:root {
  --bg: #f4f6fa;
  --panel: #ffffff;
  --panel-2: #f1f4f9;
  --line: #dfe4ec;
  --line-2: #eef1f6;
  --text: #1b2231;
  --muted: #6b7688;
  --accent: #2563eb;
  --pos: #2563eb;
  --neg: #ea580c;
  --sv: #d97706;
  --radius: 10px;
  --shadow: 0 1px 2px rgba(20, 30, 55, .05), 0 6px 18px rgba(20, 30, 55, .05);
}

* { box-sizing: border-box; }
[hidden] { display: none !important; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font: 14px/1.55 "Segoe UI", system-ui, -apple-system, "Helvetica Neue", Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
}

code, kbd {
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: .92em;
  background: var(--panel-2);
  border: 1px solid var(--line);
  border-radius: 4px;
  padding: 0 4px;
}

/* ---------- thanh tiêu đề + tab ---------- */
.topbar {
  display: flex; align-items: center; justify-content: space-between; flex-wrap: wrap; gap: 12px;
  padding: 14px 24px 0;
  background: var(--panel);
  border-bottom: 1px solid var(--line);
}
.brand { display: flex; align-items: baseline; gap: 10px; padding-bottom: 12px; }
.brand h1 { margin: 0; font-size: 18px; letter-spacing: -.2px; }
.brand .sub { color: var(--muted); font-size: 12.5px; }

.tabs { display: flex; gap: 4px; }
.tabs button {
  background: none; border: none; border-bottom: 2px solid transparent;
  font: 600 13px/1 inherit; color: var(--muted);
  padding: 10px 14px 12px; cursor: pointer;
}
.tabs button:hover { color: var(--text); }
.tabs button.active { color: var(--accent); border-bottom-color: var(--accent); }

/* ---------- bố cục ---------- */
.layout {
  display: grid;
  grid-template-columns: 260px minmax(420px, 1fr) 290px;
  gap: 16px;
  padding: 18px 24px 28px;
  align-items: start;
}

.panel {
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 14px 16px 16px;
}
.panel h2 {
  margin: 2px 0 12px;
  font-size: 11px; font-weight: 700;
  text-transform: uppercase; letter-spacing: .9px;
  color: var(--muted);
}
.panel h2:not(:first-child) { margin-top: 22px; padding-top: 16px; border-top: 1px solid var(--line-2); }

.field { margin-bottom: 18px; }
.lbl {
  display: flex; align-items: center; gap: 6px;
  font-size: 12px; font-weight: 600; color: var(--muted);
  margin-bottom: 6px;
}
.msg { margin: 8px 0 0; font-size: 11.5px; color: var(--muted); }
.msg.warn { color: #b45309; }

/* ---------- nút ? và popover ---------- */
button.help {
  width: 15px; height: 15px; flex: none; padding: 0;
  border: 1px solid var(--line); border-radius: 50%;
  background: var(--panel-2); color: var(--muted);
  font: 700 10px/13px inherit; text-align: center; cursor: help;
}
button.help:hover, button.help[aria-expanded="true"] {
  background: var(--accent); border-color: var(--accent); color: #fff;
}
.tip {
  position: absolute; z-index: 50; max-width: 300px;
  background: #1b2231; color: #eaeef6;
  border-radius: 8px; padding: 10px 12px;
  font-size: 12px; line-height: 1.5;
  box-shadow: 0 8px 26px rgba(15, 23, 42, .28);
}
.tip code, .tip kbd { background: rgba(255,255,255,.12); border-color: rgba(255,255,255,.2); color: #fff; }
.tip b { color: #fff; }

/* ---------- điều khiển ---------- */
button {
  font: inherit; color: var(--text);
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 8px; padding: 7px 10px; cursor: pointer;
}
button:hover { border-color: #bcc6d6; }
button.primary { background: var(--accent); border-color: var(--accent); color: #fff; font-weight: 600; }
button.primary:hover { background: #1d4ed8; }
button.ghost { background: var(--panel-2); }

.segmented { display: flex; }
.segmented button { border-radius: 0; flex: 1; padding: 7px 4px; font-size: 12.5px; }
.segmented button:first-child { border-radius: 8px 0 0 8px; }
.segmented button:last-child { border-radius: 0 8px 8px 0; }
.segmented button + button { border-left: none; }
.segmented button.active { background: var(--accent); border-color: var(--accent); color: #fff; font-weight: 600; }
#modeGroup button[data-mode="add-pos"].active { background: var(--pos); border-color: var(--pos); }
#modeGroup button[data-mode="add-neg"].active { background: var(--neg); border-color: var(--neg); }

.row2 { display: flex; gap: 8px; margin-top: 8px; }
.row2 button { flex: 1; }

select, .filebtn {
  width: 100%; font: inherit; color: var(--text);
  background: var(--panel); border: 1px solid var(--line);
  border-radius: 8px; padding: 7px 9px;
}
.filebtn { display: block; text-align: center; cursor: pointer; background: var(--panel-2); }
.filebtn:hover { border-color: #bcc6d6; }

