mk-key--accent:active{ background: var(--amber-hover); }

  .mk-key--muted{ color: var(--chalk-dim); font-size: 12px; letter-spacing: .03em; }

  .mk-key-with-menu{
    position: relative;
    display: grid;
    grid-template-columns: 1fr auto;
    gap: 2px;
  }
  .mk-key-with-menu .mk-key{ width: 100%; }
  .mk-key-caret{
    background: transparent;
    border: 1px solid var(--rule-strong);
    border-left: none;
    color: var(--chalk-dim);
    width: 20px;
    border-radius: 0 8px 8px 0;
    cursor: pointer;
    font-size: 10px;
  }
  .mk-key-caret:hover{ color: var(--amber); border-color: var(--amber); }
  .mk-key-caret:focus-visible{ outline: 2px solid var(--amber); outline-offset: 1px; }

  .mk-menu-wrapper{ position: relative; }
  .mk-menu-btn{
    width: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 6px;
    background: var(--panel-2);
    border: 1px solid var(--rule-strong);
    color: var(--chalk);
    font-family: 'IBM Plex Sans', sans-serif;
    font-size: 13px;
    min-height: 38px;
    padding: 6px 10px;
    border-radius: 8px;
    cursor: pointer;
    transition: all .12s ease;
  }
  .mk-menu-btn:hover{ border-color: var(--amber); color: var(--amber); }
  .mk-menu-btn:focus-visible{ outline: 2px solid var(--amber); outline-offset: 1px; }
  .mk-menu-btn[aria-expanded="true"]{ border-color: var(--amber); color: var(--amber); }
  .mk-menu-caret{ font-size: 10px; opacity: .7; }

  .mk-popover{
    position: absolute;
    z-index: 30;
    top: calc(100% + 6px);
    left: 0;
    min-width: 220px;
    max-width: min(92vw, 360px);
    background: var(--panel);
    border: 1px solid var(--rule-strong);
    border-radius: 10px;
    box-shadow: 0 8px 24px rgba(0,0,0,.18);
    padding: 10px;
    display: flex;
    flex-direction: column;
    gap: 4px;
  }
  .mk-popover--grid{ gap: 8px; }

  .mk-popover-heading{
    font-family: 'IBM Plex Mono', monospace;
    font-size: 10px;
    letter-spacing: .05em;
    text-transform: uppercase;
    color: var(--chalk-dim);
    margin-top: 4px;
  }
  .mk-popover-heading:first-child{ margin-top: 0; }

  .mk-popover-row{
    display: flex;
    flex-wrap: wrap;
    gap: 4px;
  }
  .mk-popover-row--wrap{ max-width: 100%; }

  .mk-popover-item{
    background: transparent;
    border: 1px solid var(--rule-strong);
    color: var(--chalk);
    font-family: 'IBM Plex Mono', monospace;
    font-size: 13px;
    min-width: 40px;
    min-height: 36px;
    padding: 6px 8px;
    border-radius: 7px;
    cursor: pointer;
    transition: all .1s ease;
  }
  .mk-popover-item:hover{ border-color: var(--amber); color: var(--amber); background: var(--amber-dim); }
  .mk-popover-item:active{ transform: scale(.94); }
  .mk-popover-item:focus-visible{ outline: 2px solid var(--amber); outline-offset: 1px; }
  .mk-popover-item--symbol{ min-width: 36px; font-size: 15px; }

  .mk-popover-more{
    align-self: flex-start;
    background: transparent;
    border: none;
    color: var(--chalk-dim);
    font-family: 'IBM Plex Sans', sans-serif;
    font-size: 12px;
    cursor: pointer;
    padding: 4px 2px;
    margin-top: 2px;
  }
  .mk-popover-more:hover{ color: var(--amber); }

  @media (max-width: 480px){
    .mk-row{ grid-template-columns: repeat(auto-fit, minmax(36px, 1fr)); }
    .mk-key{ min-height: 44px; font-size: 16px; } /* comfortable touch target */
    .mk-popover{ left: 50%; transform: translateX(-50%); max-width: 94vw; }
  }

  /* ---------- derivative chain cards ---------- */
  .chain-card{ position: relative; }
  .chain-card .badge-row:empty{ display:none; }

  .card-actions{
    display:flex;
    gap:8px;
    flex-wrap:wrap;
    margin-top:14px;
  }

  .ladder-wrap{ margin-top:18px; }

  .check-panel{
    margin-top:18px;
    margin-bottom:0;
    padding:20px 22px;
  }
  .check-panel .check-hint{
    color: var(--chalk-dim);
    font-size: 13px;
    line-height:1.5;
    margin: 0 0 12px;
  }

  .calculate-next-btn{
    display:inline-flex;
    align-items:center;
    justify-content:center;
    width:100%;
    padding:14px;
    font-size:14px;
    margin-bottom:28px;
  }