Function upsertCssRule

packages/sdk/src/engine/cssWriter.ts:117–150

(
  css: string,
  selector: string,
  styles: Record<string, string | null>,
)

 * Returns the modified CSS string. Returns `css` unchanged when nothing changed.
 */
export function upsertCssRule(
  css: string,
  selector: string,
  styles: Record<string, string | null>,
): string {
  const normalized = normalizeSelector(selector);
  const rules = parseCssRules(css);
  const idx = rules.findIndex((r) => normalizeSelector(r.selector) === normalized);

  if (idx !== -1) {
    const rule = rules[idx]!;
    const decls = parseDeclarations(rule.body);
    for (const [prop, value] of Object.entries(styles)) {
      if (value === null) {
        delete decls[prop];
      } else {
        decls[prop] = value;
      }
    }
    const newRuleText = `${rule.selector} {${serializeDeclarations(decls)}}`;
    return css.slice(0, rule.start) + newRuleText + css.slice(rule.end);
  }

  // Append new rule — skip if all values are null (nothing to write).
  const newDecls: Record<string, string> = {};
  for (const [prop, value] of Object.entries(styles)) {
    if (value !== null) newDecls[prop] = value;
  }
  if (!Object.keys(newDecls).length) return css;

  const newRuleText = `${selector} {${serializeDeclarations(newDecls)}}`;
  const sep = css.length > 0 && !css.endsWith("\n") ? "\n" : "";
  return css + sep + newRuleText + "\n";
}