.username-group { position: relative; }

  .field-tip {
    position: absolute;
    top: 40%;
    left: calc(60% + 10px);        /* 紧挨输入框右侧 */
    transform: translateY(-50%);
    width: 260px;
    background: #fff;
    border: 1px solid #e5e7eb;
    border-radius: 8px;
    box-shadow: 0 8px 24px rgba(0,0,0,0.08);
    padding: 10px 12px;
    font-size: 12px;
    line-height: 1.5;
    color: #111827;

    /* 初始隐藏 */
    opacity: 0;
    visibility: hidden;
    pointer-events: none;
    transition: opacity .15s ease, visibility .15s ease;
    z-index: 10;
  }
  .field-tip::after {                /* 小三角箭头 */
    content: "";
    position: absolute;
    top: 50%;
    left: -6px;
    transform: translateY(-50%);
    border-width: 6px;
    border-style: solid;
    border-color: transparent #e5e7eb transparent transparent;
  }
  .field-tip::before {               /* 箭头内层白色 */
    content: "";
    position: absolute;
    top: 50%;
    left: -5px;
    transform: translateY(-50%);
    border-width: 5px;
    border-style: solid;
    border-color: transparent #fff transparent transparent;
    z-index: 1;
  }
  .field-tip strong { display: block; margin-bottom: 6px; }
  .field-tip ul { margin: 0; padding-left: 16px; }
  .field-tip li { margin: 2px 0; color: #374151; }
  .field-tip span { font-weight: 600; color: #111827; }

  /* 显示状态 */
  .field-tip.is-visible {
    opacity: 1;
    visibility: visible;
  }

  /* 小屏时把提示框放到输入框下方，避免遮挡 */
  @media (max-width: 640px) {
    .field-tip {
      top: calc(60% + 8px);
      left: 0;
      transform: none;
      width: min(92vw, 320px);
    }
    .field-tip::after,
    .field-tip::before {
      display: none;
    }
  }