.progression {
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
  background: var(--layer-01);
}

/* ---- stage tabs ---- */
.tabs {
  display: flex;
  flex-wrap: wrap;
  gap: 0;
  border-bottom: 1px solid var(--border);
  background: var(--bg);
}
.tab {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  height: var(--control-height);
  padding: 0 var(--space-4);
  font: inherit;
  font-size: 13px;
  color: var(--text-secondary);
  background: transparent;
  border: none;
  border-bottom: 2px solid transparent;
  cursor: pointer;
  white-space: nowrap;
}
.tab:hover {
  color: var(--text);
  background: var(--control-hover-fill);
}
.tab:focus-visible {
  outline: 2px solid var(--focus);
  outline-offset: -2px;
}
.tabActive {
  color: var(--text);
  border-bottom-color: var(--accent);
}
.tabNum {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 18px;
  height: 18px;
  font-size: 11px;
  border-radius: 50%;
  background: var(--field);
  color: var(--text-secondary);
}
.tabActive .tabNum {
  background: var(--accent);
  color: var(--accent-contrast);
}

/* ---- the two-pane stage ---- */
.panes {
  display: grid;
  grid-template-columns: 1fr 1fr;
  align-items: stretch;
}
.source {
  margin: 0;
  padding: var(--space-4) var(--space-5);
  max-height: 420px;
  overflow: auto;
  font-family: var(--font-mono);
  font-size: 12px;
  line-height: 1.6;
  color: var(--text);
  background: var(--field);
  border-right: 1px solid var(--border);
}
.source:focus-visible {
  outline: 2px solid var(--focus);
  outline-offset: -2px;
}
.line {
  display: block;
  white-space: pre;
  padding: 0 var(--space-3);
  margin: 0 calc(-1 * var(--space-3));
}
/* The changed lines for this stage — what its edit did. */
.added {
  background: var(--accent-soft);
  box-shadow: inset 2px 0 0 var(--accent);
}
/* A block (not a centering flexbox): the chart host must inherit a definite
   width from the grid column, or Vega's width:"container" fit collapses to ~0
   (the §2 trap in docs/embedding-vega-lite.md). min-width:0 lets the column
   shrink below the chart's natural content width. */
.chartPane {
  min-width: 0;
  padding: var(--space-5);
}
.chart {
  width: 100%;
  min-height: 180px;
}

/* The per-stage hand-off into the app: the stage's spec, self-contained, as a
   snippet — opened in a new tab so the lesson keeps its place. */
.openLink {
  display: inline-block;
  margin-top: var(--space-3);
  font-size: 13px;
  color: var(--accent);
  text-decoration: none;
}
.openLink:hover {
  text-decoration: underline;
}

.note {
  padding: var(--space-4) var(--space-5);
  border-top: 1px solid var(--border);
  font-size: 14px;
  color: var(--text-secondary);
  background: var(--bg);
}
.note p {
  margin: 0;
}
.note code {
  font-family: var(--font-mono);
  font-size: 0.9em;
  color: var(--text);
}

/* Stack the panes on narrow screens. */
@media (max-width: 720px) {
  .panes {
    grid-template-columns: 1fr;
  }
  .source {
    border-right: none;
    border-bottom: 1px solid var(--border);
    max-height: 280px;
  }
}