// Converts single line breaks to BR tags, multiple line breaks to P tags. Does nothing more.
public static string PlainTextToHtml(string text)
{
    if (string.IsNullOrWhiteSpace(text)) return text?.Trim();
    text = text.Trim();

    // Remove blanks from otherwise completely empty lines.
    text = Regex.Replace(text, @"^\s+$", string.Empty, RegexOptions.Multiline);

    text = text.Replace("\r\n", "\r");
    text = text.Replace("\n", "\r");
    text = text.Replace("\r", "\r\n");

    // Codense multiple blank lines.
    text = Regex.Replace(text, "\n{3,}", Environment.NewLine + Environment.NewLine);

    text = HttpUtility.HtmlEncode(text);

    text = text.Replace("\r\n", "\r");
    text = text.Replace("\n", "\r");

    text = text.Replace("\r\r", @"</p><p>");

    text = text.Replace("\r", @"<br>");

    text = @"<p>" + text + @"</p>";

    return text;
}