.slider { margin-bottom: 15px; }
.slider label { display: flex; align-items: center; gap: 6px; font-size: 12px; color: var(--muted); font-weight: 600; }
.slider output { margin-left: auto; color: var(--text); font-variant-numeric: tabular-nums; }
input[type=range] { width: 100%; accent-color: var(--accent); margin: 5px 0 0; }
.check { display: flex; align-items: center; gap: 6px; font-size: 12px; color: var(--muted); margin-top: 7px; font-weight: 400; }
.slider.off { display: none; }

.adv { border-top: 1px solid var(--line-2); padding-top: 10px; }
.adv summary { cursor: pointer; font-size: 12px; color: var(--muted); margin-bottom: 12px; }
.actions { margin-top: 14px; }

/* ---------- khung vẽ ---------- */
.plot-head { display: flex; flex-wrap: wrap; align-items: baseline; justify-content: space-between; gap: 8px; }
.plot-head h2 { margin-bottom: 8px; }
.toggles { display: flex; flex-wrap: wrap; gap: 12px; font-size: 12px; color: var(--muted); }
.toggles label { display: flex; align-items: center; gap: 5px; cursor: pointer; }

.canvas-wrap { position: relative; border: 1px solid var(--line); border-radius: 8px; overflow: hidden; background: #fff; }
canvas { display: block; width: 100%; height: auto; cursor: crosshair; touch-action: none; }
.drop-hint {
  position: absolute; inset: 0; display: none; align-items: center; justify-content: center;
  background: rgba(37, 99, 235, .1); border: 2px dashed var(--accent);
  font-weight: 600; color: var(--accent);
}
.canvas-wrap.dragover .drop-hint { display: flex; }

.plot-foot { display: flex; justify-content: space-between; align-items: center; margin-top: 9px; font-size: 12px; color: var(--muted); flex-wrap: wrap; gap: 8px; }
.legend { display: flex; align-items: center; gap: 6px; }
.legend .dot { width: 10px; height: 10px; border-radius: 50%; display: inline-block; margin-left: 10px; }
.legend .dot:first-child { margin-left: 0; }
.dot.pos { background: var(--pos); }
.dot.neg { background: var(--neg); }
.dot.sv { background: transparent; border: 2px solid var(--sv); }
.cursor-info { font-variant-numeric: tabular-nums; }

/* ---------- bảng kết quả ---------- */
.stats { width: 100%; border-collapse: collapse; font-size: 12.5px; }
.stats th, .stats td { text-align: left; padding: 6px 0; border-bottom: 1px solid var(--line-2); vertical-align: top; }
.stats th { font-weight: 500; color: var(--muted); width: 54%; }
.stats th .help { margin-left: 3px; vertical-align: 1px; }
.stats td { font-variant-numeric: tabular-nums; }

/* ---------- trang hướng dẫn ---------- */
.help-page {
  max-width: 760px; margin: 0 auto; padding: 26px 24px 60px;
  font-size: 14.5px;
}
.help-page h2 {
  font-size: 15px; margin: 34px 0 8px;
  padding-bottom: 6px; border-bottom: 1px solid var(--line);
}
.help-page h2:first-child { margin-top: 6px; }
.help-page p { margin: 0 0 12px; color: #2f3849; }
.help-page .eq {
  background: var(--panel); border: 1px solid var(--line); border-radius: 8px;
  padding: 12px 14px; text-align: center; font-family: ui-monospace, Menlo, Consolas, monospace;
  font-size: 13.5px;
}
.cmp { margin: 0 0 12px; padding-left: 20px; color: #2f3849; }
.cmp li { margin-bottom: 6px; }

.fig { margin: 6px 0 20px; }
.fig svg { width: 100%; height: auto; background: var(--panel); border: 1px solid var(--line); border-radius: 8px; }
.fig figcaption { margin-top: 8px; font-size: 12.5px; color: var(--muted); }
.d-bound { stroke: #1b2231; stroke-width: 2; fill: none; }
.d-bound-curve { stroke: #1b2231; stroke-width: 2; fill: none; stroke-dasharray: none; }
.d-margin { stroke: #98a3b6; stroke-width: 1.2; stroke-dasharray: 6 4; }
.d-box { fill: none; stroke: var(--line); }
.d-pos circle { fill: var(--pos); }
.d-neg circle { fill: var(--neg); }
.d-svring circle { fill: none; stroke: var(--sv); stroke-width: 2.2; }
.d-ann text { fill: var(--muted); font: 11px system-ui, sans-serif; }
.d-arrow { stroke: var(--muted); stroke-width: 1; }
.d-cap { fill: var(--muted); font: 11px system-ui, sans-serif; }
.d-map { fill: var(--muted); font: italic 600 15px system-ui, sans-serif; }

.ktab { width: 100%; border-collapse: collapse; font-size: 13px; margin: 4px 0 16px; background: var(--panel); }
.ktab th, .ktab td { text-align: left; padding: 8px 10px; border: 1px solid var(--line); vertical-align: top; }
.ktab th { background: var(--panel-2); font-weight: 600; font-size: 12px; color: var(--muted); }

@media (max-width: 1080px) {
  .layout { grid-template-columns: 1fr; }
}