@layer collapse {
  /* The panel-header is the collapsed-state title strip. While the panel
     is expanded, the in-content H1 (injected by `renderer.injectTitleHeading`)
     is the visible title, so the header element stays hidden to avoid
     duplicating it. */
  .panel:not([data-collapsed="true"]) .panel-header { display: none; }

  .panel[data-collapsed="true"] .panel-content,
  .panel[data-collapsed="true"] .panel-backlinks { display: none; }

  .panel[data-collapsed="true"] .panel-header {
    writing-mode: vertical-rl;
    height: 100%;
    padding: var(--space-sm);
  }
}